Overhaul implementation of for-generators (#844)

Motivation:
* fix known bugs and limitations of for-generators
* improve code health by removing complex workarounds

Changes:
* simplify AstBuilder code related to for-generators
  * track for-generators via `SymbolTable.enterForGenerator()`
  * add `RestoreForBindingsNode` during initial AST construction
    instead of calling `MemberNode.replaceBody()` later on
  * simplify some unnecessarily complex code
* remove workarounds and band-aids such as:
  * `isInIterable`
  * `executeAndSetEagerly`
  * adding dummy slots in `AmendFunctionNode`
* overhaul implementation of for-generators
  * store keys and values of for-generator iterations in regular instead of auxiliary frame slots
    * set them via `TypeNode.executeAndSet()`
    * `ResolveVariableNode` no longer needs to search auxiliary slots
    * `Read(Enclosing)AuxiliarySlot` is no longer needed
  * at the start of each for-generator iteration, create a new `VirtualFrame`
    that is a copy of the current frame (arguments + slots)
    and stores the iteration key and value in additional slots.
  * execute for-generator iteration with the newly created frame
    * `childNode.execute(newFrame)`
    * Pkl objects created during the iteration will materialize this frame
  * store newly created frames in `owner.extraStorage`
    if their for-generator slots may be accessed when a generated member is executed
    * resolving variable names to for-generator variables at parse time would make this analysis more precise
  * when a generated member is executed,
	  * retrieve the corresponding frame stored in `owner.extraStorage`
	  * copy the retrieved frame's for-generator slots into slots of the current frame

Result:
* for-generators are implemented in a correct, reasonably simple, and reasonably efficient way
  * complexity is fully contained within package `generator` and `AstBuilder`
* for-generator keys and values can be accessed from all nested scopes:
  * key and value expressions of generated members
  * condition expressions of nested when-generators
  * iterable expressions of nested for-generators
* for-generator keys and values can be accessed from within objects created by the expressions listed above
* sibling for-generators can use the same key/value variable names
* parent/child for-generators can use the same key/value variable names
* fixes https://github.com/apple/pkl/issues/741

pkl-core/src/main/java/org/pkl/core/ast/MemberNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,7 +17,6 @@ package org.pkl.core.ast;
 
 import com.oracle.truffle.api.frame.FrameDescriptor;
 import com.oracle.truffle.api.source.SourceSection;
-import java.util.function.Function;
 import org.pkl.core.ast.member.DefaultPropertyBodyNode;
 import org.pkl.core.runtime.VmExceptionBuilder;
 import org.pkl.core.runtime.VmLanguage;
@@ -43,10 +42,6 @@ public abstract class MemberNode extends PklRootNode {
     return bodyNode;
   }
 
-  public final void replaceBody(Function<ExpressionNode, ExpressionNode> replacer) {
-    bodyNode = insert(replacer.apply(bodyNode));
-  }
-
   protected final VmExceptionBuilder exceptionBuilder() {
     return new VmExceptionBuilder().withSourceSection(getHeaderSection());
   }

pkl-core/src/main/java/org/pkl/core/ast/VmModifier.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -55,9 +55,6 @@ public final class VmModifier {
 
   public static final int GLOB = 0x1000;
 
-  // To be removed when https://github.com/apple/pkl/issues/741 is fixed
-  public static final int IS_IN_ITERABLE = 0x100000;
-
   // modifier sets
 
   public static final int NONE = 0;
@@ -137,10 +134,6 @@ public final class VmModifier {
     return (modifiers & ENTRY) != 0;
   }
 
-  public static boolean isInIterable(int modifiers) {
-    return (modifiers & IS_IN_ITERABLE) != 0;
-  }
-
   public static boolean isType(int modifiers) {
     return (modifiers & (CLASS | TYPE_ALIAS | IMPORT)) != 0 && (modifiers & GLOB) == 0;
   }

pkl-core/src/main/java/org/pkl/core/ast/builder/AstBuilder.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,6 @@ import java.io.IOException;
 import java.net.URI;
 import java.net.URISyntaxException;
 import java.util.*;
-import java.util.function.Function;
 import java.util.function.Supplier;
 import java.util.stream.Collectors;
 import org.antlr.v4.runtime.ParserRuleContext;
@@ -40,7 +39,6 @@ import org.pkl.core.TypeParameter.Variance;
 import org.pkl.core.ast.*;
 import org.pkl.core.ast.builder.SymbolTable.AnnotationScope;
 import org.pkl.core.ast.builder.SymbolTable.ClassScope;
-import org.pkl.core.ast.builder.SymbolTable.EntryScope;
 import org.pkl.core.ast.builder.SymbolTable.Scope;
 import org.pkl.core.ast.expression.binary.*;
 import org.pkl.core.ast.expression.generator.*;
@@ -852,20 +850,20 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
 
   @Override
   public GeneratorPropertyNode visitObjectProperty(ObjectPropertyContext ctx) {
-    checkHasNoForGenerator(ctx, "forGeneratorCannotGenerateProperties");
+    checkNotInsideForGenerator(ctx, "forGeneratorCannotGenerateProperties");
     var member = doVisitObjectProperty(ctx);
     return GeneratorPropertyNodeGen.create(member);
   }
 
   @Override
   public GeneratorMemberNode visitObjectMethod(ObjectMethodContext ctx) {
-    checkHasNoForGenerator(ctx, "forGeneratorCannotGenerateMethods");
+    checkNotInsideForGenerator(ctx, "forGeneratorCannotGenerateMethods");
     var member = doVisitObjectMethod(ctx);
     return GeneratorPropertyNodeGen.create(member);
   }
 
-  private void checkHasNoForGenerator(ParserRuleContext ctx, String errorMessageKey) {
+  private void checkNotInsideForGenerator(ParserRuleContext ctx, String errorMessageKey) {
-    if (symbolTable.getCurrentScope().getForGeneratorVariables().isEmpty()) {
+    if (!symbolTable.getCurrentScope().isForGeneratorScope()) {
       return;
     }
     var forExprCtx = ctx.getParent();
@@ -880,12 +878,19 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
 
   @Override
   public GeneratorMemberNode visitMemberPredicate(MemberPredicateContext ctx) {
-    var keyNodeAndMember = doVisitMemberPredicate(ctx);
+    if (ctx.err1 == null && ctx.err2 == null) {
-    var keyNode = keyNodeAndMember.first;
+      throw missingDelimiter("]]", ctx.k.stop.getStopIndex() + 1);
-    var member = keyNodeAndMember.second;
+    } else if (ctx.err1 != null
-    insertWriteForGeneratorVarsToFrameSlotsNode(member.getMemberNode());
+        && (ctx.err2 == null || ctx.err1.getStartIndex() != ctx.err2.getStartIndex() - 1)) {
+      // There shouldn't be any whitespace between the first and second ']'.
+      throw wrongDelimiter("]]", "]", ctx.err1.getStartIndex());
+    }
 
-    return GeneratorPredicateMemberNodeGen.create(keyNode, member);
+    var keyNode = symbolTable.enterCustomThisScope(scope -> visitExpr(ctx.k));
+    var member = doVisitObjectEntryBody(createSourceSection(ctx), keyNode, ctx.v, ctx.objectBody());
+    var isFrameStored =
+        member.getMemberNode() != null && symbolTable.getCurrentScope().isForGeneratorScope();
+    return GeneratorPredicateMemberNodeGen.create(keyNode, member, isFrameStored);
   }
 
   @Override
@@ -893,43 +898,23 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
     var keyNodeAndMember = doVisitObjectEntry(ctx);
     var keyNode = keyNodeAndMember.first;
     var member = keyNodeAndMember.second;
-    insertWriteForGeneratorVarsToFrameSlotsNode(member.getMemberNode());
+    var isFrameStored =
-
+        member.getMemberNode() != null && symbolTable.getCurrentScope().isForGeneratorScope();
-    return GeneratorEntryNodeGen.create(keyNode, member);
+    return GeneratorEntryNodeGen.create(keyNode, member, isFrameStored);
   }
 
   @Override
   public GeneratorMemberNode visitObjectSpread(ObjectSpreadContext ctx) {
-    var scope = symbolTable.getCurrentScope();
-    var visitingIterable = scope.isVisitingIterable();
-    scope.setVisitingIterable(true);
     var expr = visitExpr(ctx.expr());
-    scope.setVisitingIterable(visitingIterable);
     return GeneratorSpreadNodeGen.create(createSourceSection(ctx), expr, ctx.QSPREAD() != null);
   }
 
-  private void insertWriteForGeneratorVarsToFrameSlotsNode(@Nullable MemberNode memberNode) {
-    if (memberNode == null) return; // member has constant value
-
-    var descriptor = memberNode.getFrameDescriptor();
-    var forGeneratorVars = symbolTable.getCurrentScope().getForGeneratorVariables();
-    if (forGeneratorVars.isEmpty()) {
-      return; // node is not within a for generator
-    }
-    var slots = new int[forGeneratorVars.size()];
-    var i = 0;
-    for (var variable : forGeneratorVars) {
-      slots[i] = descriptor.findOrAddAuxiliarySlot(variable);
-      i++;
-    }
-    memberNode.replaceBody((bodyNode) -> new WriteForVariablesNode(slots, bodyNode));
-  }
-
   @Override
   public GeneratorElementNode visitObjectElement(ObjectElementContext ctx) {
     var member = doVisitObjectElement(ctx);
-    insertWriteForGeneratorVarsToFrameSlotsNode(member.getMemberNode());
+    var isFrameStored =
-    return GeneratorElementNodeGen.create(member);
+        member.getMemberNode() != null && symbolTable.getCurrentScope().isForGeneratorScope();
+    return GeneratorElementNodeGen.create(member, isFrameStored);
   }
 
   private GeneratorMemberNode[] doVisitForWhenBody(ObjectBodyContext ctx) {
@@ -953,18 +938,6 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
     return new GeneratorWhenNode(sourceSection, visitExpr(ctx.e), thenNodes, elseNodes);
   }
 
-  private int pushForGeneratorVariableContext(ParameterContext ctx) {
-    var currentScope = symbolTable.getCurrentScope();
-    var slot = currentScope.pushForGeneratorVariableContext(ctx);
-    if (slot == -1) {
-      throw exceptionBuilder()
-          .evalError("duplicateDefinition", ctx.typedIdentifier().Identifier().getText())
-          .withSourceSection(createSourceSection(ctx))
-          .build();
-    }
-    return slot;
-  }
-
   private static boolean isIgnored(@Nullable ParameterContext param) {
     return param != null && param.UNDERSCORE() != null;
   }
@@ -972,53 +945,68 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
   @Override
   public GeneratorForNode visitForGenerator(ForGeneratorContext ctx) {
     checkClosingDelimiter(ctx.err, ")", ctx.e.stop);
-    var sourceSection = createSourceSection(ctx);
+    var keyParameter = ctx.t2 == null ? null : ctx.t1;
-    int keyVariableSlot;
+    var valueParameter = ctx.t2 == null ? ctx.t1 : ctx.t2;
-    int valueVariableSlot;
+    var keyTypedIdentifier = keyParameter == null ? null : keyParameter.typedIdentifier();
-    UnresolvedTypeNode unresolvedKeyTypeNode;
+    var valueTypedIdentifier = valueParameter == null ? null : valueParameter.typedIdentifier();
-    UnresolvedTypeNode unresolvedValueTypeNode;
+    var keyIdentifier =
+        keyTypedIdentifier == null ? null : toIdentifier(keyTypedIdentifier.Identifier());
+    var valueIdentifier =
+        valueTypedIdentifier == null ? null : toIdentifier(valueTypedIdentifier.Identifier());
+    if (valueIdentifier != null && valueIdentifier == keyIdentifier) {
+      throw exceptionBuilder()
+          .evalError("duplicateDefinition", valueIdentifier)
+          .withSourceSection(createSourceSection(valueTypedIdentifier.Identifier()))
+          .build();
+    }
     var currentScope = symbolTable.getCurrentScope();
-    var ignoreT1 = isIgnored(ctx.t1);
+    var generatorDescriptorBuilder = currentScope.newFrameDescriptorBuilder();
-    var ignoreT2 = ctx.t2 == null ? ignoreT1 : isIgnored(ctx.t2);
+    var memberDescriptorBuilder = currentScope.newForGeneratorMemberDescriptorBuilder();
-
+    var keySlot = -1;
-    if (ctx.t2 != null) {
+    var valueSlot = -1;
-      keyVariableSlot = ignoreT1 ? -1 : pushForGeneratorVariableContext(ctx.t1);
+    if (keyIdentifier != null) {
-      valueVariableSlot = ignoreT2 ? -1 : pushForGeneratorVariableContext(ctx.t2);
+      keySlot = generatorDescriptorBuilder.addSlot(FrameSlotKind.Illegal, keyIdentifier, null);
-      unresolvedKeyTypeNode =
+      memberDescriptorBuilder.addSlot(FrameSlotKind.Illegal, keyIdentifier, null);
-          ignoreT1 ? null : visitTypeAnnotation(ctx.t1.typedIdentifier().typeAnnotation());
-      unresolvedValueTypeNode =
-          ignoreT2 ? null : visitTypeAnnotation(ctx.t2.typedIdentifier().typeAnnotation());
-    } else {
-      keyVariableSlot = -1;
-      valueVariableSlot = ignoreT1 ? -1 : pushForGeneratorVariableContext(ctx.t1);
-      unresolvedKeyTypeNode = null;
-      unresolvedValueTypeNode =
-          ignoreT1 ? null : visitTypeAnnotation(ctx.t1.typedIdentifier().typeAnnotation());
     }
-
+    if (valueIdentifier != null) {
-    var scope = symbolTable.getCurrentScope();
+      valueSlot = generatorDescriptorBuilder.addSlot(FrameSlotKind.Illegal, valueIdentifier, null);
-    var visitingIterable = scope.isVisitingIterable();
+      memberDescriptorBuilder.addSlot(FrameSlotKind.Illegal, valueIdentifier, null);
-    scope.setVisitingIterable(true);
+    }
+    var unresolvedKeyTypeNode =
+        keyTypedIdentifier == null
+            ? null
+            : visitTypeAnnotation(keyTypedIdentifier.typeAnnotation());
+    var unresolvedValueTypeNode =
+        valueTypedIdentifier == null
+            ? null
+            : visitTypeAnnotation(valueTypedIdentifier.typeAnnotation());
+    // if possible, initialize immediately to avoid later insert
+    var keyTypeNode =
+        unresolvedKeyTypeNode == null && keySlot != -1
+            ? new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection())
+                .initWriteSlotNode(keySlot)
+            : null;
+    // if possible, initialize immediately to avoid later insert
+    var valueTypeNode =
+        unresolvedValueTypeNode == null && valueSlot != -1
+            ? new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection())
+                .initWriteSlotNode(valueSlot)
+            : null;
     var iterableNode = visitExpr(ctx.e);
-    scope.setVisitingIterable(visitingIterable);
+    var memberNodes =
-    var memberNodes = doVisitForWhenBody(ctx.objectBody());
+        symbolTable.enterForGenerator(
-    if (keyVariableSlot != -1) {
+            generatorDescriptorBuilder,
-      currentScope.popForGeneratorVariable();
+            memberDescriptorBuilder,
-    }
+            scope -> doVisitForWhenBody(ctx.objectBody()));
-    if (valueVariableSlot != -1) {
-      currentScope.popForGeneratorVariable();
-    }
-    //noinspection ConstantConditions
     return GeneratorForNodeGen.create(
-        sourceSection,
+        createSourceSection(ctx),
-        keyVariableSlot,
+        generatorDescriptorBuilder.build(),
-        valueVariableSlot,
         iterableNode,
         unresolvedKeyTypeNode,
         unresolvedValueTypeNode,
         memberNodes,
-        ctx.t2 != null && !ignoreT1,
+        keyTypeNode,
-        !ignoreT2);
+        valueTypeNode);
   }
 
   private void checkSpaceSeparatedObjectMembers(ObjectBodyContext objectBodyContext) {
@@ -1200,15 +1188,13 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
   }
 
   private ObjectMember doVisitObjectElement(ObjectElementContext ctx) {
+    var isForGeneratorScope = symbolTable.getCurrentScope().isForGeneratorScope();
     return symbolTable.enterEntry(
         null,
         scope -> {
           var elementNode = visitExpr(ctx.expr());
 
-          var modifier =
+          var modifier = VmModifier.ELEMENT;
-              scope.isVisitingIterable()
-                  ? VmModifier.ELEMENT | VmModifier.IS_IN_ITERABLE
-                  : VmModifier.ELEMENT;
           var member =
               new ObjectMember(
                   createSourceSection(ctx),
@@ -1220,6 +1206,9 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
           if (elementNode instanceof ConstantNode constantNode) {
             member.initConstantValue(constantNode);
           } else {
+            if (isForGeneratorScope) {
+              elementNode = new RestoreForBindingsNode(elementNode);
+            }
             member.initMemberNode(
                 ElementOrEntryNodeGen.create(
                     language, scope.buildFrameDescriptor(), member, elementNode));
@@ -1229,21 +1218,6 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
         });
   }
 
