Add inference for let bindings (#1824)

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

@@ -109,6 +109,7 @@ import org.pkl.core.ast.expression.literal.MapLiteralNode;
 import org.pkl.core.ast.expression.literal.PropertiesLiteralNodeGen;
 import org.pkl.core.ast.expression.literal.SetLiteralNode;
 import org.pkl.core.ast.expression.literal.TrueLiteralNode;
+import org.pkl.core.ast.expression.member.InferParentWithinLetBindingNodeGen;
 import org.pkl.core.ast.expression.member.InferParentWithinMethodArgumentNodeGen;
 import org.pkl.core.ast.expression.member.InferParentWithinMethodNodeGen;
 import org.pkl.core.ast.expression.member.InferParentWithinObjectMethodNodeGen;
@@ -1073,6 +1074,7 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
   }
 
   private ExpressionNode doVisitNewExprWithInferredParent(NewExpr expr) {
+    var sourceSection = createSourceSection(expr.newSpan());
     ExpressionNode inferredParentNode;
 
     Node child = expr;
@@ -1083,8 +1085,8 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
     while (parent instanceof IfExpr ifExpr
             && (ifExpr.getThen() == child || ifExpr.getEls() == child)
         || parent instanceof TraceExpr
-        || parent instanceof LetExpr letExpr && letExpr.getExpr() == child) {
-
+        || parent instanceof LetExpr letExpr && letExpr.getExpr() == child
+        || parent instanceof ParenthesizedExpr) {
       child = parent;
       parent = parent.parent();
     }
@@ -1092,15 +1094,13 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
     if (parent instanceof ClassProperty || parent instanceof ObjectProperty) {
       inferredParentNode =
           InferParentWithinPropertyNodeGen.create(
-              createSourceSection(expr.newSpan()), scope.getName(), new GetOwnerNode());
+              sourceSection, scope.getName(), new GetOwnerNode());
     } else if (parent instanceof ObjectElement
         || parent instanceof ObjectEntry objectEntry && objectEntry.getValue() == child) {
       inferredParentNode =
           ApplyVmFunction1NodeGen.create(
               ReadPropertyNodeGen.create(
-                  createSourceSection(expr.newSpan()),
-                  org.pkl.core.runtime.Identifier.DEFAULT,
-                  new GetReceiverNode()),
+                  sourceSection, org.pkl.core.runtime.Identifier.DEFAULT, new GetReceiverNode()),
               new GetMemberKeyNode());
     } else if (parent instanceof ClassMethod || parent instanceof ObjectMethod) {
       var isObjectMethod =
@@ -1110,29 +1110,24 @@ public class AstBuilder extends AbstractAstBuilder<Object> {
       inferredParentNode =
           isObjectMethod
               ? InferParentWithinObjectMethodNodeGen.create(
-                  createSourceSection(expr.newSpan()), language, scopeName, new GetOwnerNode())
+                  sourceSection, language, scopeName, new GetOwnerNode())
               : InferParentWithinMethodNodeGen.create(
-                  createSourceSection(expr.newSpan()), language, scopeName, new GetOwnerNode());
+                  sourceSection, language, scopeName, new GetOwnerNode());
     } else if (parent instanceof LetExpr letExpr && letExpr.getBindingExpr() == child) {
-      // TODO correctly infer parent, e.g. `let (x: Person = new {}) ...`
-      throw exceptionBuilder()
-          .evalError("cannotInferParent")
-          .withSourceSection(createSourceSection(expr.newSpan()))
-          .build();
+      inferredParentNode = InferParentWithinLetBindingNodeGen.create(sourceSection, language);
     } else if (parent instanceof ArgumentList argumentList) {
       // cases we can't cover currently
       // - FunctionN.apply: the parameter type nodes are not stored
       // - pkl.base intrinsic constructors: List(), Set(), Map(), Bytes()
       // - generic methods: pkl.base#Pair(), etc.
       // these will throw cannotInferParent at runtime
-      var sourceSection = createSourceSection(expr.newSpan());
       var argIndex = argumentList.getArguments().indexOf(child);
       inferredParentNode =
           InferParentWithinMethodArgumentNodeGen.create(sourceSection, language, argIndex);
     } else {
       throw exceptionBuilder()
           .evalError("cannotInferParent")
-          .withSourceSection(createSourceSection(expr.newSpan()))
+          .withSourceSection(sourceSection)
           .build();
     }
 

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

@@ -49,22 +49,29 @@ public abstract class LetExprNode extends ExpressionNode {
     this.slot = slot;
   }
 
