pkl-parser: Represent "no children" as an empty list instead of null (#1513)

Motivation:
Facilitate the use of the NullAway checker as part of moving to
JSpecify.

Changes:
- represent "no children" as `List.of()` instead of null
- remove obsolete `children != null` assertions
  - NullAway intentionally ignores such assertions
- remove "no children" special-casing where no longer necessary

Result:
- cleaner code with similar performance
- removed a barrier to using the NullAway checker

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

@@ -219,9 +219,6 @@ public class PowerAssertions {
       return parserNode;
     }
     var children = parserNode.children();
-    if (children == null) {
-      return null;
-    }
     for (var child : children) {
       var found = findParserNode(node, child, offset);
       if (found != null) {

pkl-parser/src/main/java/org/pkl/parser/BaseParserVisitor.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -465,7 +465,6 @@ public abstract class BaseParserVisitor<T> implements ParserVisitor<T> {
   private T visitChildren(Node node) {
     T result = defaultValue();
     var children = node.children();
-    if (children == null) return result;
     for (var child : children) {
       if (child != null) {
         result = aggregateResult(result, child.accept(this));

pkl-parser/src/main/java/org/pkl/parser/syntax/AbstractNode.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -23,22 +23,20 @@ import org.pkl.parser.util.Nullable;
 
 public abstract class AbstractNode implements Node {
   protected final Span span;
-  protected final @Nullable List<? extends @Nullable Node> children;
+  protected final List<? extends @Nullable Node> children;
   protected @Nullable Node parent;
 
-  public AbstractNode(Span span, @Nullable List<? extends @Nullable Node> children) {
+  public AbstractNode(Span span, List<? extends @Nullable Node> children) {
     this.span = span;
-    if (children != null) {
-      this.children = Collections.unmodifiableList(children);
+    if (children.isEmpty()) {
+      // optimization: always store an unwrapped List.of()
+      this.children = List.of();
     } else {
-      this.children = null;
+      this.children = Collections.unmodifiableList(children);
     }
-
-    if (children != null) {
-      for (var node : children) {
-        if (node != null) {
-          node.setParent(this);
-        }
+    for (var node : children) {
+      if (node != null) {
+        node.setParent(this);
       }
     }
   }
@@ -59,7 +57,7 @@ public abstract class AbstractNode implements Node {
   }
 
   @Override
-  public @Nullable List<? extends @Nullable Node> children() {
+  public List<? extends @Nullable Node> children() {
     return children;
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/Annotation.java

@@ -31,14 +31,12 @@ public final class Annotation extends AbstractNode {
   }
 
   public Type getType() {
-    assert children != null;
     var ret = (Type) children.get(0);
     assert ret != null;
     return ret;
   }
 
   public @Nullable ObjectBody getBody() {
-    assert children != null;
     return (ObjectBody) children.get(1);
   }
 }

pkl-parser/src/main/java/org/pkl/parser/syntax/ArgumentList.java

@@ -33,7 +33,6 @@ public final class ArgumentList extends AbstractNode {
 
   @SuppressWarnings("unchecked")
   public List<Expr> getArguments() {
-    assert children != null;
     return (List<Expr>) children;
   }
 }

pkl-parser/src/main/java/org/pkl/parser/syntax/Class.java

@@ -36,55 +36,46 @@ public final class Class extends AbstractNode {
   }
 
   public @Nullable DocComment getDocComment() {
-    assert children != null;
     return (DocComment) children.get(0);
   }
 
   @SuppressWarnings("unchecked")
   public List<Annotation> getAnnotations() {
-    assert children != null;
     return (List<Annotation>) children.subList(1, modifiersOffset);
   }
 