-  private Pair<ExpressionNode, ObjectMember> doVisitMemberPredicate(MemberPredicateContext ctx) {
-    if (ctx.err1 == null && ctx.err2 == null) {
-      throw missingDelimiter("]]", ctx.k.stop.getStopIndex() + 1);
-    } else if (ctx.err1 != null
-        && (ctx.err2 == null || ctx.err1.getStartIndex() != ctx.err2.getStartIndex() - 1)) {
-      // There shouldn't be any whitespace between the first and second ']'.
-      throw wrongDelimiter("]]", "]", ctx.err1.getStartIndex());
-    }
-
-    var keyNode = symbolTable.enterCustomThisScope(scope -> visitExpr(ctx.k));
-
-    return symbolTable.enterEntry(
-        keyNode, objectMemberInserter(createSourceSection(ctx), keyNode, ctx.v, ctx.objectBody()));
-  }
-
   private Pair<ExpressionNode, ObjectMember> doVisitObjectEntry(ObjectEntryContext ctx) {
     checkClosingDelimiter(ctx.err1, "]", ctx.k.stop);
     if (ctx.err2 != null) {
@@ -1253,46 +1227,54 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
     }
 
     var keyNode = visitExpr(ctx.k);
-
+    var member = doVisitObjectEntryBody(createSourceSection(ctx), keyNode, ctx.v, ctx.objectBody());
-    return symbolTable.enterEntry(
+    return Pair.of(keyNode, member);
-        keyNode, objectMemberInserter(createSourceSection(ctx), keyNode, ctx.v, ctx.objectBody()));
   }
 
-  private Function<EntryScope, Pair<ExpressionNode, ObjectMember>> objectMemberInserter(
+  private ObjectMember doVisitObjectEntryBody(
       SourceSection sourceSection,
       ExpressionNode keyNode,
       @Nullable ExprContext valueCtx,
       List<? extends ObjectBodyContext> objectBodyCtxs) {
-    return scope -> {
+    var isForGeneratorScope = symbolTable.getCurrentScope().isForGeneratorScope();
-      var modifier =
+    return symbolTable.enterEntry(
-          scope.isVisitingIterable()
+        keyNode,
-              ? VmModifier.ENTRY | VmModifier.IS_IN_ITERABLE
+        scope -> {
-              : VmModifier.ENTRY;
+          var modifier = VmModifier.ENTRY;
-      var member =
+          var member =
-          new ObjectMember(
+              new ObjectMember(
-              sourceSection, keyNode.getSourceSection(), modifier, null, scope.getQualifiedName());
+                  sourceSection,
+                  keyNode.getSourceSection(),
+                  modifier,
+                  null,
+                  scope.getQualifiedName());
+          if (valueCtx != null) { // ["key"] = value
+            var valueNode = visitExpr(valueCtx);
+            if (valueNode instanceof ConstantNode constantNode) {
+              member.initConstantValue(constantNode);
+            } else {
+              if (isForGeneratorScope) {
+                valueNode = new RestoreForBindingsNode(valueNode);
+              }
+              member.initMemberNode(
+                  ElementOrEntryNodeGen.create(
+                      language, scope.buildFrameDescriptor(), member, valueNode));
+            }
+          } else { // ["key"] { ... }
+            var objectBody =
+                doVisitObjectBody(
+                    objectBodyCtxs,
+                    new ReadSuperEntryNode(unavailableSourceSection(), new GetMemberKeyNode()));
+            if (isForGeneratorScope) {
+              objectBody = new RestoreForBindingsNode(objectBody);
+            }
+            member.initMemberNode(
+                ElementOrEntryNodeGen.create(
+                    language, scope.buildFrameDescriptor(), member, objectBody));
+          }
 
-      if (valueCtx != null) { // ["key"] = value
+          return member;
-        var valueNode = visitExpr(valueCtx);
+        });
-        if (valueNode instanceof ConstantNode constantNode) {
-          member.initConstantValue(constantNode);
-        } else {
-          member.initMemberNode(
-              ElementOrEntryNodeGen.create(
-                  language, scope.buildFrameDescriptor(), member, valueNode));
-        }
-      } else { // ["key"] { ... }
-        var objectBody =
-            doVisitObjectBody(
-                objectBodyCtxs,
-                new ReadSuperEntryNode(unavailableSourceSection(), new GetMemberKeyNode()));
-        member.initMemberNode(
-            ElementOrEntryNodeGen.create(
-                language, scope.buildFrameDescriptor(), member, objectBody));
-      }
-
-      return Pair.of(keyNode, member);
-    };
   }
 
   @Override
@@ -1350,10 +1332,6 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
       result += modifier;
     }
 
-    if (symbolTable.getCurrentScope().isVisitingIterable()) {
-      result += VmModifier.IS_IN_ITERABLE;
-    }
-
     // flag modifier combinations that are never valid right away
 
     if (VmModifier.isExternal(result) && !ModuleKeys.isStdLibModule(moduleKey)) {
@@ -1986,7 +1964,6 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
               visitArgumentList(argCtx),
               MemberLookupMode.EXPLICIT_RECEIVER,
               needsConst,
-              symbolTable.getCurrentScope().isVisitingIterable(),
               PropagateNullReceiverNodeGen.create(unavailableSourceSection(), receiver),
               GetClassNodeGen.create(null)));
     }
@@ -1999,7 +1976,6 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
         visitArgumentList(argCtx),
         MemberLookupMode.EXPLICIT_RECEIVER,
         needsConst,
-        symbolTable.getCurrentScope().isVisitingIterable(),
         receiver,
         GetClassNodeGen.create(null));
   }
@@ -2074,11 +2050,7 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
       }
 
       return InvokeSuperMethodNodeGen.create(
-          sourceSection,
+          sourceSection, memberName, visitArgumentList(argCtx), needsConst);
-          memberName,
-          symbolTable.getCurrentScope().isVisitingIterable(),
-          visitArgumentList(argCtx),
-          needsConst);
     }
 
     // superproperty call
@@ -2136,8 +2108,7 @@ public final class AstBuilder extends AbstractAstBuilder<Object> {
         isBaseModule,
         scope.isCustomThisScope(),
         scope.getConstLevel(),
-        scope.getConstDepth(),
+        scope.getConstDepth());
-        scope.isVisitingIterable());
   }
 
   @Override

pkl-core/src/main/java/org/pkl/core/ast/builder/SymbolTable.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -17,15 +17,14 @@ package org.pkl.core.ast.builder;
 
 import com.oracle.truffle.api.frame.FrameDescriptor;
 import com.oracle.truffle.api.frame.FrameDescriptor.Builder;
-import com.oracle.truffle.api.frame.FrameSlotKind;
 import java.util.*;
 import java.util.function.Function;
 import org.pkl.core.TypeParameter;
 import org.pkl.core.ast.ConstantNode;
 import org.pkl.core.ast.ExpressionNode;
+import org.pkl.core.ast.expression.generator.GeneratorMemberNode;
 import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.parser.Lexer;
-import org.pkl.core.parser.antlr.PklParser.ParameterContext;
 import org.pkl.core.runtime.Identifier;
 import org.pkl.core.runtime.ModuleInfo;
 import org.pkl.core.runtime.VmDataSize;
@@ -35,8 +34,6 @@ import org.pkl.core.util.Nullable;
 public final class SymbolTable {
   private Scope currentScope;
 
-  public static Object FOR_GENERATOR_VARIABLE = new Object();
-
   public SymbolTable(ModuleInfo moduleInfo) {
     currentScope = new ModuleScope(moduleInfo);
   }
@@ -99,6 +96,19 @@ public final class SymbolTable {
         nodeFactory);
   }
 
+  public <T> T enterForGenerator(
+      FrameDescriptor.Builder frameDescriptorBuilder,
+      FrameDescriptor.Builder memberDescriptorBuilder,
+      Function<ForGeneratorScope, T> nodeFactory) {
+    return doEnter(
+        new ForGeneratorScope(
+            currentScope,
+            currentScope.qualifiedName,
+            frameDescriptorBuilder,
+            memberDescriptorBuilder),
+        nodeFactory);
+  }
+
   public <T> T enterLambda(
       FrameDescriptor.Builder frameDescriptorBuilder, Function<LambdaScope, T> nodeFactory) {
 
@@ -128,9 +138,11 @@ public final class SymbolTable {
       Function<EntryScope, T> nodeFactory) {
 
     var qualifiedName = currentScope.getQualifiedName() + currentScope.getNextEntryName(keyNode);
-
+    var builder =
-    return doEnter(
+        currentScope instanceof ForGeneratorScope forScope
-        new EntryScope(currentScope, qualifiedName, FrameDescriptor.newBuilder()), nodeFactory);
+            ? forScope.memberDescriptorBuilder
+            : FrameDescriptor.newBuilder();
+    return doEnter(new EntryScope(currentScope, qualifiedName, builder), nodeFactory);
   }
 
   public <T> T enterCustomThisScope(Function<CustomThisScope, T> nodeFactory) {
@@ -166,12 +178,10 @@ public final class SymbolTable {
     private final @Nullable Scope parent;
     private final @Nullable Identifier name;
     private final String qualifiedName;
-    private final Deque<Identifier> forGeneratorVariables = new ArrayDeque<>();
     private int lambdaCount = 0;
     private int entryCount = 0;
     private final FrameDescriptor.Builder frameDescriptorBuilder;
     private final ConstLevel constLevel;
-    private boolean isVisitingIterable;
 
     private Scope(
         @Nullable Scope parent,
@@ -188,7 +198,6 @@ public final class SymbolTable {
           parent != null && parent.constLevel.biggerOrEquals(constLevel)
               ? parent.constLevel
               : constLevel;
-      this.isVisitingIterable = parent != null && parent.isVisitingIterable;
     }
 
     public final @Nullable Scope getParent() {
@@ -212,6 +221,30 @@ public final class SymbolTable {
       return frameDescriptorBuilder.build();
     }
 
+    /**
+     * Returns a new descriptor builder that contains the same slots as the current scope's frame
+     * descriptor.
+     */
+    public FrameDescriptor.Builder newFrameDescriptorBuilder() {
+      return newFrameDescriptorBuilder(buildFrameDescriptor());
+    }
+
+    /** Returns a new descriptor builder for a {@link GeneratorMemberNode} in the current scope. */
+    public FrameDescriptor.Builder newForGeneratorMemberDescriptorBuilder() {
+      return this instanceof ForGeneratorScope forScope
+          ? newFrameDescriptorBuilder(forScope.buildMemberDescriptor())
+          : FrameDescriptor.newBuilder();
+    }
+
+    private static FrameDescriptor.Builder newFrameDescriptorBuilder(FrameDescriptor descriptor) {
+      var builder = FrameDescriptor.newBuilder();
+      for (var i = 0; i < descriptor.getNumberOfSlots(); i++) {
+        builder.addSlot(
+            descriptor.getSlotKind(i), descriptor.getSlotName(i), descriptor.getSlotInfo(i));
+      }
+      return builder;
+    }
+
     public @Nullable TypeParameter getTypeParameter(String name) {
       return null;
     }
@@ -253,35 +286,11 @@ public final class SymbolTable {
       return depth;
     }
 
-    /**
-     * Adds the for generator variable to the frame descriptor.
-     *
-     * <p>Returns {@code -1} if a for-generator variable already exists with this name.
-     */
-    public int pushForGeneratorVariableContext(ParameterContext ctx) {
-      var variable = Identifier.localProperty(ctx.typedIdentifier().Identifier().getText());
-      if (forGeneratorVariables.contains(variable)) {
-        return -1;
-      }
-      var slot =
-          frameDescriptorBuilder.addSlot(FrameSlotKind.Illegal, variable, FOR_GENERATOR_VARIABLE);
-      forGeneratorVariables.addLast(variable);
-      return slot;
-    }
-
-    public void popForGeneratorVariable() {
-      forGeneratorVariables.removeLast();
-    }
-
-    public Deque<Identifier> getForGeneratorVariables() {
-      return forGeneratorVariables;
-    }
-
     private String getNextLambdaName() {
       return "<function#" + (++skipLambdaScopes().lambdaCount) + ">";
     }
 
-    private String getNextEntryName(@Nullable ExpressionNode keyNode) {
+    protected String getNextEntryName(@Nullable ExpressionNode keyNode) {
       if (keyNode instanceof ConstantNode constantNode) {
         var value = constantNode.getValue();
         if (value instanceof String) {
@@ -336,17 +345,13 @@ public final class SymbolTable {
       return this instanceof LexicalScope;
     }
 
+    public final boolean isForGeneratorScope() {
+      return this instanceof ForGeneratorScope;
+    }
+
     public ConstLevel getConstLevel() {
       return constLevel;
     }
-
-    public void setVisitingIterable(boolean isVisitingIterable) {
-      this.isVisitingIterable = isVisitingIterable;
-    }
-
-    public boolean isVisitingIterable() {
-      return isVisitingIterable;
-    }
   }
 
   private interface LexicalScope {}
@@ -413,6 +418,30 @@ public final class SymbolTable {
     }
   }
 
+  public static final class ForGeneratorScope extends Scope implements LexicalScope {
+    private final FrameDescriptor.Builder memberDescriptorBuilder;
+
+    public ForGeneratorScope(
+        Scope parent,
+        String qualifiedName,
+        FrameDescriptor.Builder frameDescriptorBuilder,
+        FrameDescriptor.Builder memberDescriptorBuilder) {
+      super(parent, null, qualifiedName, ConstLevel.NONE, frameDescriptorBuilder);
+      this.memberDescriptorBuilder = memberDescriptorBuilder;
+    }
+
+    public FrameDescriptor buildMemberDescriptor() {
+      return memberDescriptorBuilder.build();
+    }
+
+    @Override
+    protected String getNextEntryName(@Nullable ExpressionNode keyNode) {
+      var parent = getParent();
+      assert parent != null;
+      return parent.getNextEntryName(keyNode);
+    }
+  }
+
   public static final class PropertyScope extends Scope {
     public PropertyScope(
         Scope parent,

pkl-core/src/main/java/org/pkl/core/ast/expression/binary/LetExprNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,7 +21,6 @@ import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.nodes.DirectCallNode;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
-import org.pkl.core.ast.builder.SymbolTable.CustomThisScope;
 import org.pkl.core.ast.member.FunctionNode;
 import org.pkl.core.ast.member.UnresolvedFunctionNode;
 import org.pkl.core.runtime.VmFunction;
@@ -57,7 +56,7 @@ public final class LetExprNode extends ExpressionNode {
       callNode = insert(DirectCallNode.create(functionNode.getCallTarget()));
       if (isCustomThisScope) {
         // deferred until execution time s.t. nodes of inlined type aliases get the right frame slot
-        customThisSlot = VmUtils.findAuxiliarySlot(frame, CustomThisScope.FRAME_SLOT_ID);
+        customThisSlot = VmUtils.findCustomThisSlot(frame);
       }
     }
 
@@ -71,6 +70,6 @@ public final class LetExprNode extends ExpressionNode {
 
     var value = valueNode.executeGeneric(frame);
 
-    return callNode.call(function.getThisValue(), function, false, value);
+    return callNode.call(function.getThisValue(), function, value);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorElementNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,57 +19,50 @@ import com.oracle.truffle.api.CompilerDirectives;
 import com.oracle.truffle.api.dsl.Fallback;
 import com.oracle.truffle.api.dsl.ImportStatic;
 import com.oracle.truffle.api.dsl.Specialization;
+import com.oracle.truffle.api.frame.VirtualFrame;
 import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.runtime.BaseModule;
 import org.pkl.core.runtime.VmClass;
 import org.pkl.core.runtime.VmDynamic;
 import org.pkl.core.runtime.VmListing;
-import org.pkl.core.util.EconomicMaps;
 
 @ImportStatic(BaseModule.class)
 public abstract class GeneratorElementNode extends GeneratorMemberNode {
   private final ObjectMember element;
 
-  protected GeneratorElementNode(ObjectMember element) {
+  protected GeneratorElementNode(ObjectMember element, boolean isFrameStored) {
-    super(element.getSourceSection());
+    super(element.getSourceSection(), isFrameStored);
     this.element = element;
   }
 
   @Specialization
   @SuppressWarnings("unused")
-  protected void evalDynamic(VmDynamic parent, ObjectData data) {
+  protected void evalDynamic(VirtualFrame frame, VmDynamic parent, ObjectData data) {
-    addElement(data);
+    data.addElement(frame, element, this);
   }
 
   @Specialization
   @SuppressWarnings("unused")
-  protected void evalListing(VmListing parent, ObjectData data) {
+  protected void evalListing(VirtualFrame frame, VmListing parent, ObjectData data) {
-    addElement(data);
+    data.addElement(frame, element, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = "parent == getDynamicClass()")
-  protected void evalDynamicClass(VmClass parent, ObjectData data) {
+  protected void evalDynamicClass(VirtualFrame frame, VmClass parent, ObjectData data) {
-    addElement(data);
+    data.addElement(frame, element, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = "parent == getListingClass()")
-  protected void evalListingClass(VmClass parent, ObjectData data) {
+  protected void evalListingClass(VirtualFrame frame, VmClass parent, ObjectData data) {
-    addElement(data);
+    data.addElement(frame, element, this);
   }
 
   @Fallback
   @SuppressWarnings("unused")
-  void fallback(Object parent, ObjectData data) {
+  void fallback(VirtualFrame frame, Object parent, ObjectData data) {
     CompilerDirectives.transferToInterpreter();
     throw exceptionBuilder().evalError("objectCannotHaveElement", parent).build();
   }
-
-  private void addElement(ObjectData data) {
-    long index = data.length;
-    EconomicMaps.put(data.members, index, element);
-    data.length += 1;
-    data.persistForBindings(index);
-  }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorEntryNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,15 +25,14 @@ import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.runtime.*;
 import org.pkl.core.runtime.VmException.ProgramValue;
-import org.pkl.core.util.EconomicMaps;
 
 @ImportStatic(BaseModule.class)
 public abstract class GeneratorEntryNode extends GeneratorMemberNode {
   @Child private ExpressionNode keyNode;
   private final ObjectMember member;
 