-  private TypeNode getTypeNode(VirtualFrame frame) {
-    if (typeNode == null) {
-      CompilerDirectives.transferToInterpreterAndInvalidate();
-      if (unresolvedTypeNode != null) {
-        typeNode = unresolvedTypeNode.execute(frame);
-      } else {
-        typeNode = new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection());
-      }
+  public TypeNode getTypeNode(VirtualFrame frame) {
+    if (typeNode != null) return typeNode;
+
+    CompilerDirectives.transferToInterpreterAndInvalidate();
+    if (unresolvedTypeNode != null && slot >= 0) {
+      typeNode = unresolvedTypeNode.execute(frame);
+    } else {
+      typeNode = new TypeNode.UnknownTypeNode(VmUtils.unavailableSourceSection());
+    }
+    if (slot >= 0) {
       typeNode.initWriteSlotNode(slot);
       frame.getFrameDescriptor().setSlotKind(slot, typeNode.getFrameSlotKind());
-      insert(typeNode);
     }
     assert typeNode != null;
-    return typeNode;
+    return insert(typeNode);
   }
 
+  public String getQualifiedName() {
+    return qualifiedName;
+  }
+
+  public abstract ExpressionNode getBindingNode();
+
   @Specialization
   protected Object eval(VirtualFrame frame, Object value) {
     if (slot != -1) {

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

@@ -0,0 +1,77 @@
+/*
+ * Copyright © 2026 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.member;
+
+import com.oracle.truffle.api.dsl.Cached;
+import com.oracle.truffle.api.dsl.Cached.Shared;
+import com.oracle.truffle.api.dsl.Specialization;
+import com.oracle.truffle.api.frame.VirtualFrame;
+import com.oracle.truffle.api.nodes.Node;
+import com.oracle.truffle.api.source.SourceSection;
+import org.pkl.core.ast.expression.binary.LetExprNode;
+import org.pkl.core.ast.type.TypeNode;
+import org.pkl.core.runtime.VmLanguage;
+
+public abstract class InferParentWithinLetBindingNode extends AbstractInferParentNode {
+  public InferParentWithinLetBindingNode(SourceSection sourceSection, VmLanguage language) {
+    super(sourceSection, language);
+  }
+
+  protected LetExprNode getLetNode() {
+    Node child = this;
+    LetExprNode letNode = null;
+    for (var node = getParent(); node != null; node = node.getParent()) {
+      if (node instanceof LetExprNode let && let.getBindingNode() == child) {
+        letNode = let;
+        break;
+      }
+      child = node;
+    }
+    assert letNode != null
+        : "AstBuilder created an InferParentWithinLetBindingNode outside of a let binding";
+    return letNode;
+  }
+
+  // keep specializations in sync with other AbstractInferParentNode subclasses
+
+  @Specialization(guards = {"typeNode.isFinalType()"})
+  protected final Object evalCached(
+      @SuppressWarnings("unused") VirtualFrame frame,
+      @Cached(value = "getLetNode()", neverDefault = true, adopt = false)
+          @Shared
+          @SuppressWarnings("unused")
+          LetExprNode letNode,
+      @Cached(value = "letNode.getTypeNode(frame)", neverDefault = true, adopt = false)
+          @SuppressWarnings("unused")
+          TypeNode typeNode,
+      @Cached(
+              value =
+                  "getDefaultValue(frame, typeNode, letNode.getSourceSection(), letNode.getQualifiedName())",
+              neverDefault = true)
+          Object defaultValue) {
+    return defaultValue;
+  }
+
+  @Specialization
+  protected final Object eval(
+      VirtualFrame frame,
+      @Cached(value = "getLetNode()", neverDefault = true, adopt = false) @Shared
+          LetExprNode letNode,
+      @Cached(value = "letNode.getTypeNode(frame)", neverDefault = true, adopt = false)
+          TypeNode typeNode) {
+    return getDefaultValue(frame, typeNode, letNode.getSourceSection(), letNode.getQualifiedName());
+  }
+}

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

@@ -20,6 +20,7 @@ import com.oracle.truffle.api.CompilerDirectives.CompilationFinal;
 import com.oracle.truffle.api.CompilerDirectives.TruffleBoundary;
 import com.oracle.truffle.api.dsl.Cached;
 import com.oracle.truffle.api.dsl.Fallback;
+import com.oracle.truffle.api.dsl.Idempotent;
 import com.oracle.truffle.api.dsl.Specialization;
 import com.oracle.truffle.api.frame.FrameDescriptor;
 import com.oracle.truffle.api.frame.FrameSlotKind;
@@ -140,6 +141,7 @@ public abstract class TypeNode extends PklNode {
     return null;
   }
 