   @SuppressWarnings("unchecked")
   public List<Modifier> getModifiers() {
-    assert children != null;
     return (List<Modifier>) children.subList(modifiersOffset, keywordOffset);
   }
 
   public Keyword getClassKeyword() {
-    assert children != null;
     var ret = (Keyword) children.get(keywordOffset);
     assert ret != null;
     return ret;
   }
 
   public Identifier getName() {
-    assert children != null;
     var ret = (Identifier) children.get(keywordOffset + 1);
     assert ret != null;
     return ret;
   }
 
   public @Nullable TypeParameterList getTypeParameterList() {
-    assert children != null;
     return (TypeParameterList) children.get(keywordOffset + 2);
   }
 
   public @Nullable Type getSuperClass() {
-    assert children != null;
     return (Type) children.get(keywordOffset + 3);
   }
 
   public @Nullable ClassBody getBody() {
-    assert children != null;
     return (ClassBody) children.get(keywordOffset + 4);
   }
 
   @SuppressWarnings("DuplicatedCode")
   public Span getHeaderSpan() {
     Span start = null;
-    assert children != null;
     for (var i = modifiersOffset; i < children.size(); i++) {
       var child = children.get(i);
       if (child != null) {

pkl-parser/src/main/java/org/pkl/parser/syntax/ClassBody.java

@@ -34,7 +34,6 @@ public final class ClassBody extends AbstractNode {
 
   public List<ClassProperty> getProperties() {
     var props = new ArrayList<ClassProperty>();
-    assert children != null;
     for (var child : children) {
       if (child instanceof ClassProperty prop) {
         props.add(prop);
@@ -45,7 +44,6 @@ public final class ClassBody extends AbstractNode {
 
   public List<ClassMethod> getMethods() {
     var methods = new ArrayList<ClassMethod>();
-    assert children != null;
     for (var child : children) {
       if (child instanceof ClassMethod method) {
         methods.add(method);

pkl-parser/src/main/java/org/pkl/parser/syntax/ClassMethod.java

@@ -39,48 +39,40 @@ public final class ClassMethod extends AbstractNode {
   }
 
   public @Nullable DocComment getDocComment() {
-    assert children != null;
     return (DocComment) children.get(0);
   }
 
   @SuppressWarnings("unchecked")
   public List<Annotation> getAnnotations() {
-    assert children != null;
     return (List<Annotation>) children.subList(1, modifiersOffset);
   }
 
   @SuppressWarnings("unchecked")
   public List<Modifier> getModifiers() {
-    assert children != null;
     return (List<Modifier>) children.subList(modifiersOffset, nameOffset);
   }
 
   public Identifier getName() {
-    assert children != null;
     var ret = (Identifier) children.get(nameOffset);
     assert ret != null;
     return ret;
   }
 
   public @Nullable TypeParameterList getTypeParameterList() {
-    assert children != null;
     return (TypeParameterList) children.get(nameOffset + 1);
   }
 
   public ParameterList getParameterList() {
-    assert children != null;
     var ret = (ParameterList) children.get(nameOffset + 2);
     assert ret != null;
     return ret;
   }
 
   public @Nullable TypeAnnotation getTypeAnnotation() {
-    assert children != null;
     return (TypeAnnotation) children.get(nameOffset + 3);
   }
 
   public @Nullable Expr getExpr() {
-    assert children != null;
     return (Expr) children.get(nameOffset + 4);
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/ClassProperty.java

@@ -36,42 +36,35 @@ public final class ClassProperty extends AbstractNode {
   }
 
   public @Nullable DocComment getDocComment() {
-    assert children != null;
     return (DocComment) children.get(0);
   }
 
   @SuppressWarnings("unchecked")
   public List<Annotation> getAnnotations() {
-    assert children != null;
     return (List<Annotation>) children.subList(1, modifiersOffset);
   }
 
   @SuppressWarnings("unchecked")
   public List<Modifier> getModifiers() {
-    assert children != null;
     return (List<Modifier>) children.subList(modifiersOffset, nameOffset);
   }
 
   public Identifier getName() {
-    assert children != null;
     var ret = (Identifier) children.get(nameOffset);
     assert ret != null;
     return ret;
   }
 
   public @Nullable TypeAnnotation getTypeAnnotation() {
-    assert children != null;
     return (TypeAnnotation) children.get(nameOffset + 1);
   }
 
   public @Nullable Expr getExpr() {
-    assert children != null;
     return (Expr) children.get(nameOffset + 2);
   }
 