-  protected GeneratorEntryNode(ExpressionNode keyNode, ObjectMember member) {
+  protected GeneratorEntryNode(ExpressionNode keyNode, ObjectMember member, boolean isFrameStored) {
-    super(member.getSourceSection());
+    super(member.getSourceSection(), isFrameStored);
     this.keyNode = keyNode;
     this.member = member;
   }
@@ -84,7 +83,7 @@ public abstract class GeneratorEntryNode extends GeneratorMemberNode {
 
   private void addRegularEntry(VirtualFrame frame, ObjectData data) {
     var key = keyNode.executeGeneric(frame);
-    doAdd(key, data);
+    data.addMember(frame, key, member, this);
   }
 
   private void addListingEntry(VirtualFrame frame, ObjectData data, int parentLength) {
@@ -108,15 +107,6 @@ public abstract class GeneratorEntryNode extends GeneratorMemberNode {
           .build();
     }
 
-    doAdd(index, data);
+    data.addMember(frame, index, member, this);
-  }
-
-  private void doAdd(Object key, ObjectData data) {
-    if (EconomicMaps.put(data.members, key, member) != null) {
-      CompilerDirectives.transferToInterpreter();
-      throw duplicateDefinition(key, member);
-    }
-
-    data.persistForBindings(key);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorForNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,62 +16,51 @@
 package org.pkl.core.ast.expression.generator;
 
 import com.oracle.truffle.api.CompilerDirectives;
-import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
+import com.oracle.truffle.api.Truffle;
 import com.oracle.truffle.api.dsl.Fallback;
 import com.oracle.truffle.api.dsl.Specialization;
+import com.oracle.truffle.api.frame.FrameDescriptor;
 import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.nodes.ExplodeLoop;
 import com.oracle.truffle.api.source.SourceSection;
-import java.util.*;
 import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.type.TypeNode;
 import org.pkl.core.ast.type.UnresolvedTypeNode;
-import org.pkl.core.ast.type.VmTypeMismatchException;
 import org.pkl.core.runtime.*;
-import org.pkl.core.util.LateInit;
 import org.pkl.core.util.Nullable;
-import org.pkl.core.util.Pair;
 
 public abstract class GeneratorForNode extends GeneratorMemberNode {
-  private final int keySlot;
+  private final FrameDescriptor generatorDescriptor;
-  private final int valueSlot;
   @Child private ExpressionNode iterableNode;
   @Child private @Nullable UnresolvedTypeNode unresolvedKeyTypeNode;
   @Child private @Nullable UnresolvedTypeNode unresolvedValueTypeNode;
   @Children private final GeneratorMemberNode[] childNodes;
   @Child private @Nullable TypeNode keyTypeNode;
-  @Child @LateInit private TypeNode valueTypeNode;
+  @Child private @Nullable TypeNode valueTypeNode;
 
   public GeneratorForNode(
       SourceSection sourceSection,
-      int keySlot,
+      FrameDescriptor generatorDescriptor,
-      int valueSlot,
       ExpressionNode iterableNode,
+      // null if for-generator doesn't bind key or `keyTypeNode` is passed instead of this node
       @Nullable UnresolvedTypeNode unresolvedKeyTypeNode,
+      // null if for-generator doesn't bind value or `valueTypeNode` is passed instead of this node
       @Nullable UnresolvedTypeNode unresolvedValueTypeNode,
+      // If this node can be constructed at parse time,
+      // it should be passed instead of `unresolvedKeyTypeNode`.
       GeneratorMemberNode[] childNodes,
-      boolean hasKeyIdentifier,
+      @Nullable TypeNode keyTypeNode,
-      boolean hasValueIdentifier) {
+      // If this node can be constructed at parse time,
-
+      // it should be passed instead of `unresolvedValueTypeNode`.
-    super(sourceSection);
+      @Nullable TypeNode valueTypeNode) {
-    this.keySlot = keySlot;
+    super(sourceSection, false);
-    this.valueSlot = valueSlot;
+    this.generatorDescriptor = generatorDescriptor;
     this.iterableNode = iterableNode;
     this.unresolvedKeyTypeNode = unresolvedKeyTypeNode;
     this.unresolvedValueTypeNode = unresolvedValueTypeNode;
     this.childNodes = childNodes;
-
+    this.keyTypeNode = keyTypeNode;
-    // initialize now if possible to save later insert()
+    this.valueTypeNode = valueTypeNode;
-    if (unresolvedKeyTypeNode == null && hasKeyIdentifier) {
-      keyTypeNode =
-          new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection())
-              .initWriteSlotNode(keySlot);
-    }
-    if (unresolvedValueTypeNode == null && hasValueIdentifier) {
-      valueTypeNode =
-          new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection())
-              .initWriteSlotNode(valueSlot);
-    }
   }
 
   protected abstract void executeWithIterable(
@@ -79,6 +68,7 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
 
   @Override
   public final void execute(VirtualFrame frame, Object parent, ObjectData data) {
+    initialize(frame);
     executeWithIterable(frame, parent, data, iterableNode.executeGeneric(frame));
   }
 
@@ -99,41 +89,33 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
 
   @Specialization
   protected void eval(VirtualFrame frame, Object parent, ObjectData data, VmList iterable) {
-    initTypeNodes(frame);
-    long idx = 0;
-    for (Object element : iterable) {
-      executeIteration(frame, parent, data, idx++, element);
-    }
-    resetFrameSlots(frame);
-  }
-
-  @Specialization
-  protected void eval(VirtualFrame frame, Object parent, ObjectData data, VmMap iterable) {
-    initTypeNodes(frame);
-    for (var entry : iterable) {
-      executeIteration(frame, parent, data, VmUtils.getKey(entry), VmUtils.getValue(entry));
-    }
-    resetFrameSlots(frame);
-  }
-
-  @Specialization
-  protected void eval(VirtualFrame frame, Object parent, ObjectData data, VmSet iterable) {
-    initTypeNodes(frame);
     long idx = 0;
     for (var element : iterable) {
       executeIteration(frame, parent, data, idx++, element);
     }
-    resetFrameSlots(frame);
+  }
+
+  @Specialization
+  protected void eval(VirtualFrame frame, Object parent, ObjectData data, VmMap iterable) {
+    for (var entry : iterable) {
+      executeIteration(frame, parent, data, VmUtils.getKey(entry), VmUtils.getValue(entry));
+    }
+  }
+
+  @Specialization
+  protected void eval(VirtualFrame frame, Object parent, ObjectData data, VmSet iterable) {
+    long idx = 0;
+    for (var element : iterable) {
+      executeIteration(frame, parent, data, idx++, element);
+    }
   }
 
   @Specialization
   protected void eval(VirtualFrame frame, Object parent, ObjectData data, VmIntSeq iterable) {
-    initTypeNodes(frame);
     var length = iterable.getLength();
     for (long key = 0, value = iterable.start; key < length; key++, value += iterable.step) {
       executeIteration(frame, parent, data, key, value);
     }
-    resetFrameSlots(frame);
   }
 
   @Fallback
@@ -147,84 +129,55 @@ public abstract class GeneratorForNode extends GeneratorMemberNode {
         .build();
   }
 
-  @SuppressWarnings("ForLoopReplaceableByForEach")
   private void doEvalObject(VirtualFrame frame, VmObject iterable, Object parent, ObjectData data) {
-    initTypeNodes(frame);
+    iterable.forceAndIterateMemberValues(
-    var members = evaluateMembers(iterable);
-    for (int i = 0; i < members.size(); i++) {
-      var member = members.get(i);
-      executeIteration(frame, parent, data, member.first, member.second);
-    }
-    resetFrameSlots(frame);
-  }
-
-  private void resetFrameSlots(VirtualFrame frame) {
-    if (keySlot != -1) {
-      frame.clear(keySlot);
-    }
-    if (valueSlot != -1) {
-      frame.clear(valueSlot);
-    }
-  }
-
-  private void initTypeNodes(VirtualFrame frame) {
-    if (unresolvedKeyTypeNode != null) {
-      CompilerDirectives.transferToInterpreter();
-      keyTypeNode = insert(unresolvedKeyTypeNode.execute(frame)).initWriteSlotNode(keySlot);
-      unresolvedKeyTypeNode = null;
-    }
-
-    if (unresolvedValueTypeNode != null) {
-      CompilerDirectives.transferToInterpreter();
-      valueTypeNode = insert(unresolvedValueTypeNode.execute(frame)).initWriteSlotNode(valueSlot);
-      unresolvedValueTypeNode = null;
-    }
-  }
-
-  /**
-   * Evaluate members upfront to make sure that `childNode.execute()` is not behind a Truffle
-   * boundary.
-   */
-  @TruffleBoundary
-  private List<Pair<Object, Object>> evaluateMembers(VmObject object) {
-    var members = new ArrayList<Pair<Object, Object>>();
-    object.forceAndIterateMemberValues(
         (key, member, value) -> {
-          members.add(Pair.of(member.isProp() ? key.toString() : key, value));
+          var convertedKey = member.isProp() ? key.toString() : key;
+          // TODO: Executing iteration behind a Truffle boundary is bad for performance.
+          // This and similar cases will be fixed in an upcoming PR that replaces method
+          // `(forceAnd)iterateMemberValues` with cursor-based external iterators.
+          executeIteration(frame, parent, data, convertedKey, value);
           return true;
         });
-    return members;
   }
 
   @ExplodeLoop
   private void executeIteration(
       VirtualFrame frame, Object parent, ObjectData data, Object key, Object value) {
 
-    try {
+    // GraalJS uses the same implementation technique here:
-      if (keyTypeNode != null) {
+    // https://github.com/oracle/graaljs/blob/44a11ce6e87/graal-js/src/com.oracle.truffle.js/
-        keyTypeNode.executeAndSet(frame, key);
+    // src/com/oracle/truffle/js/nodes/function/IterationScopeNode.java#L86-L88
-      }
+    var newFrame =
-      if (valueTypeNode != null) {
+        Truffle.getRuntime().createVirtualFrame(frame.getArguments(), generatorDescriptor);
-        valueTypeNode.executeAndSet(frame, value);
+    // the locals in `frame` (if any) are function arguments and/or outer for-generator bindings
-      }
+    VmUtils.copyLocals(frame, 0, newFrame, 0, frame.getFrameDescriptor().getNumberOfSlots());
-    } catch (VmTypeMismatchException e) {
+    if (keyTypeNode != null) {
-      CompilerDirectives.transferToInterpreter();
+      keyTypeNode.executeAndSet(newFrame, key);
-      throw e.toVmException();
     }
-
+    if (valueTypeNode != null) {
-    Object[] prevBindings = null;
+      valueTypeNode.executeAndSet(newFrame, value);
-    if (keyTypeNode != null && valueTypeNode != null) {
-      prevBindings = data.addForBinding(key, value);
-    } else if (valueTypeNode != null) {
-      prevBindings = data.addForBinding(value);
-    } else if (keyTypeNode != null) {
-      prevBindings = data.addForBinding(key);
     }
-
+    //noinspection ForLoopReplaceableByForEach
-    for (var childNode : childNodes) {
+    for (int i = 0; i < childNodes.length; i++) {
-      childNode.execute(frame, parent, data);
+      childNodes[i].execute(newFrame, parent, data);
     }
+  }
 
-    data.resetForBindings(prevBindings);
+  private void initialize(VirtualFrame frame) {
+    if (unresolvedKeyTypeNode != null) {
+      CompilerDirectives.transferToInterpreterAndInvalidate();
+      var keySlot = frame.getFrameDescriptor().getNumberOfSlots();
+      keyTypeNode = insert(unresolvedKeyTypeNode.execute(frame)).initWriteSlotNode(keySlot);
+      generatorDescriptor.setSlotKind(keySlot, keyTypeNode.getFrameSlotKind());
+      unresolvedKeyTypeNode = null;
+    }
+    if (unresolvedValueTypeNode != null) {
+      CompilerDirectives.transferToInterpreterAndInvalidate();
+      var valueSlot = frame.getFrameDescriptor().getNumberOfSlots() + (keyTypeNode != null ? 1 : 0);
+      valueTypeNode = insert(unresolvedValueTypeNode.execute(frame)).initWriteSlotNode(valueSlot);
+      generatorDescriptor.setSlotKind(valueSlot, valueTypeNode.getFrameSlotKind());
+      unresolvedValueTypeNode = null;
+    }
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorMemberNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -16,25 +16,23 @@
 package org.pkl.core.ast.expression.generator;
 
 import com.oracle.truffle.api.CompilerDirectives;
-import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
-import com.oracle.truffle.api.CompilerDirectives.ValueType;
 import com.oracle.truffle.api.dsl.Idempotent;
 import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.source.SourceSection;
-import java.util.Arrays;
-import org.graalvm.collections.EconomicMap;
 import org.pkl.core.ast.PklNode;
 import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.runtime.Identifier;
 import org.pkl.core.runtime.VmClass;
 import org.pkl.core.runtime.VmException;
 import org.pkl.core.runtime.VmException.ProgramValue;
-import org.pkl.core.util.EconomicMaps;
+import org.pkl.core.runtime.VmUtils;
-import org.pkl.core.util.Nullable;
 
 public abstract class GeneratorMemberNode extends PklNode {
-  protected GeneratorMemberNode(SourceSection sourceSection) {
+  final boolean isFrameStored;
+
+  protected GeneratorMemberNode(SourceSection sourceSection, boolean isFrameStored) {
     super(sourceSection);
+    this.isFrameStored = isFrameStored;
   }
 
   public abstract void execute(VirtualFrame frame, Object parent, ObjectData data);
@@ -54,79 +52,39 @@ public abstract class GeneratorMemberNode extends PklNode {
   }
 
   @Idempotent
-  protected boolean checkIsValidTypedProperty(VmClass clazz, ObjectMember member) {
+  @SuppressWarnings("SameReturnValue")
-    if (member.isLocal() || clazz.hasProperty(member.getName())) return true;
+  protected final boolean checkIsValidTypedProperty(VmClass clazz, ObjectMember member) {
+    if (member.isLocal()) return true;
+    var memberName = member.getName();
+    var classProperty = clazz.getProperty(memberName);
+    if (classProperty != null && !classProperty.isConstOrFixed()) return true;
 
     CompilerDirectives.transferToInterpreter();
-    throw exceptionBuilder()
+    if (classProperty == null) {
-        .cannotFindProperty(clazz.getPrototype(), member.getName(), false, false)
+      var exception =
-        .withSourceSection(member.getHeaderSection())
+          exceptionBuilder()
-        .build();
+              .cannotFindProperty(clazz.getPrototype(), memberName, false, false)
-  }
+              .build();
-
+      if (member.getHeaderSection().isAvailable()) {
-  /**
+        exception
-   * <code>
+            .getInsertedStackFrames()
-   * x = new Mapping { for (i in IntSeq(1, 3)) for (key, value in Map(4, "Pigeon", 6, "Barn Owl")) [i *
+            .put(
-   * key] = value.reverse() }
+                getRootNode().getCallTarget(),
-   * </code>
+                VmUtils.createStackFrame(member.getHeaderSection(), member.getQualifiedName()));
-   *
-   * <p>The above code results in - 1 MemberNode for `value.reverse()` - 1 ObjectMember for `[i *
-   * key] = value.reverse()` - 1 ObjectData.members map with 6 identical ObjectMember values keyed
-   * by `i * key` - 1 ObjectData.forBindings map with 6 distinct arrays keyed by `i * key` Each
-   * array contains three elements, namely the current values for `i`, `key`, and `value`. - 1
-   * VmMapping whose `members` field holds `ObjectData.members` and whose `extraStorage` field holds
-   * `ObjectData.forBindings`. - 3 `FrameSlot`s for `i`, `key`, and `value`
-   */
-  @ValueType
-  public static final class ObjectData {
-    // member count is exact iff every for/when body has exactly one member
-    ObjectData(int minMemberCount, int length) {
-      this.members = EconomicMaps.create(minMemberCount);
-      this.length = length;
-    }
-
-    final EconomicMap<Object, ObjectMember> members;
-
-    // For-bindings keyed by object member key.
-    // (There is only one ObjectMember instance per lexical member definition,
-    // hence can't store a member's for-bindings there.)
-    final EconomicMap<Object, Object[]> forBindings = EconomicMap.create();
-
-    int length;
-
-    private Object @Nullable [] currentForBindings;
-
-    @TruffleBoundary
-    Object @Nullable [] addForBinding(Object value) {
-      var result = currentForBindings;
-      if (currentForBindings == null) {
-        currentForBindings = new Object[] {value};
-      } else {
-        currentForBindings = Arrays.copyOf(currentForBindings, currentForBindings.length + 1);
-        currentForBindings[currentForBindings.length - 1] = value;
       }
-      return result;
+      throw exception;
     }
-
+    assert classProperty.isConstOrFixed();
-    @TruffleBoundary
+    var errMsg =
-    Object @Nullable [] addForBinding(Object key, Object value) {
+        classProperty.isConst() ? "cannotAssignConstProperty" : "cannotAssignFixedProperty";
-      var result = currentForBindings;
+    var exception = exceptionBuilder().evalError(errMsg, memberName).build();
-      if (currentForBindings == null) {
+    if (member.getHeaderSection().isAvailable()) {
-        currentForBindings = new Object[] {key, value};
+      exception
-      } else {
+          .getInsertedStackFrames()
-        currentForBindings = Arrays.copyOf(currentForBindings, currentForBindings.length + 2);
+          .put(
-        currentForBindings[currentForBindings.length - 2] = key;
+              getRootNode().getCallTarget(),
-        currentForBindings[currentForBindings.length - 1] = value;
+              VmUtils.createStackFrame(member.getHeaderSection(), member.getQualifiedName()));
-      }
-      return result;
-    }
-
-    void persistForBindings(Object key) {
-      EconomicMaps.put(forBindings, key, currentForBindings);
-    }
-
-    void resetForBindings(Object @Nullable [] bindings) {
-      currentForBindings = bindings;
     }
+    throw exception;
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorObjectLiteralNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -27,7 +27,6 @@ import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.nodes.ExplodeLoop;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
-import org.pkl.core.ast.expression.generator.GeneratorMemberNode.ObjectData;
 import org.pkl.core.ast.expression.literal.AmendFunctionNode;
 import org.pkl.core.ast.expression.literal.ObjectLiteralNode;
 import org.pkl.core.ast.type.UnresolvedTypeNode;
@@ -73,34 +72,31 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
 