+  @Idempotent
   public final boolean isFinalType() {
     var ret = new MutableBoolean(true);
     acceptTypeNode(

pkl-core/src/test/files/LanguageSnippetTests/input/basic/letTyped.pkl

@@ -1,5 +1,9 @@
+open module letTyped
+
 import "pkl:test"
 
+y: Int = 0
+
 res1 =
   let (x: Int = 42)
     x + 1
@@ -10,26 +14,81 @@ res2 =
 
 res3 =
   let (x: Duration(unit == "min") = 3.min + 2.min)
-    let (y: Float(isPositive) = 1.2 + 1.3)
-      x * y
+  let (y: Float(isPositive) = 1.2 + 1.3)
+    x * y
 
 res4 =
-  let (xs: List<Null|Boolean|"Pigeon"> = List("Pigeon", true, null))
+  let (xs: List<Null | Boolean | "Pigeon"> = List("Pigeon", true, null))
     xs
 
-res5 = test.catch(() ->
-  let (x: String = 42)
-    x + 1)
+res5 =
+  test.catch(() ->
+    let (x: String = 42)
+      x + 1
+  )
 
-res6 = test.catch(() ->
-  let (str: String(endsWith("A")) = "Pigeon".reverse())
-    str.reverse())
+res6 =
+  test.catch(() ->
+    let (str: String(endsWith("A")) = "Pigeon".reverse())
+      str.reverse()
+  )
 
-res7 = test.catch(() ->
-  let (x: Duration(unit == "min") = 3.min + 2.min)
+res7 =
+  test.catch(() ->
+    let (x: Duration(unit == "min") = 3.min + 2.min)
     let (y: Float(!isPositive) = 1.2 + 1.3)
-      x * y)
+      x * y
+  )
 
-res8 = test.catch(() ->
-  let (xs: List<Null|Boolean|"Pigeon"> = List("Barn Owl", true, null))
-    xs)
+res8 =
+  test.catch(() ->
+    let (xs: List<Null | Boolean | "Pigeon"> = List("Barn Owl", true, null))
+      xs
+  )
+
+// test basic `new {}` inference
+
+class Foo {
+  y: Int
+}
+
+res9 =
+  let (x: Foo = new { y = 1 })
+    x.y
+
+// test inference with let in binding expression
+
+res10 =
+  let (x: Foo = let (z = 0) new { y = z })
+    x.y
+res10a =
+  let (x: Foo = (new { y = 0 }))
+    x.y
+res10b =
+  let (x = new { y = 0 })
+    x.getClass().toString()
+
+// test inference against underscore binding
+
+res11 = let (_ = new { y = 1 }) 0 // binding is a new Dynamic
+
+// test inference of non-instantiable types
+
+abstract class Bar {
+  y: Int
+}
+
+res12a = test.catch(() -> let (x: Int = new { y = 1 }) x)
+res12b = test.catch(() -> let (x: Bar = new { y = 1 }) x)
+res12c = test.catch(() -> let (x: nothing = new { y = 1 }) x)
+
+// test inference of a self type
+
+function make(_y: Int): module =
+  let (z: module = new { y = _y })
+    z
+
+class Child extends module
+
+res13a = make(2).getClass().toString()
+res13b = new Child {}.make(3).getClass().toString()

pkl-core/src/test/files/LanguageSnippetTests/output/basic/letTyped.pcf

@@ -1,3 +1,4 @@
+y = 0
 res1 = 43
 res2 = "Pigeon"
 res3 = 12.5.min
@@ -5,4 +6,14 @@ res4 = List("Pigeon", true, null)
 res5 = "Expected value of type `String`, but got type `Int`. Value: 42"
 res6 = "Type constraint `endsWith(\"A\")` violated. Value: \"noegiP\""
 res7 = "Type constraint `!isPositive` violated. Value: 2.5"
-res8 = "Expected value of type `Null|Boolean|\"Pigeon\"`, but got type `String`. Value: \"Barn Owl\""
+res8 = "Expected value of type `Null | Boolean | \"Pigeon\"`, but got type `String`. Value: \"Barn Owl\""
+res9 = 1
+res10 = 0
+res10a = 0
+res10b = "Dynamic"
+res11 = 0
+res12a = "Cannot instantiate, or amend an instance of, external class `Int`."
+res12b = "Cannot instantiate abstract class `letTyped#Bar`."
+res12c = "Cannot instantiate type `nothing`."
+res13a = "letTyped"
+res13b = "letTyped#Child"