   @SuppressWarnings("unchecked")
   public List<ObjectBody> getBodyList() {
-    assert children != null;
     return (List<ObjectBody>) children.subList(nameOffset + 3, children.size());
   }
 }

pkl-parser/src/main/java/org/pkl/parser/syntax/DocComment.java

@@ -23,7 +23,7 @@ public final class DocComment extends AbstractNode {
   private final List<Span> spans;
 
   public DocComment(List<Span> spans) {
-    super(spans.get(0).endWith(spans.get(spans.size() - 1)), null);
+    super(spans.get(0).endWith(spans.get(spans.size() - 1)), List.of());
     this.spans = spans;
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/Expr.java

@@ -25,13 +25,13 @@ import org.pkl.parser.util.Nullable;
 @SuppressWarnings("unused")
 public abstract sealed class Expr extends AbstractNode {
 
-  public Expr(Span span, @Nullable List<? extends @Nullable Node> children) {
+  public Expr(Span span, List<? extends @Nullable Node> children) {
     super(span, children);
   }
 
   public static final class ThisExpr extends Expr {
     public ThisExpr(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
 
     @Override
@@ -42,7 +42,7 @@ public abstract sealed class Expr extends AbstractNode {
 
   public static final class OuterExpr extends Expr {
     public OuterExpr(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
 
     @Override
@@ -53,7 +53,7 @@ public abstract sealed class Expr extends AbstractNode {
 
   public static final class ModuleExpr extends Expr {
     public ModuleExpr(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
 
     @Override
@@ -64,7 +64,7 @@ public abstract sealed class Expr extends AbstractNode {
 
   public static final class NullLiteralExpr extends Expr {
     public NullLiteralExpr(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
 
     @Override
@@ -77,7 +77,7 @@ public abstract sealed class Expr extends AbstractNode {
     private final boolean b;
 
     public BoolLiteralExpr(boolean b, Span span) {
-      super(span, null);
+      super(span, List.of());
       this.b = b;
     }
 
@@ -95,7 +95,7 @@ public abstract sealed class Expr extends AbstractNode {
     private final String number;
 
     public IntLiteralExpr(String number, Span span) {
-      super(span, null);
+      super(span, List.of());
       this.number = number;
     }
 
@@ -113,7 +113,7 @@ public abstract sealed class Expr extends AbstractNode {
     private final String number;
 
     public FloatLiteralExpr(String number, Span span) {
-      super(span, null);
+      super(span, List.of());
       this.number = number;
     }
 
@@ -137,7 +137,6 @@ public abstract sealed class Expr extends AbstractNode {
 
     public final boolean hasInterpolation() {
       var children = children();
-      assert children != null;
       return children.size() > 1;
     }
   }
@@ -160,7 +159,6 @@ public abstract sealed class Expr extends AbstractNode {
 
     @SuppressWarnings("unchecked")
     public List<StringPart> getParts() {
-      assert children != null;
       return (List<StringPart>) children;
     }
 
@@ -191,7 +189,6 @@ public abstract sealed class Expr extends AbstractNode {
 
     @SuppressWarnings("unchecked")
     public List<StringPart> getParts() {
-      assert children != null;
       return (List<StringPart>) children;
     }
 
@@ -215,7 +212,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -233,7 +229,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -254,7 +249,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public StringConstant getImportStr() {
-      assert children != null;
       var ret = (StringConstant) children.get(0);
       assert ret != null;
       return ret;
@@ -279,7 +273,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -308,14 +301,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Identifier getIdentifier() {
-      assert children != null;
       var ret = (Identifier) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public @Nullable ArgumentList getArgumentList() {
-      assert children != null;
       return (ArgumentList) children.get(1);
     }
   }
@@ -339,14 +330,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Identifier getIdentifier() {
-      assert children != null;
       var ret = (Identifier) children.get(1);
       assert ret != null;
       return ret;
@@ -357,7 +346,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public @Nullable ArgumentList getArgumentList() {
-      assert children != null;
       return (ArgumentList) children.get(2);
     }
   }
@@ -373,14 +361,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Identifier getIdentifier() {
-      assert children != null;
       var ret = (Identifier) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public @Nullable ArgumentList getArgumentList() {
-      assert children != null;
       return (ArgumentList) children.get(1);
     }
   }
@@ -396,7 +382,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getArg() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -414,14 +399,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Expr getArg() {
-      assert children != null;
       var ret = (Expr) children.get(1);
       assert ret != null;
       return ret;
@@ -439,21 +422,18 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getCond() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Expr getThen() {
-      assert children != null;
       var ret = (Expr) children.get(1);
       assert ret != null;
       return ret;
     }
 