   @Specialization(guards = "checkObjectCannotHaveParameters()")
   protected VmDynamic evalDynamic(VirtualFrame frame, VmDynamic parent) {
-    var data = createData(frame, parent, parent.getLength());
+    var data = executeChildren(frame, parent, parent.getLength());
-    var result = new VmDynamic(frame.materialize(), parent, data.members, data.length);
+    var result = new VmDynamic(frame.materialize(), parent, data.members(), data.length());
-    result.setExtraStorage(data.forBindings);
+    return data.storeGeneratorFrames(result);
-    return result;
   }
 
   @Specialization(guards = "checkObjectCannotHaveParameters()")
   protected VmTyped evalTyped(VirtualFrame frame, VmTyped parent) {
     VmUtils.checkIsInstantiable(parent.getVmClass(), getParentNode());
-    var data = createData(frame, parent, 0);
+    var data = executeChildren(frame, parent, 0);
-    assert data.forBindings.isEmpty();
+    assert data.hasNoGeneratorFrames();
-    return new VmTyped(frame.materialize(), parent, parent.getVmClass(), data.members);
+    return new VmTyped(frame.materialize(), parent, parent.getVmClass(), data.members());
   }
 
   @Specialization(guards = "checkListingCannotHaveParameters()")
   protected VmListing evalListing(VirtualFrame frame, VmListing parent) {
-    var data = createData(frame, parent, parent.getLength());
+    var data = executeChildren(frame, parent, parent.getLength());
-    var result = new VmListing(frame.materialize(), parent, data.members, data.length);
+    var result = new VmListing(frame.materialize(), parent, data.members(), data.length());
-    result.setExtraStorage(data.forBindings);
+    return data.storeGeneratorFrames(result);
-    return result;
   }
 
   @Specialization(guards = "checkMappingCannotHaveParameters()")
   protected VmMapping evalMapping(VirtualFrame frame, VmMapping parent) {
-    var data = createData(frame, parent, 0);
+    var data = executeChildren(frame, parent, 0);
-    var result = new VmMapping(frame.materialize(), parent, data.members);
+    var result = new VmMapping(frame.materialize(), parent, data.members());
-    result.setExtraStorage(data.forBindings);
+    return data.storeGeneratorFrames(result);
-    return result;
   }
 
   @Specialization(guards = "checkObjectCannotHaveParameters()")
@@ -110,7 +106,7 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   }
 
   @Specialization(guards = "checkIsValidFunctionAmendment(parent)")
-  protected Object evalFunction(
+  protected VmFunction evalFunction(
       VirtualFrame frame,
       VmFunction parent,
       @Cached(value = "createAmendFunctionNode(frame)", neverDefault = true)
@@ -120,41 +116,34 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   }
 
   @Specialization(guards = {"parent == getDynamicClass()", "checkObjectCannotHaveParameters()"})
-  protected VmDynamic evalDynamicClass(
+  protected VmDynamic evalDynamicClass(VirtualFrame frame, VmClass parent) {
-      VirtualFrame frame, @SuppressWarnings("unused") VmClass parent) {
+    var data = executeChildren(frame, parent, 0);
-    var data = createData(frame, parent, 0);
     var result =
-        new VmDynamic(frame.materialize(), parent.getPrototype(), data.members, data.length);
+        new VmDynamic(frame.materialize(), parent.getPrototype(), data.members(), data.length());
-    result.setExtraStorage(data.forBindings);
+    return data.storeGeneratorFrames(result);
-    return result;
   }
 
   @Specialization(guards = {"parent == getMappingClass()", "checkMappingCannotHaveParameters()"})
-  protected VmMapping evalMappingClass(
+  protected VmMapping evalMappingClass(VirtualFrame frame, VmClass parent) {
-      VirtualFrame frame, @SuppressWarnings("unused") VmClass parent) {
+    var data = executeChildren(frame, parent, 0);
-    var data = createData(frame, parent, 0);
+    var result = new VmMapping(frame.materialize(), parent.getPrototype(), data.members());
-    var result = new VmMapping(frame.materialize(), parent.getPrototype(), data.members);
+    return data.storeGeneratorFrames(result);
-    result.setExtraStorage(data.forBindings);
-    return result;
   }
 
   @Specialization(guards = {"parent == getListingClass()", "checkListingCannotHaveParameters()"})
-  protected VmListing evalListingClass(
+  protected VmListing evalListingClass(VirtualFrame frame, VmClass parent) {
-      VirtualFrame frame, @SuppressWarnings("unused") VmClass parent) {
+    var data = executeChildren(frame, parent, 0);
-    var data = createData(frame, parent, 0);
     var result =
-        new VmListing(frame.materialize(), parent.getPrototype(), data.members, data.length);
+        new VmListing(frame.materialize(), parent.getPrototype(), data.members(), data.length());
-    result.setExtraStorage(data.forBindings);
+    return data.storeGeneratorFrames(result);
-    return result;
   }
 
   @Specialization(guards = {"isTypedObjectClass(parent)", "checkObjectCannotHaveParameters()"})
-  protected VmTyped evalTypedObjectClass(
+  protected VmTyped evalTypedObjectClass(VirtualFrame frame, VmClass parent) {
-      VirtualFrame frame, @SuppressWarnings("unused") VmClass parent) {
     VmUtils.checkIsInstantiable(parent, getParentNode());
-    var data = createData(frame, parent, 0);
+    var data = executeChildren(frame, parent, 0);
-    assert data.forBindings.isEmpty();
+    assert data.hasNoGeneratorFrames();
-    return new VmTyped(frame.materialize(), parent.getPrototype(), parent, data.members);
+    return new VmTyped(frame.materialize(), parent.getPrototype(), parent, data.members());
   }
 
   @Fallback
@@ -200,9 +189,9 @@ public abstract class GeneratorObjectLiteralNode extends ObjectLiteralNode {
   }
 
   @ExplodeLoop
-  private ObjectData createData(VirtualFrame frame, Object parent, int parentLength) {
+  private ObjectData executeChildren(VirtualFrame frame, Object parent, int parentLength) {
-    var data = new ObjectData(memberNodes.length, parentLength);
+    var data = new ObjectData(parentLength);
-    for (GeneratorMemberNode memberNode : memberNodes) {
+    for (var memberNode : memberNodes) {
       memberNode.execute(frame, parent, data);
     }
     return data;

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorPredicateMemberNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,8 +34,9 @@ public abstract class GeneratorPredicateMemberNode extends GeneratorMemberNode {
 
   @CompilationFinal private int customThisSlot = -1;
 
-  protected GeneratorPredicateMemberNode(ExpressionNode predicateNode, ObjectMember member) {
+  protected GeneratorPredicateMemberNode(
-    super(member.getSourceSection());
+      ExpressionNode predicateNode, ObjectMember member, boolean isFrameStored) {
+    super(member.getSourceSection(), isFrameStored);
     this.predicateNode = predicateNode;
     this.member = member;
   }
@@ -110,7 +111,7 @@ public abstract class GeneratorPredicateMemberNode extends GeneratorMemberNode {
 
         try {
           var isApplicable = predicateNode.executeBoolean(frame);
-          if (isApplicable) doAdd(key, data);
+          if (isApplicable) data.addMember(frame, key, this.member, this);
         } catch (UnexpectedResultException e) {
           CompilerDirectives.transferToInterpreter();
           throw exceptionBuilder()
@@ -134,13 +135,4 @@ public abstract class GeneratorPredicateMemberNode extends GeneratorMemberNode {
           frame.getFrameDescriptor().findOrAddAuxiliarySlot(CustomThisScope.FRAME_SLOT_ID);
     }
   }
-
-  private void doAdd(Object key, ObjectData data) {
-    if (EconomicMaps.put(data.members, key, member) != null) {
-      CompilerDirectives.transferToInterpreter();
-      throw duplicateDefinition(key, member);
-    }
-
-    data.persistForBindings(key);
-  }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorPropertyNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,67 +20,67 @@ import com.oracle.truffle.api.dsl.Fallback;
 import com.oracle.truffle.api.dsl.Idempotent;
 import com.oracle.truffle.api.dsl.ImportStatic;
 import com.oracle.truffle.api.dsl.Specialization;
+import com.oracle.truffle.api.frame.VirtualFrame;
 import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.runtime.*;
-import org.pkl.core.util.EconomicMaps;
 
 @ImportStatic({BaseModule.class, GeneratorObjectLiteralNode.class})
 public abstract class GeneratorPropertyNode extends GeneratorMemberNode {
   protected final ObjectMember member;
 
   protected GeneratorPropertyNode(ObjectMember member) {
-    super(member.getSourceSection());
+    super(member.getSourceSection(), false);
     this.member = member;
     assert member.isProp();
   }
 
   @Specialization
   @SuppressWarnings("unused")
-  protected void evalDynamic(VmDynamic parent, ObjectData data) {
+  protected void evalDynamic(VirtualFrame frame, VmDynamic parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = "checkIsValidTypedProperty(parent.getVmClass(), member)")
-  protected void evalTyped(VmTyped parent, ObjectData data) {
+  protected void evalTyped(VirtualFrame frame, VmTyped parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = "checkIsValidMappingProperty()")
-  protected void evalMapping(VmMapping parent, ObjectData data) {
+  protected void evalMapping(VirtualFrame frame, VmMapping parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = "checkIsValidListingProperty()")
-  protected void evalListing(VmListing parent, ObjectData data) {
+  protected void evalListing(VirtualFrame frame, VmListing parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = "parent == getDynamicClass()")
-  protected void evalDynamicClass(VmClass parent, ObjectData data) {
+  protected void evalDynamicClass(VirtualFrame frame, VmClass parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = {"parent == getMappingClass()", "checkIsValidMappingProperty()"})
-  protected void evalMappingClass(VmClass parent, ObjectData data) {
+  protected void evalMappingClass(VirtualFrame frame, VmClass parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(guards = {"parent == getListingClass()", "checkIsValidListingProperty()"})
-  protected void evalListingClass(VmClass parent, ObjectData data) {
+  protected void evalListingClass(VirtualFrame frame, VmClass parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @SuppressWarnings("unused")
   @Specialization(
       guards = {"isTypedObjectClass(parent)", "checkIsValidTypedProperty(parent, member)"})
-  protected void evalTypedObjectClass(VmClass parent, ObjectData data) {
+  protected void evalTypedObjectClass(VirtualFrame frame, VmClass parent, ObjectData data) {
-    addProperty(data);
+    data.addProperty(frame, member, this);
   }
 
   @Fallback
@@ -116,11 +116,4 @@ public abstract class GeneratorPropertyNode extends GeneratorMemberNode {
         .withSourceSection(member.getHeaderSection())
         .build();
   }
-
-  private void addProperty(ObjectData data) {
-    if (EconomicMaps.put(data.members, member.getName(), member) == null) return;
-
-    CompilerDirectives.transferToInterpreter();
-    throw duplicateDefinition(member.getName(), member);
-  }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorSpreadNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -19,7 +19,6 @@ import static org.pkl.core.runtime.BaseModule.getListingClass;
 import static org.pkl.core.runtime.BaseModule.getMappingClass;
 
 import com.oracle.truffle.api.CompilerDirectives;
-import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
 import com.oracle.truffle.api.dsl.Fallback;
 import com.oracle.truffle.api.dsl.ImportStatic;
 import com.oracle.truffle.api.dsl.Specialization;
@@ -29,9 +28,11 @@ import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.runtime.BaseModule;
 import org.pkl.core.runtime.Identifier;
+import org.pkl.core.runtime.Iterators.TruffleIterator;
 import org.pkl.core.runtime.VmClass;
 import org.pkl.core.runtime.VmCollection;
 import org.pkl.core.runtime.VmDynamic;
+import org.pkl.core.runtime.VmException;
 import org.pkl.core.runtime.VmException.ProgramValue;
 import org.pkl.core.runtime.VmIntSeq;
 import org.pkl.core.runtime.VmListing;
@@ -41,8 +42,6 @@ import org.pkl.core.runtime.VmNull;
 import org.pkl.core.runtime.VmObject;
 import org.pkl.core.runtime.VmTyped;
 import org.pkl.core.runtime.VmUtils;
-import org.pkl.core.util.EconomicMaps;
-import org.pkl.core.util.MutableLong;
 
 @ImportStatic(BaseModule.class)
 public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
@@ -51,7 +50,7 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
 
   public GeneratorSpreadNode(
       SourceSection sourceSection, ExpressionNode iterableNode, boolean nullable) {
-    super(sourceSection);
+    super(sourceSection, false);
     this.iterableNode = iterableNode;
     this.nullable = nullable;
   }
@@ -84,78 +83,82 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
 
   @Specialization(guards = "!iterable.isTyped()")
   @SuppressWarnings("unused")
-  protected void eval(VmDynamic parent, ObjectData data, VmObject iterable) {
+  protected void eval(VirtualFrame frame, VmDynamic parent, ObjectData data, VmObject iterable) {
-    doEvalDynamic(data, iterable);
+    doEvalDynamic(frame, data, iterable);
   }
 
   @Specialization(guards = "!iterable.isTyped()")
   @SuppressWarnings("unused")
-  protected void eval(VmListing parent, ObjectData data, VmObject iterable) {
+  protected void eval(VirtualFrame frame, VmListing parent, ObjectData data, VmObject iterable) {
-    doEvalListing(data, iterable);
+    doEvalListing(frame, data, iterable);
   }
 
   @Specialization(guards = "!iterable.isTyped()")
   @SuppressWarnings("unused")
-  protected void eval(VmMapping parent, ObjectData data, VmObject iterable) {
+  protected void eval(VirtualFrame frame, VmMapping parent, ObjectData data, VmObject iterable) {
-    doEvalMapping(data, iterable);
+    doEvalMapping(frame, data, iterable);
   }
 
   @Specialization(guards = {"parent == getDynamicClass()", "!iterable.isTyped()"})
   @SuppressWarnings("unused")
-  protected void evalDynamicClass(VmClass parent, ObjectData data, VmObject iterable) {
+  protected void evalDynamicClass(
-    doEvalDynamic(data, iterable);
+      VirtualFrame frame, VmClass parent, ObjectData data, VmObject iterable) {
+    doEvalDynamic(frame, data, iterable);
   }
 
   @Specialization(guards = {"parent == getListingClass()", "!iterable.isTyped()"})
   @SuppressWarnings("unused")
-  protected void evalListingClass(VmClass parent, ObjectData data, VmObject iterable) {
+  protected void evalListingClass(
-    doEvalListing(data, iterable);
+      VirtualFrame frame, VmClass parent, ObjectData data, VmObject iterable) {
+    doEvalListing(frame, data, iterable);
   }
 
   @Specialization(guards = {"parent == getMappingClass()", "!iterable.isTyped()"})
   @SuppressWarnings("unused")
-  protected void evalMappingClass(VmClass parent, ObjectData data, VmObject iterable) {
+  protected void evalMappingClass(
-    doEvalMapping(data, iterable);
+      VirtualFrame frame, VmClass parent, ObjectData data, VmObject iterable) {
+    doEvalMapping(frame, data, iterable);
   }
 
   @Specialization(guards = {"isTypedObjectClass(parent)", "!iterable.isTyped()"})
-  protected void evalTypedClass(VmClass parent, ObjectData data, VmObject iterable) {
+  protected void evalTypedClass(
-    doEvalTyped(parent, data, iterable);
+      VirtualFrame frame, VmClass parent, ObjectData data, VmObject iterable) {
+    doEvalTyped(frame, parent, data, iterable);
   }
 
   @Specialization(guards = {"!iterable.isTyped()"})
-  protected void eval(VmTyped parent, ObjectData data, VmObject iterable) {
+  protected void eval(VirtualFrame frame, VmTyped parent, ObjectData data, VmObject iterable) {
-    doEvalTyped(parent.getVmClass(), data, iterable);
+    doEvalTyped(frame, parent.getVmClass(), data, iterable);
   }
 
   @Specialization
-  protected void eval(VmObject parent, ObjectData data, VmMap iterable) {
+  protected void eval(VirtualFrame frame, VmObject parent, ObjectData data, VmMap iterable) {
-    doEvalMap(parent.getVmClass(), data, iterable);
+    doEvalMap(frame, parent.getVmClass(), data, iterable);
   }
 
   @Specialization
-  protected void eval(VmClass parent, ObjectData data, VmMap iterable) {
+  protected void eval(VirtualFrame frame, VmClass parent, ObjectData data, VmMap iterable) {
-    doEvalMap(parent, data, iterable);
+    doEvalMap(frame, parent, data, iterable);
   }
 
   @Specialization
-  protected void eval(VmObject parent, ObjectData data, VmCollection iterable) {
+  protected void eval(VirtualFrame frame, VmObject parent, ObjectData data, VmCollection iterable) {
-    doEvalCollection(parent.getVmClass(), data, iterable);
+    doEvalCollection(frame, parent.getVmClass(), data, iterable);
   }
 
   @Specialization
-  protected void eval(VmClass parent, ObjectData data, VmCollection iterable) {
+  protected void eval(VirtualFrame frame, VmClass parent, ObjectData data, VmCollection iterable) {
-    doEvalCollection(parent, data, iterable);
+    doEvalCollection(frame, parent, data, iterable);
   }
 