     public Expr getEls() {
-      assert children != null;
       var ret = (Expr) children.get(2);
       assert ret != null;
       return ret;
@@ -471,21 +451,18 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Parameter getParameter() {
-      assert children != null;
       var ret = (Parameter) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Expr getBindingExpr() {
-      assert children != null;
       var ret = (Expr) children.get(1);
       assert ret != null;
       return ret;
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(2);
       assert ret != null;
       return ret;
@@ -503,14 +480,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public ParameterList getParameterList() {
-      assert children != null;
       var ret = (ParameterList) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(1);
       assert ret != null;
       return ret;
@@ -528,7 +503,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -546,12 +520,10 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public @Nullable Type getType() {
-      assert children != null;
       return (Type) children.get(0);
     }
 
     public ObjectBody getBody() {
-      assert children != null;
       var ret = (ObjectBody) children.get(1);
       assert ret != null;
       return ret;
@@ -573,14 +545,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public ObjectBody getBody() {
-      assert children != null;
       var ret = (ObjectBody) children.get(1);
       assert ret != null;
       return ret;
@@ -598,7 +568,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -616,7 +585,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -634,7 +602,6 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -655,14 +622,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getLeft() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Expr getRight() {
-      assert children != null;
       var ret = (Expr) children.get(1);
       assert ret != null;
       return ret;
@@ -709,14 +674,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Type getType() {
-      assert children != null;
       var ret = (Type) children.get(1);
       assert ret != null;
       return ret;
@@ -734,14 +697,12 @@ public abstract sealed class Expr extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public Type getType() {
-      assert children != null;
       var ret = (Type) children.get(1);
       assert ret != null;
       return ret;

pkl-parser/src/main/java/org/pkl/parser/syntax/ExtendsOrAmendsClause.java

@@ -35,7 +35,6 @@ public final class ExtendsOrAmendsClause extends AbstractNode {
   }
 
   public StringConstant getUrl() {
-    assert children != null;
     var ret = (StringConstant) children.get(0);
     assert ret != null;
     return ret;

pkl-parser/src/main/java/org/pkl/parser/syntax/Identifier.java

@@ -15,6 +15,7 @@
  */
 package org.pkl.parser.syntax;
 
+import java.util.List;
 import java.util.Objects;
 import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
@@ -23,7 +24,7 @@ public final class Identifier extends AbstractNode {
   private final String value;
 
   public Identifier(String value, Span span) {
-    super(span, null);
+    super(span, List.of());
     this.value = value;
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/ImportClause.java

@@ -36,7 +36,6 @@ public final class ImportClause extends AbstractNode {
   }
 
   public StringConstant getImportStr() {
-    assert children != null;
     var ret = (StringConstant) children.get(0);
     assert ret != null;
     return ret;
@@ -47,7 +46,6 @@ public final class ImportClause extends AbstractNode {
   }
 
   public @Nullable Identifier getAlias() {
-    assert children != null;
     return (Identifier) children.get(1);
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/Keyword.java

@@ -15,13 +15,14 @@
  */
 package org.pkl.parser.syntax;
 
+import java.util.List;
 import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
 
 public final class Keyword extends AbstractNode {
 
   public Keyword(Span span) {
-    super(span, null);
+    super(span, List.of());
   }
 
   @Override

pkl-parser/src/main/java/org/pkl/parser/syntax/Modifier.java

@@ -15,6 +15,7 @@
  */
 package org.pkl.parser.syntax;
 
+import java.util.List;
 import java.util.Objects;
 import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
@@ -23,7 +24,7 @@ public final class Modifier extends AbstractNode {
   private final ModifierValue value;
 
   public Modifier(ModifierValue value, Span span) {
-    super(span, null);
+    super(span, List.of());
     this.value = value;
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/Module.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -32,13 +32,11 @@ public final class Module extends AbstractNode {
   }
 
   public @Nullable ModuleDecl getDecl() {
-    assert children != null;
     // children can be empty when there was a parse error
     return children.isEmpty() ? null : (ModuleDecl) children.get(0);
   }
 
   public List<ImportClause> getImports() {
-    assert children != null;
     if (children.size() < 2) return List.of();
     var res = new ArrayList<ImportClause>();
     for (int i = 1; i < children.size(); i++) {
@@ -55,7 +53,6 @@ public final class Module extends AbstractNode {
 
   public List<org.pkl.parser.syntax.Class> getClasses() {
     var res = new ArrayList<org.pkl.parser.syntax.Class>();
-    assert children != null;
     for (var child : children) {
       if (child instanceof Class clazz) {
         res.add(clazz);
@@ -66,7 +63,6 @@ public final class Module extends AbstractNode {
 
   public List<TypeAlias> getTypeAliases() {
     var res = new ArrayList<TypeAlias>();
-    assert children != null;
     for (var child : children) {
       if (child instanceof TypeAlias typeAlias) {
         res.add(typeAlias);
@@ -77,7 +73,6 @@ public final class Module extends AbstractNode {
 
   public List<ClassProperty> getProperties() {
     var res = new ArrayList<ClassProperty>();
-    assert children != null;
     for (var child : children) {
       if (child instanceof ClassProperty classProperty) {
         res.add(classProperty);
@@ -88,7 +83,6 @@ public final class Module extends AbstractNode {
 
   public List<ClassMethod> getMethods() {
     var res = new ArrayList<ClassMethod>();
-    assert children != null;
     for (var child : children) {
       if (child instanceof ClassMethod classMethod) {
         res.add(classMethod);

pkl-parser/src/main/java/org/pkl/parser/syntax/ModuleDecl.java

@@ -36,41 +36,34 @@ public final class ModuleDecl extends AbstractNode {
   }
 
   public @Nullable DocComment getDocComment() {
-    assert children != null;
     return (DocComment) children.get(0);
   }
 
   @SuppressWarnings("unchecked")
   public List<Annotation> getAnnotations() {
-    assert children != null;
     return (List<Annotation>) children.subList(1, modifiersOffset);
   }
 
   @SuppressWarnings("unchecked")
   public List<Modifier> getModifiers() {
-    assert children != null;
     return (List<Modifier>) children.subList(modifiersOffset, nameOffset);
   }
 
   public @Nullable Keyword getModuleKeyword() {
-    assert children != null;
     return (Keyword) children.get(nameOffset);
   }
 
   public @Nullable QualifiedIdentifier getName() {
-    assert children != null;
     return (QualifiedIdentifier) children.get(nameOffset + 1);
   }
 
   public @Nullable ExtendsOrAmendsClause getExtendsOrAmendsDecl() {
-    assert children != null;
     return (ExtendsOrAmendsClause) children.get(nameOffset + 2);
   }
 
   @SuppressWarnings("DuplicatedCode")
   public Span headerSpan() {
     Span start = null;
-    assert children != null;
     for (var i = modifiersOffset; i < children.size(); i++) {
       var child = children.get(i);
       if (child != null) {

pkl-parser/src/main/java/org/pkl/parser/syntax/Node.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -28,7 +28,6 @@ public interface Node {
 
   void setParent(Node parent);
 
-  @Nullable
   List<? extends @Nullable Node> children();
 
   <T> T accept(ParserVisitor<? extends T> visitor);

pkl-parser/src/main/java/org/pkl/parser/syntax/ObjectBody.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2025-2026 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.
@@ -34,13 +34,11 @@ public final class ObjectBody extends AbstractNode {
 