   @Specialization
-  protected void eval(VmObject parent, ObjectData data, VmIntSeq iterable) {
+  protected void eval(VirtualFrame frame, VmObject parent, ObjectData data, VmIntSeq iterable) {
-    doEvalIntSeq(parent.getVmClass(), data, iterable);
+    doEvalIntSeq(frame, parent.getVmClass(), data, iterable);
   }
 
   @Specialization
-  protected void eval(VmClass parent, ObjectData data, VmIntSeq iterable) {
+  protected void eval(VirtualFrame frame, VmClass parent, ObjectData data, VmIntSeq iterable) {
-    doEvalIntSeq(parent, data, iterable);
+    doEvalIntSeq(frame, parent, data, iterable);
   }
 
   @Fallback
@@ -174,64 +177,55 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
     throw builder.build();
   }
 
-  protected void doEvalDynamic(ObjectData data, VmObject iterable) {
+  protected void doEvalDynamic(VirtualFrame frame, ObjectData data, VmObject iterable) {
-    var length = new MutableLong(data.length);
     iterable.forceAndIterateMemberValues(
         (key, member, value) -> {
           if (member.isElement()) {
-            EconomicMaps.put(data.members, length.getAndIncrement(), createMember(member, value));
+            data.addElement(frame, createMember(member, value), this);
           } else {
-            if (EconomicMaps.put(data.members, key, createMember(member, value)) != null) {
+            data.addMember(frame, key, createMember(member, value), this);
-              duplicateMember(key, member);
-            }
           }
           return true;
         });
-    data.length = (int) length.get();
   }
 
-  private void doEvalMapping(ObjectData data, VmObject iterable) {
+  private void doEvalMapping(VirtualFrame frame, ObjectData data, VmObject iterable) {
     iterable.forceAndIterateMemberValues(
         (key, member, value) -> {
           if (member.isElement() || member.isProp()) {
             cannotHaveMember(BaseModule.getMappingClass(), member);
           }
-          if (EconomicMaps.put(data.members, key, createMember(member, value)) != null) {
+          data.addMember(frame, key, createMember(member, value), this);
-            duplicateMember(key, member);
-          }
           return true;
         });
   }
 
-  private void doEvalListing(ObjectData data, VmObject iterable) {
+  private void doEvalListing(VirtualFrame frame, ObjectData data, VmObject iterable) {
-    var length = new MutableLong(data.length);
     iterable.forceAndIterateMemberValues(
         (key, member, value) -> {
           if (member.isEntry() || member.isProp()) {
             cannotHaveMember(getListingClass(), member);
           }
-          EconomicMaps.put(data.members, length.getAndIncrement(), createMember(member, value));
+          data.addElement(frame, createMember(member, value), this);
           return true;
         });
-    data.length = (int) length.get();
   }
 
-  private void doEvalTyped(VmClass clazz, ObjectData data, VmObject iterable) {
+  private void doEvalTyped(VirtualFrame frame, VmClass clazz, ObjectData data, VmObject iterable) {
     iterable.forceAndIterateMemberValues(
         (key, member, value) -> {
           if (member.isElement() || member.isEntry()) {
             cannotHaveMember(clazz, member);
           }
-          checkTypedProperty(clazz, member);
+          checkIsValidTypedProperty(clazz, member);
-          if (EconomicMaps.put(data.members, key, createMember(member, value)) != null) {
+          data.addProperty(frame, createMember(member, value), this);
-            duplicateMember(key, member);
-          }
           return true;
         });
   }
 
   // handles both `List` and `Set`
-  private void doEvalCollection(VmClass parent, ObjectData data, VmCollection iterable) {
+  private void doEvalCollection(
+      VirtualFrame frame, VmClass parent, ObjectData data, VmCollection iterable) {
     if (isTypedObjectClass(parent) || parent == getMappingClass()) {
       CompilerDirectives.transferToInterpreter();
       throw exceptionBuilder()
@@ -241,10 +235,10 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
           .withProgramValue("Value", iterable)
           .build();
     }
-    spreadIterable(data, iterable);
+    spreadIterable(frame, data, iterable);
   }
 
-  private void doEvalMap(VmClass parent, ObjectData data, VmMap iterable) {
+  private void doEvalMap(VirtualFrame frame, VmClass parent, ObjectData data, VmMap iterable) {
     if (isTypedObjectClass(parent) || parent == getListingClass()) {
       CompilerDirectives.transferToInterpreter();
       throw exceptionBuilder()
@@ -255,13 +249,12 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
     }
     for (var entry : iterable) {
       var member = VmUtils.createSyntheticObjectEntry("", VmUtils.getValue(entry));
-      if (EconomicMaps.put(data.members, VmUtils.getKey(entry), member) != null) {
+      data.addMember(frame, VmUtils.getKey(entry), member, this);
-        duplicateMember(VmUtils.getKey(entry), member);
-      }
     }
   }
 
-  private void doEvalIntSeq(VmClass parent, ObjectData data, VmIntSeq iterable) {
+  private void doEvalIntSeq(
+      VirtualFrame frame, VmClass parent, ObjectData data, VmIntSeq iterable) {
     if (isTypedObjectClass(parent) || parent == getMappingClass()) {
       CompilerDirectives.transferToInterpreter();
       throw exceptionBuilder()
@@ -270,7 +263,7 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
           .withProgramValue("Value", iterable)
           .build();
     }
-    spreadIterable(data, iterable);
+    spreadIterable(frame, data, iterable);
   }
 
   private void cannotHaveMember(VmClass clazz, ObjectMember member) {
@@ -295,7 +288,8 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
     throw exception;
   }
 
-  private void duplicateMember(Object key, ObjectMember member) {
+  @Override
+  protected VmException duplicateDefinition(Object key, ObjectMember member) {
     CompilerDirectives.transferToInterpreter();
     var exception =
         exceptionBuilder()
@@ -331,51 +325,12 @@ public abstract class GeneratorSpreadNode extends GeneratorMemberNode {
     return result;
   }
 
-  @TruffleBoundary
+  private void spreadIterable(VirtualFrame frame, ObjectData data, Iterable<?> iterable) {
-  private void spreadIterable(ObjectData data, Iterable<?> iterable) {
+    var iterator = new TruffleIterator<>(iterable);
-    var length = data.length;
+    while (iterator.hasNext()) {
-    for (var elem : iterable) {
+      var elem = iterator.next();
-      var index = length++;
+      var member = VmUtils.createSyntheticObjectElement(String.valueOf(data.length()), elem);
-      var member = VmUtils.createSyntheticObjectElement(String.valueOf(index), elem);
+      data.addElement(frame, member, this);
-      EconomicMaps.put(data.members, (long) index, member);
-    }
-    data.length = length;
-  }
-
-  protected void checkTypedProperty(VmClass clazz, ObjectMember member) {
-    if (member.isLocal()) return;
-
-    var memberName = member.getName();
-    var classProperty = clazz.getProperty(memberName);
-    if (classProperty == null) {
-      CompilerDirectives.transferToInterpreter();
-      var exception =
-          exceptionBuilder()
-              .cannotFindProperty(clazz.getPrototype(), memberName, false, false)
-              .build();
-      if (member.getHeaderSection().isAvailable()) {
-        exception
-            .getInsertedStackFrames()
-            .put(
-                getRootNode().getCallTarget(),
-                VmUtils.createStackFrame(member.getHeaderSection(), member.getQualifiedName()));
-      }
-      throw exception;
-    }
-
-    if (classProperty.isConstOrFixed()) {
-      CompilerDirectives.transferToInterpreter();
-      var errMsg =
-          classProperty.isConst() ? "cannotAssignConstProperty" : "cannotAssignFixedProperty";
-      var exception = exceptionBuilder().evalError(errMsg, memberName).build();
-      if (member.getHeaderSection().isAvailable()) {
-        exception
-            .getInsertedStackFrames()
-            .put(
-                getRootNode().getCallTarget(),
-                VmUtils.createStackFrame(member.getHeaderSection(), member.getQualifiedName()));
-      }
-      throw exception;
     }
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorWhenNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -34,7 +34,7 @@ public final class GeneratorWhenNode extends GeneratorMemberNode {
       GeneratorMemberNode[] thenNodes,
       GeneratorMemberNode[] elseNodes) {
 
-    super(sourceSection);
+    super(sourceSection, false);
     this.conditionNode = conditionNode;
     this.thenNodes = thenNodes;
     this.elseNodes = elseNodes;

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/ObjectData.java

@@ -0,0 +1,93 @@
+/*
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.ast.expression.generator;
+
+import com.oracle.truffle.api.CompilerDirectives;
+import com.oracle.truffle.api.frame.MaterializedFrame;
+import com.oracle.truffle.api.frame.VirtualFrame;
+import org.graalvm.collections.EconomicMap;
+import org.graalvm.collections.UnmodifiableEconomicMap;
+import org.pkl.core.ast.member.ObjectMember;
+import org.pkl.core.runtime.VmObject;
+import org.pkl.core.runtime.VmUtils;
+import org.pkl.core.util.EconomicMaps;
+
+/** Data collected by {@link GeneratorObjectLiteralNode} to generate a `VmObject`. */
+public final class ObjectData {
+  /** The object's members. */
+  private final EconomicMap<Object, ObjectMember> members;
+
+  /**
+   * The frames that were active when `members` were generated. Only a subset of members have their
+   * frames stored ({@link GeneratorMemberNode#isFrameStored}). Frames are stored in
+   * `owner.extraStorage` and retrieved by `RestoreForBindingsNode` when members are executed
+   */
+  private final EconomicMap<Object, MaterializedFrame> generatorFrames;
+
+  /** The object's number of elements. */
+  private int length;
+
+  ObjectData(int parentLength) {
+    // optimize for memory usage by not estimating minimum size
+    members = EconomicMaps.create();
+    generatorFrames = EconomicMaps.create();
+    length = parentLength;
+  }
+
+  UnmodifiableEconomicMap<Object, ObjectMember> members() {
+    return members;
+  }
+
+  int length() {
+    return length;
+  }
+
+  boolean hasNoGeneratorFrames() {
+    return generatorFrames.isEmpty();
+  }
+
+  void addElement(VirtualFrame frame, ObjectMember member, GeneratorMemberNode node) {
+    addMember(frame, (long) length, member, node);
+    length += 1;
+  }
+
+  void addProperty(VirtualFrame frame, ObjectMember member, GeneratorMemberNode node) {
+    addMember(frame, member.getName(), member, node);
+  }
+
+  void addMember(VirtualFrame frame, Object key, ObjectMember member, GeneratorMemberNode node) {
+    if (EconomicMaps.put(members, key, member) != null) {
+      CompilerDirectives.transferToInterpreter();
+      throw node.duplicateDefinition(key, member);
+    }
+    if (node.isFrameStored) {
+      EconomicMaps.put(generatorFrames, key, frame.materialize());
+    }
+  }
+
+  <T extends VmObject> T storeGeneratorFrames(T object) {
+    object.setExtraStorage(generatorFrames);
+    return object;
+  }
+
+  static MaterializedFrame getGeneratorFrame(VirtualFrame frame) {
+    @SuppressWarnings("unchecked")
+    var map = (EconomicMap<Object, MaterializedFrame>) VmUtils.getOwner(frame).getExtraStorage();
+    var result = EconomicMaps.get(map, VmUtils.getMemberKey(frame));
+    assert result != null;
+    return result;
+  }
+}

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/RestoreForBindingsNode.java

@@ -0,0 +1,46 @@
+/*
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.pkl.core.ast.expression.generator;
+
+import com.oracle.truffle.api.frame.VirtualFrame;
+import org.pkl.core.ast.ExpressionNode;
+import org.pkl.core.runtime.VmUtils;
+
+/**
+ * Restores for-generator variable bindings when a member generated by a for-generator is executed.
+ */
+public final class RestoreForBindingsNode extends ExpressionNode {
+  private @Child ExpressionNode child;
+
+  public RestoreForBindingsNode(ExpressionNode child) {
+    super(child.getSourceSection());
+    this.child = child;
+  }
+
+  @Override
+  public Object executeGeneric(VirtualFrame frame) {
+    var generatorFrame = ObjectData.getGeneratorFrame(frame);
+    var numSlots = frame.getFrameDescriptor().getNumberOfSlots();
+    // This value is constant and could be a constructor argument.
+    var startSlot = generatorFrame.getFrameDescriptor().getNumberOfSlots() - numSlots;
+    assert startSlot >= 0;
+    // Copy locals that are for-generator variables into this frame.
+    // Slots before `startSlot` (if any) are function arguments
+    // and must not be copied to preserve scoping rules.
+    VmUtils.copyLocals(generatorFrame, startSlot, frame, 0, numSlots);
+    return child.executeGeneric(frame);
+  }
+}

pkl-core/src/main/java/org/pkl/core/ast/expression/generator/WriteForVariablesNode.java

@@ -1,52 +0,0 @@
-/*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.pkl.core.ast.expression.generator;
-
-import com.oracle.truffle.api.frame.VirtualFrame;
-import com.oracle.truffle.api.nodes.ExplodeLoop;
-import org.graalvm.collections.UnmodifiableEconomicMap;
-import org.pkl.core.ast.ExpressionNode;
-import org.pkl.core.runtime.VmUtils;
-import org.pkl.core.util.EconomicMaps;
-
-public final class WriteForVariablesNode extends ExpressionNode {
-  private final int[] auxiliarySlots;
-  @Child private ExpressionNode childNode;
-
-  public WriteForVariablesNode(int[] auxiliarySlots, ExpressionNode childNode) {
-    this.auxiliarySlots = auxiliarySlots;
-    this.childNode = childNode;
-  }
-
-  @Override
-  @ExplodeLoop
-  public Object executeGeneric(VirtualFrame frame) {
-    var extraStorage = VmUtils.getOwner(frame).getExtraStorage();
-    assert extraStorage instanceof UnmodifiableEconomicMap;
-
-    @SuppressWarnings("unchecked")
-    var forBindings = (UnmodifiableEconomicMap<Object, Object[]>) extraStorage;
-    var bindings = EconomicMaps.get(forBindings, VmUtils.getMemberKey(frame));
-    assert bindings != null;
-    assert bindings.length == auxiliarySlots.length;
-
-    for (var i = 0; i < auxiliarySlots.length; i++) {
-      frame.setAuxiliarySlot(auxiliarySlots[i], bindings[i]);
-    }
-
-    return childNode.executeGeneric(frame);
-  }
-}

pkl-core/src/main/java/org/pkl/core/ast/expression/literal/AmendFunctionNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -26,8 +26,6 @@ import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.PklNode;
 import org.pkl.core.ast.PklRootNode;
 import org.pkl.core.ast.SimpleRootNode;
-import org.pkl.core.ast.builder.SymbolTable;
-import org.pkl.core.ast.builder.SymbolTable.CustomThisScope;
 import org.pkl.core.ast.frame.ReadFrameSlotNodeGen;
 import org.pkl.core.ast.member.FunctionNode;
 import org.pkl.core.ast.member.Lambda;
@@ -40,10 +38,7 @@ public final class AmendFunctionNode extends PklNode {
   private final PklRootNode initialFunctionRootNode;
   @CompilationFinal private int customThisSlot = -1;
 