   @SuppressWarnings("unchecked")
   public List<Parameter> getParameters() {
-    assert children != null;
     return (List<Parameter>) children.subList(0, membersOffset);
   }
 
   @SuppressWarnings("unchecked")
   public List<ObjectMember> getMembers() {
-    assert children != null;
     return (List<ObjectMember>) children.subList(membersOffset, children.size());
   }
 }

pkl-parser/src/main/java/org/pkl/parser/syntax/ObjectMember.java

@@ -25,7 +25,7 @@ import org.pkl.parser.util.Nullable;
 @SuppressWarnings("unused")
 public abstract sealed class ObjectMember extends AbstractNode {
 
-  public ObjectMember(Span span, @Nullable List<? extends @Nullable Node> children) {
+  public ObjectMember(Span span, List<? extends @Nullable Node> children) {
     super(span, children);
   }
 
@@ -40,7 +40,6 @@ public abstract sealed class ObjectMember extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -62,30 +61,25 @@ public abstract sealed class ObjectMember extends AbstractNode {
 
     @SuppressWarnings("unchecked")
     public List<Modifier> getModifiers() {
-      assert children != null;
       return (List<Modifier>) children.subList(0, identifierOffset);
     }
 
     public Identifier getIdentifier() {
-      assert children != null;
       var ret = (Identifier) children.get(identifierOffset);
       assert ret != null;
       return ret;
     }
 
     public @Nullable TypeAnnotation getTypeAnnotation() {
-      assert children != null;
       return (TypeAnnotation) children.get(identifierOffset + 1);
     }
 
     public @Nullable Expr getExpr() {
-      assert children != null;
       return (Expr) children.get(identifierOffset + 2);
     }
 
     @SuppressWarnings("unchecked")
     public List<ObjectBody> getBodyList() {
-      assert children != null;
       return (List<ObjectBody>) children.subList(identifierOffset + 3, children.size());
     }
   }
@@ -105,43 +99,36 @@ public abstract sealed class ObjectMember extends AbstractNode {
 
     @SuppressWarnings("unchecked")
     public List<Modifier> getModifiers() {
-      assert children != null;
       return (List<Modifier>) children.subList(0, identifierOffset);
     }
 
     public Keyword getFunctionKeyword() {
-      assert children != null;
       var ret = (Keyword) children.get(identifierOffset);
       assert ret != null;
       return ret;
     }
 
     public Identifier getIdentifier() {
-      assert children != null;
       var ret = (Identifier) children.get(identifierOffset + 1);
       assert ret != null;
       return ret;
     }
 
     public @Nullable TypeParameterList getTypeParameterList() {
-      assert children != null;
       return (TypeParameterList) children.get(identifierOffset + 2);
     }
 
     public ParameterList getParamList() {
-      assert children != null;
       var ret = (ParameterList) children.get(identifierOffset + 3);
       assert ret != null;
       return ret;
     }
 
     public @Nullable TypeAnnotation getTypeAnnotation() {
-      assert children != null;
       return (TypeAnnotation) children.get(identifierOffset + 4);
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(identifierOffset + 5);
       assert ret != null;
       return ret;
@@ -150,7 +137,6 @@ public abstract sealed class ObjectMember extends AbstractNode {
     @SuppressWarnings("DuplicatedCode")
     public Span headerSpan() {
       Span start = null;
-      assert children != null;
       for (var child : children) {
         if (child != null) {
           start = child.span();
@@ -182,20 +168,17 @@ public abstract sealed class ObjectMember extends AbstractNode {
     }
 
     public Expr getPred() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public @Nullable Expr getExpr() {
-      assert children != null;
       return (Expr) children.get(1);
     }
 
     @SuppressWarnings("unchecked")
     public List<ObjectBody> getBodyList() {
-      assert children != null;
       return (List<ObjectBody>) children.subList(2, children.size());
     }
   }
@@ -211,20 +194,17 @@ public abstract sealed class ObjectMember extends AbstractNode {
     }
 
     public Expr getKey() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public @Nullable Expr getValue() {
-      assert children != null;
       return (Expr) children.get(1);
     }
 