-  public AmendFunctionNode(
+  public AmendFunctionNode(ObjectLiteralNode hostNode, TypeNode[] parameterTypeNodes) {
-      ObjectLiteralNode hostNode,
-      TypeNode[] parameterTypeNodes,
-      FrameDescriptor hostFrameDescriptor) {
     super(hostNode.getSourceSection());
 
     isCustomThisScope = hostNode.isCustomThisScope;
@@ -61,39 +56,7 @@ public final class AmendFunctionNode extends PklNode {
     } else {
       parameterSlots = new int[0];
     }
-    var hasForGenVars = false;
+    var objectToAmendSlot = builder.addSlot(FrameSlotKind.Object, null, null);
-    for (var i = 0; i < hostFrameDescriptor.getNumberOfSlots(); i++) {
-      var slotInfo = hostFrameDescriptor.getSlotInfo(i);
-      // Copy for-generator variables from the outer frame descriptor into inner lambda.
-      //
-      // We need to do this because at parse time within AstBuilder, we inject for-generator
-      // variables into the frame descriptor of the containing root node.
-      // The expectation is that when GeneratorForNode executes, it writes for-generator variables
-      // into these slots.
-      //
-      // In the case of an amend function node, AstBuilder can't determine out that there is another
-      // frame (e.g. with `new Mixin { ... }` syntax), so it injects for-generator vars into the
-      // wrong frame.
-      //
-      // As a remedy, we simply copy outer variables into this frame if there are any for generator
-      // variables.
-      //
-      // We need to preserve the frame slot index, so we insert dummy identifiers
-      // for other slots that aren't for generator variables.
-      if (slotInfo != null && slotInfo.equals(SymbolTable.FOR_GENERATOR_VARIABLE)) {
-        if (!hasForGenVars) {
-          hasForGenVars = true;
-          for (var j = 0; j < i; j++) {
-            builder.addSlot(FrameSlotKind.Illegal, Identifier.DUMMY, null);
-          }
-        }
-        builder.addSlot(
-            hostFrameDescriptor.getSlotKind(i), hostFrameDescriptor.getSlotName(i), null);
-      } else if (hasForGenVars) {
-        builder.addSlot(FrameSlotKind.Illegal, Identifier.DUMMY, null);
-      }
-    }
-    var objectToAmendSlot = builder.addSlot(FrameSlotKind.Object, new Object(), null);
     var frameDescriptor = builder.build();
 
     var subsequentFunctionRootNode =
@@ -138,7 +101,7 @@ public final class AmendFunctionNode extends PklNode {
   public VmFunction execute(VirtualFrame frame, VmFunction functionToAmend) {
     if (isCustomThisScope && customThisSlot == -1) {
       CompilerDirectives.transferToInterpreterAndInvalidate();
-      customThisSlot = VmUtils.findAuxiliarySlot(frame, CustomThisScope.FRAME_SLOT_ID);
+      customThisSlot = VmUtils.findCustomThisSlot(frame);
     }
     return new VmFunction(
         frame.materialize(),
@@ -184,8 +147,7 @@ public final class AmendFunctionNode extends PklNode {
       var arguments = new Object[frameArguments.length];
       arguments[0] = functionToAmend.getThisValue();
       arguments[1] = functionToAmend;
-      arguments[2] = false;
+      System.arraycopy(frameArguments, 2, arguments, 2, frameArguments.length - 2);
-      System.arraycopy(frameArguments, 3, arguments, 3, frameArguments.length - 3);
 
       var valueToAmend = callNode.call(functionToAmend.getCallTarget(), arguments);
       if (!(valueToAmend instanceof VmFunction newFunctionToAmend)) {

pkl-core/src/main/java/org/pkl/core/ast/expression/literal/FunctionLiteralNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -20,7 +20,6 @@ import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
 import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
-import org.pkl.core.ast.builder.SymbolTable.CustomThisScope;
 import org.pkl.core.ast.member.FunctionNode;
 import org.pkl.core.ast.member.UnresolvedFunctionNode;
 import org.pkl.core.runtime.VmFunction;
@@ -48,7 +47,7 @@ public final class FunctionLiteralNode extends ExpressionNode {
       CompilerDirectives.transferToInterpreterAndInvalidate();
       functionNode = unresolvedFunctionNode.execute(frame);
       if (isCustomThisScope) {
-        customThisSlot = VmUtils.findAuxiliarySlot(frame, CustomThisScope.FRAME_SLOT_ID);
+        customThisSlot = VmUtils.findCustomThisSlot(frame);
       }
     }
 

pkl-core/src/main/java/org/pkl/core/ast/expression/literal/ObjectLiteralNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -66,7 +66,7 @@ public abstract class ObjectLiteralNode extends ExpressionNode {
         parametersDescriptor == null
             ? new TypeNode[0]
             : VmUtils.resolveParameterTypes(frame, parametersDescriptor, parameterTypes);
-    return new AmendFunctionNode(this, resolvedParameterTypes, frame.getFrameDescriptor());
+    return new AmendFunctionNode(this, resolvedParameterTypes);
   }
 
   @Idempotent

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InvokeMethodDirectNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -28,7 +28,6 @@ public final class InvokeMethodDirectNode extends ExpressionNode {
   private final VmObjectLike owner;
   @Child private ExpressionNode receiverNode;
   @Children private final ExpressionNode[] argumentNodes;
-  private final boolean isInIterable;
 
   @Child private DirectCallNode callNode;
 
@@ -36,14 +35,12 @@ public final class InvokeMethodDirectNode extends ExpressionNode {
       SourceSection sourceSection,
       ClassMethod method,
       ExpressionNode receiverNode,
-      ExpressionNode[] argumentNodes,
+      ExpressionNode[] argumentNodes) {
-      boolean isInIterable) {
 
     super(sourceSection);
     this.owner = method.getOwner();
     this.receiverNode = receiverNode;
     this.argumentNodes = argumentNodes;
-    this.isInIterable = isInIterable;
 
     callNode = DirectCallNode.create(method.getCallTarget(sourceSection));
   }
@@ -51,12 +48,11 @@ public final class InvokeMethodDirectNode extends ExpressionNode {
   @Override
   @ExplodeLoop
   public Object executeGeneric(VirtualFrame frame) {
-    var args = new Object[3 + argumentNodes.length];
+    var args = new Object[2 + argumentNodes.length];
     args[0] = receiverNode.executeGeneric(frame);
     args[1] = owner;
-    args[2] = isInIterable;
     for (var i = 0; i < argumentNodes.length; i++) {
-      args[3 + i] = argumentNodes[i].executeGeneric(frame);
+      args[2 + i] = argumentNodes[i].executeGeneric(frame);
     }
 
     return callNode.call(args);

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InvokeMethodLexicalNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,33 +33,29 @@ public final class InvokeMethodLexicalNode extends ExpressionNode {
   private final int levelsUp;
 
   @Child private DirectCallNode callNode;
-  private final boolean isInIterable;
 
   InvokeMethodLexicalNode(
       SourceSection sourceSection,
       CallTarget callTarget,
       int levelsUp,
-      ExpressionNode[] argumentNodes,
+      ExpressionNode[] argumentNodes) {
-      boolean isInIterable) {
 
     super(sourceSection);
     this.levelsUp = levelsUp;
     this.argumentNodes = argumentNodes;
 
     callNode = DirectCallNode.create(callTarget);
-    this.isInIterable = isInIterable;
   }
 
   @Override
   @ExplodeLoop
   public Object executeGeneric(VirtualFrame frame) {
-    var args = new Object[3 + argumentNodes.length];
+    var args = new Object[2 + argumentNodes.length];
     var enclosingFrame = getEnclosingFrame(frame);
     args[0] = VmUtils.getReceiver(enclosingFrame);
     args[1] = VmUtils.getOwner(enclosingFrame);
-    args[2] = isInIterable;
     for (var i = 0; i < argumentNodes.length; i++) {
-      args[3 + i] = argumentNodes[i].executeGeneric(frame);
+      args[2 + i] = argumentNodes[i].executeGeneric(frame);
     }
 
     return callNode.call(args);

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InvokeMethodVirtualNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -45,31 +45,27 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
   @Children private final ExpressionNode[] argumentNodes;
   private final MemberLookupMode lookupMode;
   private final boolean needsConst;
-  private final boolean isInIterable;
 
   protected InvokeMethodVirtualNode(
       SourceSection sourceSection,
       Identifier methodName,
       ExpressionNode[] argumentNodes,
       MemberLookupMode lookupMode,
-      boolean needsConst,
+      boolean needsConst) {
-      boolean isInIterable) {
 
     super(sourceSection);
     this.methodName = methodName;
     this.argumentNodes = argumentNodes;
     this.lookupMode = lookupMode;
     this.needsConst = needsConst;
-    this.isInIterable = isInIterable;
   }
 
   protected InvokeMethodVirtualNode(
       SourceSection sourceSection,
       Identifier methodName,
       ExpressionNode[] argumentNodes,
-      MemberLookupMode lookupMode,
+      MemberLookupMode lookupMode) {
-      boolean isInIterable) {
+    this(sourceSection, methodName, argumentNodes, lookupMode, false);
-    this(sourceSection, methodName, argumentNodes, lookupMode, false, isInIterable);
   }
 
   /**
@@ -89,12 +85,11 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
           RootCallTarget cachedCallTarget,
       @Cached("create(cachedCallTarget)") DirectCallNode callNode) {
 
-    var args = new Object[3 + argumentNodes.length];
+    var args = new Object[2 + argumentNodes.length];
     args[0] = receiver.getThisValue();
     args[1] = receiver;
-    args[2] = isInIterable;
     for (var i = 0; i < argumentNodes.length; i++) {
-      args[3 + i] = argumentNodes[i].executeGeneric(frame);
+      args[2 + i] = argumentNodes[i].executeGeneric(frame);
     }
 
     return callNode.call(args);
@@ -109,12 +104,11 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
       @SuppressWarnings("unused") VmClass receiverClass,
       @Exclusive @Cached("create()") IndirectCallNode callNode) {
 
-    var args = new Object[3 + argumentNodes.length];
+    var args = new Object[2 + argumentNodes.length];
     args[0] = receiver.getThisValue();
     args[1] = receiver;
-    args[2] = isInIterable;
     for (var i = 0; i < argumentNodes.length; i++) {
-      args[3 + i] = argumentNodes[i].executeGeneric(frame);
+      args[2 + i] = argumentNodes[i].executeGeneric(frame);
     }
 
     return callNode.call(receiver.getCallTarget(), args);
@@ -130,12 +124,11 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
       @Cached("resolveMethod(receiverClass)") ClassMethod method,
       @Cached("create(method.getCallTarget(sourceSection))") DirectCallNode callNode) {
 
-    var args = new Object[3 + argumentNodes.length];
+    var args = new Object[2 + argumentNodes.length];
     args[0] = receiver;
     args[1] = method.getOwner();
-    args[2] = isInIterable;
     for (var i = 0; i < argumentNodes.length; i++) {
-      args[3 + i] = argumentNodes[i].executeGeneric(frame);
+      args[2 + i] = argumentNodes[i].executeGeneric(frame);
     }
 
     return callNode.call(args);
@@ -150,12 +143,11 @@ public abstract class InvokeMethodVirtualNode extends ExpressionNode {
       @Exclusive @Cached("create()") IndirectCallNode callNode) {
 
     var method = resolveMethod(receiverClass);
-    var args = new Object[3 + argumentNodes.length];
+    var args = new Object[2 + argumentNodes.length];
     args[0] = receiver;
     args[1] = method.getOwner();
-    args[2] = isInIterable;
     for (var i = 0; i < argumentNodes.length; i++) {
-      args[3 + i] = argumentNodes[i].executeGeneric(frame);
+      args[2 + i] = argumentNodes[i].executeGeneric(frame);
     }
 
     // Deprecation should not report here (getCallTarget(sourceSection)), as this happens for each

pkl-core/src/main/java/org/pkl/core/ast/expression/member/InvokeSuperMethodNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -30,18 +30,15 @@ import org.pkl.core.runtime.VmUtils;
 public abstract class InvokeSuperMethodNode extends ExpressionNode {
   private final Identifier methodName;
   @Children private final ExpressionNode[] argumentNodes;
-  private final boolean isInIterable;
   private final boolean needsConst;
 
   protected InvokeSuperMethodNode(
       SourceSection sourceSection,
       Identifier methodName,
-      boolean isInIterable,
       ExpressionNode[] argumentNodes,
       boolean needsConst) {
 
     super(sourceSection);
-    this.isInIterable = isInIterable;
     this.needsConst = needsConst;
 
     assert !methodName.isLocalMethod();
@@ -57,12 +54,11 @@ public abstract class InvokeSuperMethodNode extends ExpressionNode {
       @Cached(value = "findSupermethod(frame)", neverDefault = true) ClassMethod supermethod,
       @Cached("create(supermethod.getCallTarget(sourceSection))") DirectCallNode callNode) {
 
-    var args = new Object[3 + argumentNodes.length];
+    var args = new Object[2 + argumentNodes.length];
     args[0] = VmUtils.getReceiverOrNull(frame);
     args[1] = supermethod.getOwner();
-    args[2] = isInIterable;
     for (int i = 0; i < argumentNodes.length; i++) {
-      args[3 + i] = argumentNodes[i].executeGeneric(frame);
+      args[2 + i] = argumentNodes[i].executeGeneric(frame);
     }
 
     return callNode.call(args);

pkl-core/src/main/java/org/pkl/core/ast/expression/member/ResolveMethodNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -50,7 +50,6 @@ public final class ResolveMethodNode extends ExpressionNode {
   private final boolean isCustomThisScope;
   private final ConstLevel constLevel;
   private final int constDepth;
-  private final boolean isInIterable;
 
   public ResolveMethodNode(
       SourceSection sourceSection,
@@ -59,8 +58,7 @@ public final class ResolveMethodNode extends ExpressionNode {
       boolean isBaseModule,
       boolean isCustomThisScope,
       ConstLevel constLevel,
-      int constDepth,
+      int constDepth) {
-      boolean isInIterable) {
 
     super(sourceSection);
 
@@ -70,7 +68,6 @@ public final class ResolveMethodNode extends ExpressionNode {
     this.isCustomThisScope = isCustomThisScope;
     this.constLevel = constLevel;
     this.constDepth = constDepth;
-    this.isInIterable = isInIterable;
   }
 
   @Override
@@ -94,11 +91,7 @@ public final class ResolveMethodNode extends ExpressionNode {
           assert localMethod.isLocal();
           checkConst(currOwner, localMethod, levelsUp);
           return new InvokeMethodLexicalNode(
-              sourceSection,
+              sourceSection, localMethod.getCallTarget(sourceSection), levelsUp, argumentNodes);
-              localMethod.getCallTarget(sourceSection),
-              levelsUp,
-              argumentNodes,
-              isInIterable);
         }
         var method = currOwner.getVmClass().getDeclaredMethod(methodName);
         if (method != null) {
@@ -106,11 +99,7 @@ public final class ResolveMethodNode extends ExpressionNode {
           checkConst(currOwner, method, levelsUp);
           if (method.getDeclaringClass().isClosed()) {
             return new InvokeMethodLexicalNode(
-                sourceSection,
+                sourceSection, method.getCallTarget(sourceSection), levelsUp, argumentNodes);
-                method.getCallTarget(sourceSection),
-                levelsUp,
-                argumentNodes,
-                isInIterable);
           }
 
           //noinspection ConstantConditions
@@ -119,7 +108,6 @@ public final class ResolveMethodNode extends ExpressionNode {
               methodName,
               argumentNodes,
               MemberLookupMode.IMPLICIT_LEXICAL,
-              isInIterable,
               levelsUp == 0 ? new GetReceiverNode() : new GetEnclosingReceiverNode(levelsUp),
               GetClassNodeGen.create(null));
         }
@@ -134,7 +122,7 @@ public final class ResolveMethodNode extends ExpressionNode {
               (CallTarget) localMethod.getCallTarget().call(currOwner, currOwner);
 
           return new InvokeMethodLexicalNode(
-              sourceSection, methodCallTarget, levelsUp, argumentNodes, isInIterable);
+              sourceSection, methodCallTarget, levelsUp, argumentNodes);
         }
       }
 
@@ -150,7 +138,7 @@ public final class ResolveMethodNode extends ExpressionNode {
       if (method != null) {
         assert !method.isLocal();
         return new InvokeMethodDirectNode(
-            sourceSection, method, new ConstantValueNode(baseModule), argumentNodes, isInIterable);
+            sourceSection, method, new ConstantValueNode(baseModule), argumentNodes);
       }
     }
 
@@ -170,7 +158,6 @@ public final class ResolveMethodNode extends ExpressionNode {
         argumentNodes,
         MemberLookupMode.IMPLICIT_THIS,
         needsConst,
-        isInIterable,
         VmUtils.createThisNode(VmUtils.unavailableSourceSection(), isCustomThisScope),
         GetClassNodeGen.create(null));
   }

pkl-core/src/main/java/org/pkl/core/ast/expression/primary/CustomThisNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -21,7 +21,6 @@ import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.nodes.NodeInfo;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
-import org.pkl.core.ast.builder.SymbolTable.CustomThisScope;
 import org.pkl.core.runtime.VmUtils;
 
 /** `this` inside `CustomThisScope` (type constraint, object member predicate). */
@@ -38,7 +37,7 @@ public final class CustomThisNode extends ExpressionNode {
     if (customThisSlot == -1) {
       CompilerDirectives.transferToInterpreterAndInvalidate();
       // deferred until execution time s.t. nodes of inlined type aliases get the right frame slot
-      customThisSlot = VmUtils.findAuxiliarySlot(frame, CustomThisScope.FRAME_SLOT_ID);
+      customThisSlot = VmUtils.findCustomThisSlot(frame);
     }
     return frame.getAuxiliarySlot(customThisSlot);
   }

pkl-core/src/main/java/org/pkl/core/ast/expression/primary/ResolveVariableNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -24,8 +24,6 @@ import org.pkl.core.ast.MemberLookupMode;
 import org.pkl.core.ast.builder.ConstLevel;
 import org.pkl.core.ast.expression.member.ReadLocalPropertyNode;
 import org.pkl.core.ast.expression.member.ReadPropertyNodeGen;
-import org.pkl.core.ast.frame.ReadAuxiliarySlotNode;
-import org.pkl.core.ast.frame.ReadEnclosingAuxiliarySlotNode;
 import org.pkl.core.ast.frame.ReadEnclosingFrameSlotNodeGen;
 import org.pkl.core.ast.frame.ReadFrameSlotNodeGen;
 import org.pkl.core.ast.member.Member;
@@ -34,14 +32,23 @@ import org.pkl.core.runtime.*;
 /**
  * Resolves a variable name, for example `foo` in `x = foo`.
  *
- * <p>A variable name can refer to any of the following: - a (potentially `local`) property in the
+ * <p>A variable name can refer to any of the following:
- * lexical scope - a method or lambda parameter in the lexical scope - a base module property - a
+ *
- * property accessible through `this`
+ * <ul>
+ *   <li>a method/lambda parameter or for-generator/let-expression variable in the lexical scope
+ *   <li>a (potentially `local`) property in the lexical scope
+ *   <li>a `pkl.base` module property
+ *   <li>a property accessible through `this`
+ * </ul>
  *
  * <p>This node's task is to make a one-time decision between these alternatives for the call site
  * it represents.
  */
-// TODO: Move this to parse time (required for supporting local variables, more efficient)
+// TODO: Move this to parse time
+// * more capable because more information is available
+//   and AST customization beyond replacing this node is possible
+// * useful for runtime AST transformations, for example to implement property-based testing
+// * more efficient
 //
 // TODO: In REPL, undo replace if environment changes to make the following work.
 // Perhaps instrumenting this node in REPL would be a good solution.
@@ -82,49 +89,18 @@ public final class ResolveVariableNode extends ExpressionNode {
     CompilerDirectives.transferToInterpreter();
 
     var localPropertyName = variableName.toLocalProperty();
-
-    // search the frame for auxiliary slots carrying this variable (placed by
-    // `WriteForVariablesNode`)
-    var variableSlot = VmUtils.findAuxiliarySlot(frame, localPropertyName);
-    if (variableSlot == -1) {
-      variableSlot = VmUtils.findAuxiliarySlot(frame, variableName);
-    }
-    if (variableSlot != -1) {
-      return new ReadAuxiliarySlotNode(getSourceSection(), variableSlot);
-    }
-    // search the frame for slots carrying this variable
-    variableSlot = VmUtils.findSlot(frame, localPropertyName);
-    if (variableSlot == -1) {
-      variableSlot = VmUtils.findSlot(frame, variableName);
-    }
-    if (variableSlot != -1) {
-      return ReadFrameSlotNodeGen.create(getSourceSection(), variableSlot);
-    }
-
     var currFrame = frame;
     var currOwner = VmUtils.getOwner(currFrame);
     var levelsUp = 0;
 
-    // Search lexical scope for a matching method/lambda parameter, `for` generator variable, or
+    // Search lexical scope for a matching function parameter, for-generator variable, or object
-    // object property.
+    // property.
     do {
-      var parameterSlot = VmUtils.findSlot(currFrame, variableName);
+      var slot = findFrameSlot(currFrame, variableName, localPropertyName);
-      if (parameterSlot == -1) {
+      if (slot != -1) {
-        parameterSlot = VmUtils.findSlot(currFrame, localPropertyName);
-      }
-      if (parameterSlot != -1) {
         return levelsUp == 0
-            ? ReadFrameSlotNodeGen.create(getSourceSection(), parameterSlot)
+            ? ReadFrameSlotNodeGen.create(getSourceSection(), slot)
-            : ReadEnclosingFrameSlotNodeGen.create(getSourceSection(), parameterSlot, levelsUp);
+            : ReadEnclosingFrameSlotNodeGen.create(getSourceSection(), slot, levelsUp);
-      }
-      var auxiliarySlot = VmUtils.findAuxiliarySlot(currFrame, variableName);
-      if (auxiliarySlot == -1) {
-        auxiliarySlot = VmUtils.findAuxiliarySlot(currFrame, localPropertyName);
-      }
-      if (auxiliarySlot != -1) {
-        return levelsUp == 0
-            ? new ReadAuxiliarySlotNode(getSourceSection(), auxiliarySlot)
-            : new ReadEnclosingAuxiliarySlotNode(getSourceSection(), auxiliarySlot, levelsUp);
       }
 
       var localMember = currOwner.getMember(localPropertyName);
@@ -136,8 +112,8 @@ public final class ResolveVariableNode extends ExpressionNode {
         var value = localMember.getConstantValue();
         if (value != null) {
           // This is the only code path that resolves local constant properties.
-          // Since this code path doesn't use ObjectMember.getCachedValue(),
+          // Since this code path doesn't call VmObject.getCachedValue(),
-          // there is no point in calling localMember.setCachedValue() either.
+          // there is no point in calling VmObject.setCachedValue() either.
           return new ConstantValueNode(sourceSection, value);
         }
 
@@ -236,4 +212,17 @@ public final class ResolveVariableNode extends ExpressionNode {
       throw exceptionBuilder().evalError("propertyMustBeConst", variableName.toString()).build();
     }
   }
+
+  private static int findFrameSlot(VirtualFrame frame, Object identifier1, Object identifier2) {
+    var descriptor = frame.getFrameDescriptor();
+    // Search backwards. The for-generator implementation exploits this
+    // to shadow a slot by appending a slot with the same name.
+    for (var i = descriptor.getNumberOfSlots() - 1; i >= 0; i--) {
+      var slotName = descriptor.getSlotName(i);
+      if (slotName == identifier1 || slotName == identifier2) {
+        return i;
+      }
+    }
+    return -1;
+  }
 }

pkl-core/src/main/java/org/pkl/core/ast/frame/ReadAuxiliarySlotNode.java

@@ -1,34 +0,0 @@
-/*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.pkl.core.ast.frame;
-
-import com.oracle.truffle.api.frame.VirtualFrame;
-import com.oracle.truffle.api.source.SourceSection;
-import org.pkl.core.ast.ExpressionNode;
-
-public class ReadAuxiliarySlotNode extends ExpressionNode {
-  private final int slot;
-
-  public ReadAuxiliarySlotNode(SourceSection sourceSection, int slot) {
-    super(sourceSection);
-    this.slot = slot;
-  }
-
-  @Override
-  public Object executeGeneric(VirtualFrame frame) {
-    return frame.getAuxiliarySlot(slot);
-  }
-}

pkl-core/src/main/java/org/pkl/core/ast/frame/ReadEnclosingAuxiliarySlotNode.java

@@ -1,51 +0,0 @@
-/*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
- *
- * Licensed under the Apache License, Version 2.0 (the "License");
- * you may not use this file except in compliance with the License.
- * You may obtain a copy of the License at
- *
- *     https://www.apache.org/licenses/LICENSE-2.0
- *
- * Unless required by applicable law or agreed to in writing, software
- * distributed under the License is distributed on an "AS IS" BASIS,
- * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
- * See the License for the specific language governing permissions and
- * limitations under the License.
- */
-package org.pkl.core.ast.frame;
-
-import com.oracle.truffle.api.frame.MaterializedFrame;
-import com.oracle.truffle.api.frame.VirtualFrame;
-import com.oracle.truffle.api.nodes.ExplodeLoop;
-import com.oracle.truffle.api.source.SourceSection;
-import org.pkl.core.ast.ExpressionNode;
-import org.pkl.core.runtime.VmUtils;
-
-public class ReadEnclosingAuxiliarySlotNode extends ExpressionNode {
-  private final int slot;
-  private final int levelsUp;
-
-  public ReadEnclosingAuxiliarySlotNode(SourceSection sourceSection, int slot, int levelsUp) {
-    super(sourceSection);
-    this.slot = slot;
-    this.levelsUp = levelsUp;
-  }
-
-  // could be factored out into a separate node s.t. it
-  // won't be repeated in case of FrameSlotTypeException
-  @ExplodeLoop
-  protected final MaterializedFrame getCapturedFrame(VirtualFrame frame) {
-    var owner = VmUtils.getOwner(frame);
-    for (var i = 0; i < levelsUp - 1; i++) {
-      owner = owner.getEnclosingOwner();
-      assert owner != null; // guaranteed by AstBuilder
-    }
-    return owner.getEnclosingFrame();
-  }
-
-  @Override
-  public Object executeGeneric(VirtualFrame frame) {
-    return getCapturedFrame(frame).getAuxiliarySlot(slot);
-  }
-}

pkl-core/src/main/java/org/pkl/core/ast/internal/ToStringNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -80,7 +80,6 @@ public abstract class ToStringNode extends UnaryExpressionNode {
         Identifier.TO_STRING,
         new ExpressionNode[] {},
         MemberLookupMode.EXPLICIT_RECEIVER,
-        false,
         null,
         null);
   }

pkl-core/src/main/java/org/pkl/core/ast/lambda/ApplyVmFunction0Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,12 +33,12 @@ public abstract class ApplyVmFunction0Node extends PklNode {
           RootCallTarget cachedCallTarget,
       @Cached("create(cachedCallTarget)") DirectCallNode callNode) {
 
-    return callNode.call(function.getThisValue(), function, false);
+    return callNode.call(function.getThisValue(), function);
   }
 
   @Specialization(replaces = "evalDirect")
   protected Object eval(VmFunction function, @Cached("create()") IndirectCallNode callNode) {
 
-    return callNode.call(function.getCallTarget(), function.getThisValue(), function, false);
+    return callNode.call(function.getCallTarget(), function.getThisValue(), function);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/lambda/ApplyVmFunction1Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -77,13 +77,13 @@ public abstract class ApplyVmFunction1Node extends ExpressionNode {
           RootCallTarget cachedCallTarget,
       @Cached("create(cachedCallTarget)") DirectCallNode callNode) {
 
-    return callNode.call(function.getThisValue(), function, false, arg1);
+    return callNode.call(function.getThisValue(), function, arg1);
   }
 
   @Specialization(replaces = "evalDirect")
   protected Object eval(
       VmFunction function, Object arg1, @Cached("create()") IndirectCallNode callNode) {
 
-    return callNode.call(function.getCallTarget(), function.getThisValue(), function, false, arg1);
+    return callNode.call(function.getCallTarget(), function.getThisValue(), function, arg1);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/lambda/ApplyVmFunction2Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -76,7 +76,7 @@ public abstract class ApplyVmFunction2Node extends PklNode {
           RootCallTarget cachedCallTarget,
       @Cached("create(cachedCallTarget)") DirectCallNode callNode) {
 
-    return callNode.call(function.getThisValue(), function, false, arg1, arg2);
+    return callNode.call(function.getThisValue(), function, arg1, arg2);
   }
 
   @Specialization(replaces = "evalDirect")
@@ -86,7 +86,6 @@ public abstract class ApplyVmFunction2Node extends PklNode {
       Object arg2,
       @Cached("create()") IndirectCallNode callNode) {
 
-    return callNode.call(
+    return callNode.call(function.getCallTarget(), function.getThisValue(), function, arg1, arg2);
-        function.getCallTarget(), function.getThisValue(), function, false, arg1, arg2);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/lambda/ApplyVmFunction3Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -36,7 +36,7 @@ public abstract class ApplyVmFunction3Node extends PklNode {
           RootCallTarget cachedCallTarget,
       @Cached("create(cachedCallTarget)") DirectCallNode callNode) {
 
-    return callNode.call(function.getThisValue(), function, false, arg1, arg2, arg3);
+    return callNode.call(function.getThisValue(), function, arg1, arg2, arg3);
   }
 
   @Specialization(replaces = "evalDirect")
@@ -48,6 +48,6 @@ public abstract class ApplyVmFunction3Node extends PklNode {
       @Cached("create()") IndirectCallNode callNode) {
 
     return callNode.call(
-        function.getCallTarget(), function.getThisValue(), function, false, arg1, arg2, arg3);
+        function.getCallTarget(), function.getThisValue(), function, arg1, arg2, arg3);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/lambda/ApplyVmFunction4Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -38,7 +38,7 @@ public abstract class ApplyVmFunction4Node extends PklNode {
           RootCallTarget cachedCallTarget,
       @Cached("create(cachedCallTarget)") DirectCallNode callNode) {
 
-    return callNode.call(function.getThisValue(), function, false, arg1, arg2, arg3, arg4);
+    return callNode.call(function.getThisValue(), function, arg1, arg2, arg3, arg4);
   }
 
   @Specialization(replaces = "evalDirect")
@@ -51,6 +51,6 @@ public abstract class ApplyVmFunction4Node extends PklNode {
       @Cached("create()") IndirectCallNode callNode) {
 
     return callNode.call(
-        function.getCallTarget(), function.getThisValue(), function, false, arg1, arg2, arg3, arg4);
+        function.getCallTarget(), function.getThisValue(), function, arg1, arg2, arg3, arg4);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/lambda/ApplyVmFunction5Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -39,7 +39,7 @@ public abstract class ApplyVmFunction5Node extends PklNode {
           RootCallTarget cachedCallTarget,
       @Cached("create(cachedCallTarget)") DirectCallNode callNode) {
 
-    return callNode.call(function.getThisValue(), function, false, arg1, arg2, arg3, arg4, arg5);
+    return callNode.call(function.getThisValue(), function, arg1, arg2, arg3, arg4, arg5);
   }
 
   @Specialization(replaces = "evalDirect")
@@ -53,14 +53,6 @@ public abstract class ApplyVmFunction5Node extends PklNode {
       @Cached("create()") IndirectCallNode callNode) {
 
     return callNode.call(
-        function.getCallTarget(),
+        function.getCallTarget(), function.getThisValue(), function, arg1, arg2, arg3, arg4, arg5);
-        function.getThisValue(),
-        function,
-        false,
-        arg1,
-        arg2,
-        arg3,
-        arg4,
-        arg5);
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/member/FunctionNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -43,11 +43,7 @@ public final class FunctionNode extends RegularMemberNode {
   // For VmObject receivers, the owner is the same as or an ancestor of the receiver.
   // For other receivers, the owner is the prototype of the receiver's class.
   // The chain of enclosing owners forms a function/property's lexical scope.
-  //
+  private static final int IMPLICIT_PARAM_COUNT = 2;
-  // For function calls only, a third implicit argument is passed; whether the call came from within
-  // an iterable node or not.
-  // This is a mitigation for an existing bug (https://github.com/apple/pkl/issues/741).
-  private static final int IMPLICIT_PARAM_COUNT = 3;
 
   private final int paramCount;
   private final int totalParamCount;
@@ -112,15 +108,9 @@ public final class FunctionNode extends RegularMemberNode {
       throw wrongArgumentCount(totalArgCount - IMPLICIT_PARAM_COUNT);
     }
 
-    var isInIterable = (boolean) frame.getArguments()[2];
-
     for (var i = 0; i < parameterTypeNodes.length; i++) {
       var argument = frame.getArguments()[IMPLICIT_PARAM_COUNT + i];
-      if (isInIterable) {
+      parameterTypeNodes[i].executeAndSet(frame, argument);
-        parameterTypeNodes[i].executeEagerlyAndSet(frame, argument);
-      } else {
-        parameterTypeNodes[i].executeAndSet(frame, argument);
-      }
     }
 
     var result = bodyNode.executeGeneric(frame);

pkl-core/src/main/java/org/pkl/core/ast/member/Member.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -130,20 +130,4 @@ public abstract class Member {
   public final boolean isLocalOrExternalOrAbstract() {
     return VmModifier.isLocalOrExternalOrAbstract(modifiers);
   }
-
-  /**
-   * Tells if this member is declared inside the iterable of a for-generator, or an object spread.
-   *
-   * <p>This is {@code true} for {@code new {}} within:
-   *
-   * <pre>{@code
-   * for (x in new Listing { new {} }) {
-   *                         ^^^^^^
-   *   // etc
-   * }
-   * }</pre>
-   */
-  public boolean isInIterable() {
-    return VmModifier.isInIterable(modifiers);
-  }
 }

pkl-core/src/main/java/org/pkl/core/ast/member/PropertyTypeNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -59,24 +59,8 @@ public final class PropertyTypeNode extends PklRootNode {
     return qualifiedPropertyName;
   }
 
-  private boolean isInIterable(VirtualFrame frame) {
-    var args = frame.getArguments();
-    return args.length >= 4 && args[3] instanceof Boolean b && b;
-  }
-
   @Override
   protected Object executeImpl(VirtualFrame frame) {
-    if (isInIterable(frame)) {
-      // There is currently a bug around resolving variables within the iterable of a for
-      // generator or spread syntax (https://github.com/apple/pkl/issues/741)
-      //
-      // Normally, mappings/listings are type-checked lazily. However, this results in said
-      // bug getting widened, for any object members declared in the iterable.
-      //
-      // As a workaround for now, prevent the bug from being any worse by ensuring that these
-      // object members are eagerly typechecked.
-      return typeNode.executeEagerly(frame, frame.getArguments()[2]);
-    }
     return typeNode.execute(frame, frame.getArguments()[2]);
   }
 

pkl-core/src/main/java/org/pkl/core/ast/member/TypeCheckedPropertyNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -55,8 +55,7 @@ public abstract class TypeCheckedPropertyNode extends RegularMemberNode {
 
     // TODO: propagate SUPER_CALL_MARKER to disable constraint (but not type) check
     if (callNode != null && VmUtils.shouldRunTypeCheck(frame)) {
-      return callNode.call(
+      return callNode.call(VmUtils.getReceiverOrNull(frame), property.getOwner(), result);
-          VmUtils.getReceiverOrNull(frame), property.getOwner(), result, member.isInIterable());
     }
 
     return result;
@@ -76,8 +75,7 @@ public abstract class TypeCheckedPropertyNode extends RegularMemberNode {
             typeAnnNode.getCallTarget(),
             VmUtils.getReceiverOrNull(frame),
             property.getOwner(),
-            result,
+            result);
-            member.isInIterable());
       }
     }
 

pkl-core/src/main/java/org/pkl/core/ast/member/TypedPropertyNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -47,10 +47,7 @@ public final class TypedPropertyNode extends RegularMemberNode {
     var propertyValue = bodyNode.executeGeneric(frame);
     if (VmUtils.shouldRunTypeCheck(frame)) {
       return typeCheckCallNode.call(
-          VmUtils.getReceiver(frame),
+          VmUtils.getReceiver(frame), VmUtils.getOwner(frame), propertyValue);
-          VmUtils.getOwner(frame),
-          propertyValue,
-          member.isInIterable());
     }
     return propertyValue;
   }

pkl-core/src/main/java/org/pkl/core/ast/type/IdentityMixinNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -54,14 +54,14 @@ public final class IdentityMixinNode extends PklRootNode {
   @Override
   protected Object executeImpl(VirtualFrame frame) {
     var arguments = frame.getArguments();
-    if (arguments.length != 4) {
+    if (arguments.length != 3) {
       CompilerDirectives.transferToInterpreter();
       throw exceptionBuilder()
-          .evalError("wrongFunctionArgumentCount", 1, arguments.length - 3)
+          .evalError("wrongFunctionArgumentCount", 1, arguments.length - 2)
           .withSourceSection(sourceSection)
           .build();
     }
-    var argument = arguments[3];
+    var argument = arguments[2];
     if (argumentTypeNode != null) {
       return argumentTypeNode.execute(frame, argument);
     }

pkl-core/src/main/java/org/pkl/core/ast/type/TypeConstraintNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -25,7 +25,6 @@ import com.oracle.truffle.api.frame.VirtualFrame;
 import com.oracle.truffle.api.source.SourceSection;
 import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.PklNode;
-import org.pkl.core.ast.builder.SymbolTable.CustomThisScope;
 import org.pkl.core.ast.lambda.ApplyVmFunction1Node;
 import org.pkl.core.runtime.BaseModule;
 import org.pkl.core.runtime.VmFunction;
@@ -87,7 +86,7 @@ public abstract class TypeConstraintNode extends PklNode {
     if (customThisSlot == -1) {
       CompilerDirectives.transferToInterpreterAndInvalidate();
       // deferred until execution time s.t. nodes of inlined type aliases get the right frame slot
-      customThisSlot = VmUtils.findAuxiliarySlot(frame, CustomThisScope.FRAME_SLOT_ID);
+      customThisSlot = VmUtils.findCustomThisSlot(frame);
     }
   }
 }

pkl-core/src/main/java/org/pkl/core/ast/type/TypeNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -95,14 +95,6 @@ public abstract class TypeNode extends PklNode {
     return execute(frame, value);
   }
 