     @SuppressWarnings("unchecked")
     public List<ObjectBody> getBodyList() {
-      assert children != null;
       return (List<ObjectBody>) children.subList(2, children.size());
     }
   }
@@ -243,7 +223,6 @@ public abstract sealed class ObjectMember extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
@@ -299,21 +278,18 @@ public abstract sealed class ObjectMember extends AbstractNode {
     }
 
     public Expr getPredicate() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public ObjectBody getThenClause() {
-      assert children != null;
       var ret = (ObjectBody) children.get(1);
       assert ret != null;
       return ret;
     }
 
     public @Nullable ObjectBody getElseClause() {
-      assert children != null;
       return (ObjectBody) children.get(2);
     }
   }
@@ -330,26 +306,22 @@ public abstract sealed class ObjectMember extends AbstractNode {
     }
 
     public Parameter getP1() {
-      assert children != null;
       var ret = (Parameter) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public @Nullable Parameter getP2() {
-      assert children != null;
       return (Parameter) children.get(1);
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(2);
       assert ret != null;
       return ret;
     }
 
     public ObjectBody getBody() {
-      assert children != null;
       var ret = (ObjectBody) children.get(3);
       assert ret != null;
       return ret;

pkl-parser/src/main/java/org/pkl/parser/syntax/Parameter.java

@@ -23,7 +23,7 @@ import org.pkl.parser.util.Nullable;
 
 public abstract sealed class Parameter extends AbstractNode {
 
-  public Parameter(Span span, @Nullable List<? extends @Nullable Node> children) {
+  public Parameter(Span span, List<? extends @Nullable Node> children) {
     super(span, children);
   }
 
@@ -34,7 +34,7 @@ public abstract sealed class Parameter extends AbstractNode {
 
   public static final class Underscore extends Parameter {
     public Underscore(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
   }
 
@@ -45,14 +45,12 @@ public abstract sealed class Parameter extends AbstractNode {
     }
 
     public Identifier getIdentifier() {
-      assert children != null;
       var ret = (Identifier) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public @Nullable TypeAnnotation getTypeAnnotation() {
-      assert children != null;
       return (TypeAnnotation) children.get(1);
     }
   }

pkl-parser/src/main/java/org/pkl/parser/syntax/ParameterList.java

@@ -32,7 +32,6 @@ public final class ParameterList extends AbstractNode {
 
   @SuppressWarnings("unchecked")
   public List<Parameter> getParameters() {
-    assert children != null;
     return (List<Parameter>) children;
   }
 }

pkl-parser/src/main/java/org/pkl/parser/syntax/QualifiedIdentifier.java

@@ -1,5 +1,5 @@
 /*
- * Copyright © 2024-2025 Apple Inc. and the Pkl project authors. All rights reserved.
+ * Copyright © 2024-2026 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.
@@ -32,7 +32,6 @@ public final class QualifiedIdentifier extends AbstractNode {
 
   @SuppressWarnings("unchecked")
   public List<Identifier> getIdentifiers() {
-    assert children != null;
     return (List<Identifier>) children;
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/ReplInput.java

@@ -32,7 +32,6 @@ public final class ReplInput extends AbstractNode {
 
   @SuppressWarnings("unchecked")
   public List<Node> getNodes() {
-    assert children != null;
     return (List<Node>) children;
   }
 }

pkl-parser/src/main/java/org/pkl/parser/syntax/StringConstant.java

@@ -15,6 +15,7 @@
  */
 package org.pkl.parser.syntax;
 
+import java.util.List;
 import java.util.Objects;
 import org.pkl.parser.ParserVisitor;
 import org.pkl.parser.Span;
@@ -24,7 +25,7 @@ public final class StringConstant extends AbstractNode {
   private final String string;
 
   public StringConstant(String string, Span span) {
-    super(span, null);
+    super(span, List.of());
     this.string = string;
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/StringPart.java