-  /**
-   * Checks if {@code value} conforms to this type.
-   *
-   * <p>If {@code value} is conforming, sets {@code slot} to {@code value}. Otherwise, throws a
-   * {@link VmTypeMismatchException}.
-   */
-  public abstract Object executeEagerlyAndSet(VirtualFrame frame, Object value);
-
   // method arguments are used when default value contains a root node
   public @Nullable Object createDefaultValue(
       VmLanguage language,
@@ -226,11 +218,6 @@ public abstract class TypeNode extends PklNode {
       frame.setLong(slot, (long) value);
       return value;
     }
-
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      return executeAndSet(frame, value);
-    }
   }
 
   public abstract static class ObjectSlotTypeNode extends FrameSlotTypeNode {
@@ -248,13 +235,6 @@ public abstract class TypeNode extends PklNode {
       frame.setObject(slot, result);
       return result;
     }
-
-    @Override
-    public final Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      var result = executeEagerly(frame, value);
-      frame.setObject(slot, result);
-      return result;
-    }
   }
 
   /**
@@ -288,13 +268,6 @@ public abstract class TypeNode extends PklNode {
       writeSlotNode.executeWithValue(frame, result);
       return result;
     }
-
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      var result = executeEagerly(frame, value);
-      writeSlotNode.executeWithValue(frame, result);
-      return result;
-    }
   }
 
   /** The `unknown` type. */
@@ -360,11 +333,6 @@ public abstract class TypeNode extends PklNode {
       throw PklBugException.unreachableCode();
     }
 
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      return executeAndSet(frame, value);
-    }
-
     @Override
     public FrameSlotKind getFrameSlotKind() {
       return FrameSlotKind.Illegal;
@@ -2435,22 +2403,6 @@ public abstract class TypeNode extends PklNode {
       }
     }
 
-    /** See docstring on {@link TypeAliasTypeNode#execute}. */
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      var prevOwner = VmUtils.getOwner(frame);
-      var prevReceiver = VmUtils.getReceiver(frame);
-      setOwner(frame, VmUtils.getOwner(typeAlias.getEnclosingFrame()));
-      setReceiver(frame, VmUtils.getReceiver(typeAlias.getEnclosingFrame()));
-
-      try {
-        return aliasedTypeNode.executeEagerlyAndSet(frame, value);
-      } finally {
-        setOwner(frame, prevOwner);
-        setReceiver(frame, prevReceiver);
-      }
-    }
-
     @Override
     @TruffleBoundary
     public @Nullable Object createDefaultValue(
@@ -2586,13 +2538,6 @@ public abstract class TypeNode extends PklNode {
       return ret;
     }
 
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      var ret = executeEagerly(frame, value);
-      childNode.executeEagerlyAndSet(frame, ret);
-      return ret;
-    }
-
     @Override
     public @Nullable Object createDefaultValue(
         VmLanguage language, SourceSection headerSection, String qualifiedName) {
@@ -2740,11 +2685,6 @@ public abstract class TypeNode extends PklNode {
       }
     }
 
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      return executeAndSet(frame, value);
-    }
-
     @Override
     public VmClass getVmClass() {
       return BaseModule.getNumberClass();
@@ -2813,11 +2753,6 @@ public abstract class TypeNode extends PklNode {
       return value;
     }
 
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      return executeAndSet(frame, value);
-    }
-
     @Override
     public VmClass getVmClass() {
       return BaseModule.getFloatClass();
@@ -2858,11 +2793,6 @@ public abstract class TypeNode extends PklNode {
       return value;
     }
 
-    @Override
-    public Object executeEagerlyAndSet(VirtualFrame frame, Object value) {
-      return executeAndSet(frame, value);
-    }
-
     @Override
     public VmClass getVmClass() {
       return BaseModule.getBooleanClass();

pkl-core/src/main/java/org/pkl/core/runtime/Identifier.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -141,9 +141,6 @@ public final class Identifier implements Comparable<Identifier> {
   // common in lambdas etc
   public static final Identifier IT = get("it");
 
-  // dummy, unrepresentable identifier
-  public static final Identifier DUMMY = get("`#_");
-
   private final String name;
 
   private Identifier(String name) {

pkl-core/src/main/java/org/pkl/core/runtime/VmFunction.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -55,7 +55,7 @@ public final class VmFunction extends VmObjectLike {
   // if call site is a node, use ApplyVmFunction1Node.execute() or DirectCallNode.call() instead of
   // this method
   public Object apply(Object arg1) {
-    return getCallTarget().call(thisValue, this, false, arg1);
+    return getCallTarget().call(thisValue, this, arg1);
   }
 
   public String applyString(Object arg1) {
@@ -69,7 +69,7 @@ public final class VmFunction extends VmObjectLike {
   // if call site is a node, use ApplyVmFunction2Node.execute() or DirectCallNode.call() instead of
   // this method
   public Object apply(Object arg1, Object arg2) {
-    return getCallTarget().call(thisValue, this, false, arg1, arg2);
+    return getCallTarget().call(thisValue, this, arg1, arg2);
   }
 
   public VmFunction copy(

pkl-core/src/main/java/org/pkl/core/runtime/VmObjectLike.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -23,8 +23,8 @@ import org.pkl.core.ast.member.ObjectMember;
 import org.pkl.core.util.Nullable;
 
 /**
- * Corresponds to `pkl.base#Object|pkl.base#Function`. The lexical scope is a hierarchy of
+ * Corresponds to `pkl.base#Object|pkl.base#Function`. The lexical scope is a chain of
- * `VmOwner`s.
+ * `VmObjectLike` instances.
  */
 public abstract class VmObjectLike extends VmValue {
   /** The frame that was active when this object was instantiated. * */
@@ -66,8 +66,8 @@ public abstract class VmObjectLike extends VmValue {
   }
 
   /**
-   * Returns the parent object in the prototype chain. For each concrete subclass X of VmOwner, the
+   * Returns the parent object in the prototype chain. For each concrete subclass X of VmObjectLike,
-   * exact return type of this method is `X|VmTyped`.
+   * the exact return type of this method is `X|VmTyped`.
    */
   public abstract @Nullable VmObjectLike getParent();
 

pkl-core/src/main/java/org/pkl/core/runtime/VmUtils.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -47,6 +47,7 @@ import org.pkl.core.ast.ExpressionNode;
 import org.pkl.core.ast.SimpleRootNode;
 import org.pkl.core.ast.VmModifier;
 import org.pkl.core.ast.builder.AstBuilder;
+import org.pkl.core.ast.builder.SymbolTable.CustomThisScope;
 import org.pkl.core.ast.expression.primary.CustomThisNode;
 import org.pkl.core.ast.expression.primary.ThisNode;
 import org.pkl.core.ast.member.*;
@@ -300,6 +301,59 @@ public final class VmUtils {
     return result;
   }
 
+  /**
+   * Copies {@code numberOfLocalsToCopy} locals from {@code sourceFrame}, starting at {@code
+   * firstSourceSlot}, to {@code firstSourceSlot}, starting at {@code firstTargetSlot}.
+   */
+  public static void copyLocals(
+      VirtualFrame sourceFrame,
+      int firstSourceSlot,
+      VirtualFrame targetFrame,
+      int firstTargetSlot,
+      int numberOfLocalsToCopy) {
+    var sourceDescriptor = sourceFrame.getFrameDescriptor();
+    var targetDescriptor = targetFrame.getFrameDescriptor();
+    // Alternatively, locals could be copied with `numberOfLocalsToCopy`
+    // `ReadFrameSlotNode/WriteFrameSlotNode`'s.
+    for (int i = 0; i < numberOfLocalsToCopy; i++) {
+      var sourceSlot = firstSourceSlot + i;
+      var targetSlot = firstTargetSlot + i;
+      // If, for a particular call site of this method,
+      // slot kinds of `sourceDescriptor` will reach a steady state,
+      // then slot kinds of `targetDescriptor` will too.
+      var slotKind = sourceDescriptor.getSlotKind(sourceSlot);
+      switch (slotKind) {
+        case Boolean -> {
+          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Boolean);
+          targetFrame.setBoolean(targetSlot, sourceFrame.getBoolean(sourceSlot));
+        }
+        case Long -> {
+          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Long);
+          targetFrame.setLong(targetSlot, sourceFrame.getLong(sourceSlot));
+        }
+        case Double -> {
+          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Double);
+          targetFrame.setDouble(targetSlot, sourceFrame.getDouble(sourceSlot));
+        }
+        case Object -> {
+          targetDescriptor.setSlotKind(targetSlot, FrameSlotKind.Object);
+          targetFrame.setObject(
+              targetSlot,
+              sourceFrame instanceof MaterializedFrame
+                  // Even though sourceDescriptor.getSlotKind is now Object,
+                  // it may have been a primitive kind when `sourceFrame`'s local was written.
+                  // Hence we need to read the local with getValue() instead of getObject().
+                  ? sourceFrame.getValue(sourceSlot)
+                  : sourceFrame.getObject(sourceSlot));
+        }
+        default -> {
+          CompilerDirectives.transferToInterpreter();
+          throw new VmExceptionBuilder().bug("Unexpected FrameSlotKind: " + slotKind).build();
+        }
+      }
+    }
+  }
+
   // A failed property type check looks as follows in the stack trace:
   // x: Int(isPositive)
   // at ...
@@ -687,9 +741,7 @@ public final class VmUtils {
   }
 
   public static TypeNode[] resolveParameterTypes(
-      VirtualFrame frame,
+      VirtualFrame frame, FrameDescriptor descriptor, UnresolvedTypeNode[] parameterTypeNodes) {
-      FrameDescriptor descriptor,
-      @Nullable UnresolvedTypeNode[] parameterTypeNodes) {
 
     var resolvedNodes = new TypeNode[parameterTypeNodes.length];
 
@@ -836,25 +888,11 @@ public final class VmUtils {
     return callNode.call(rootNode.getCallTarget(), module, module);
   }
 
-  public static int findSlot(VirtualFrame frame, Object identifier) {
+  public static int findCustomThisSlot(VirtualFrame frame) {
-    var descriptor = frame.getFrameDescriptor();
+    return frame
-    for (int i = 0; i < descriptor.getNumberOfSlots(); i++) {
+        .getFrameDescriptor()
-      if (descriptor.getSlotName(i) == identifier
+        .getAuxiliarySlots()
-          && frame.getTag(i) != FrameSlotKind.Illegal.tag
+        .getOrDefault(CustomThisScope.FRAME_SLOT_ID, -1);
-          // Truffle initializes all frame tags as `FrameSlotKind.Object`, instead of
-          // `FrameSlotKind.Illegal`. Unevaluated (in a `when` with `false` condition) `for`
-          // generators, therefore, leave their bound variables tagged as `Object`, but `null`. If
-          // another `for` generator in the same scope binds variables with the same names, they
-          // resolve the wrong slot number.
-          && (frame.getTag(i) != FrameSlotKind.Object.tag || frame.getObject(i) != null)) {
-        return i;
-      }
-    }
-    return -1;
-  }
-
-  public static int findAuxiliarySlot(VirtualFrame frame, Object identifier) {
-    return frame.getFrameDescriptor().getAuxiliarySlots().getOrDefault(identifier, -1);
   }
 
   @TruffleBoundary

pkl-core/src/main/java/org/pkl/core/stdlib/base/FunctionNodes.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -31,12 +31,11 @@ public final class FunctionNodes {
     protected Object eval(VmFunction self, VmList argList) {
       var argCount = argList.getLength();
 
-      var args = new Object[3 + argCount];
+      var args = new Object[2 + argCount];
       args[0] = self.getThisValue();
       args[1] = self;
-      args[2] = false;
 
-      var i = 3;
+      var i = 2;
       for (var arg : argList) {
         args[i++] = arg;
       }

pkl-core/src/test/files/LanguageSnippetTests/input/errors/forGeneratorDuplicateParams2.pkl

@@ -1,11 +0,0 @@
-a = List(1, 2, 3, 4)
-
-b = List("a", "b", "c", "d")
-
-foo {
-  for (_dup, i in a) {
-    for (_dup, j in b) {
-      i + j
-    }
-  }
-}

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorInFunctionBody.pkl

@@ -0,0 +1,18 @@
+function myMethod(arg) = new {
+  arg = "property" // same name as method arg
+  for (key, value in List("one", "two", arg)) { // `arg` resolves to method arg
+    [Pair(arg, key)] = // `arg` resolves to method arg
+      Pair(arg, value) // `arg` resolves to object property
+  }
+}
+
+local myLambda = (arg) -> new Dynamic {
+  arg = "property" // same name as lambda arg
+  for (key, value in List("one", "two", arg)) { // `arg` resolves to lambda arg
+    [Pair(arg, key)] = // `arg` resolves to lambda arg
+      Pair(arg, value) // `arg` resolves to object property
+  }
+}
+
+res1 = myMethod("three")
+res2 = myLambda.apply("three")

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorNestedReference2.pkl

@@ -0,0 +1,25 @@
+// https://github.com/apple/pkl/issues/741
+
+bar = new {}
+
+res1 {
+  for (i in List(1)) {
+    ...(bar) {
+      baz {
+        new { i }
+      }
+    }.baz
+  }
+}
+
+res2 {
+  for (i in List(1)) {
+    for (elem in (bar) {
+      baz {
+        new { i }
+      }
+    }.baz) {
+      elem
+    }
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/input/generators/forGeneratorVariableShadowing.pkl

@@ -0,0 +1,33 @@
+examples {
+  local a = List("1", "2", "3", "4")
+  local b = List("a", "b", "c", "d")
+  
+  ["shadow key variable"] {
+    new {
+      for (key, outerValue in a) {
+        for (key, innerValue in b) {
+          List(outerValue, key, innerValue)
+        }
+      }
+    }
+  }
+  ["shadow value variable"] {
+    new {
+      for (outerKey, value in a) {
+        for (innerKey, value in b) {
+          List(outerKey, value, innerKey)
+        }
+      }
+    }
+  }
+  ["sibling for-generators can use same variable names"] {
+    new {
+      for (key, value in a) {
+        List(key,  value)
+      }
+      for (key, value in b) {
+        List(key, value)
+      }
+    }
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/errors/forGeneratorDuplicateParams2.err

@@ -1,6 +0,0 @@
-–– Pkl Error ––
-Duplicate definition of member `_dup`.
-
-x | for (_dup, j in b) {
-         ^^^^
-at forGeneratorDuplicateParams2#foo (file:///$snippetsDir/input/errors/forGeneratorDuplicateParams2.pkl)

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorInFunctionBody.pcf

@@ -0,0 +1,12 @@
+res1 {
+  arg = "property"
+  [Pair("three", 0)] = Pair("property", "one")
+  [Pair("three", 1)] = Pair("property", "two")
+  [Pair("three", 2)] = Pair("property", "three")
+}
+res2 {
+  arg = "property"
+  [Pair("three", 0)] = Pair("property", "one")
+  [Pair("three", 1)] = Pair("property", "two")
+  [Pair("three", 2)] = Pair("property", "three")
+}

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorNestedReference2.pcf

@@ -0,0 +1,11 @@
+bar {}
+res1 {
+  new {
+    1
+  }
+}
+res2 {
+  new {
+    1
+  }
+}

pkl-core/src/test/files/LanguageSnippetTests/output/generators/forGeneratorVariableShadowing.pcf

@@ -0,0 +1,54 @@
+examples {
+  ["shadow key variable"] {
+    new {
+      List("1", 0, "a")
+      List("1", 1, "b")
+      List("1", 2, "c")
+      List("1", 3, "d")
+      List("2", 0, "a")
+      List("2", 1, "b")
+      List("2", 2, "c")
+      List("2", 3, "d")
+      List("3", 0, "a")
+      List("3", 1, "b")
+      List("3", 2, "c")
+      List("3", 3, "d")
+      List("4", 0, "a")
+      List("4", 1, "b")
+      List("4", 2, "c")
+      List("4", 3, "d")
+    }
+  }
+  ["shadow value variable"] {
+    new {
+      List(0, "a", 0)
+      List(0, "b", 1)
+      List(0, "c", 2)
+      List(0, "d", 3)
+      List(1, "a", 0)
+      List(1, "b", 1)
+      List(1, "c", 2)
+      List(1, "d", 3)
+      List(2, "a", 0)
+      List(2, "b", 1)
+      List(2, "c", 2)
+      List(2, "d", 3)
+      List(3, "a", 0)
+      List(3, "b", 1)
+      List(3, "c", 2)
+      List(3, "d", 3)
+    }
+  }
+  ["sibling for-generators can use same variable names"] {
+    new {
+      List(0, "1")
+      List(1, "2")
+      List(2, "3")
+      List(3, "4")
+      List(0, "a")
+      List(1, "b")
+      List(2, "c")
+      List(3, "d")
+    }
+  }
+}