@@ -23,7 +23,7 @@ import org.pkl.parser.util.Nullable;
 
 public abstract sealed class StringPart extends AbstractNode {
 
-  public StringPart(Span span, @Nullable List<? extends @Nullable Node> children) {
+  public StringPart(Span span, List<? extends @Nullable Node> children) {
     super(span, children);
   }
 
@@ -36,7 +36,7 @@ public abstract sealed class StringPart extends AbstractNode {
     private final String string;
 
     public StringChars(String string, Span span) {
-      super(span, null);
+      super(span, List.of());
       this.string = string;
     }
 
@@ -77,7 +77,6 @@ public abstract sealed class StringPart extends AbstractNode {
     }
 
     public Expr getExpr() {
-      assert children != null;
       var ret = (Expr) children.get(0);
       assert ret != null;
       return ret;

pkl-parser/src/main/java/org/pkl/parser/syntax/Type.java

@@ -24,13 +24,13 @@ import org.pkl.parser.util.Nullable;
 
 public abstract sealed class Type extends AbstractNode {
 
-  public Type(Span span, @Nullable List<? extends @Nullable Node> children) {
+  public Type(Span span, List<? extends @Nullable Node> children) {
     super(span, children);
   }
 
   public static final class UnknownType extends Type {
     public UnknownType(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
 
     @Override
@@ -41,7 +41,7 @@ public abstract sealed class Type extends AbstractNode {
 
   public static final class NothingType extends Type {
     public NothingType(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
 
     @Override
@@ -52,7 +52,7 @@ public abstract sealed class Type extends AbstractNode {
 
   public static final class ModuleType extends Type {
     public ModuleType(Span span) {
-      super(span, null);
+      super(span, List.of());
     }
 
     @Override
@@ -72,7 +72,6 @@ public abstract sealed class Type extends AbstractNode {
     }
 
     public StringConstant getStr() {
-      assert children != null;
       var ret = (StringConstant) children.get(0);
       assert ret != null;
       return ret;
@@ -90,14 +89,12 @@ public abstract sealed class Type extends AbstractNode {
     }
 
     public QualifiedIdentifier getName() {
-      assert children != null;
       var ret = (QualifiedIdentifier) children.get(0);
       assert ret != null;
       return ret;
     }
 
     public @Nullable TypeArgumentList getArgs() {
-      assert children != null;
       return (TypeArgumentList) children.get(1);
     }
   }
@@ -113,7 +110,6 @@ public abstract sealed class Type extends AbstractNode {
     }
 
     public Type getType() {
-      assert children != null;
       var ret = (Type) children.get(0);
       assert ret != null;
       return ret;
@@ -131,7 +127,6 @@ public abstract sealed class Type extends AbstractNode {
     }
 
     public Type getType() {
-      assert children != null;
       var ret = (Type) children.get(0);
       assert ret != null;
       return ret;
@@ -149,7 +144,6 @@ public abstract sealed class Type extends AbstractNode {
     }
 
     public Type getType() {
-      assert children != null;
       var ret = (Type) children.get(0);
       assert ret != null;
       return ret;
@@ -157,7 +151,6 @@ public abstract sealed class Type extends AbstractNode {
 
     @SuppressWarnings("unchecked")
     public List<Expr> getExprs() {
-      assert children != null;
       return (List<Expr>) children.subList(1, children.size());
     }
   }
@@ -177,7 +170,6 @@ public abstract sealed class Type extends AbstractNode {
 
     @SuppressWarnings("unchecked")
     public List<Type> getTypes() {
-      assert children != null;
       return (List<Type>) children;
     }
 
@@ -231,12 +223,10 @@ public abstract sealed class Type extends AbstractNode {
 
     @SuppressWarnings("unchecked")
     public List<Type> getArgs() {
-      assert children != null;
       return (List<Type>) children.subList(0, children.size() - 1);
     }
 
     public Type getRet() {
-      assert children != null;
       var ret = (Type) children.get(children.size() - 1);
       assert ret != null;
       return ret;

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeAlias.java

@@ -37,43 +37,36 @@ public final class TypeAlias extends AbstractNode {
   }
 
   public @Nullable DocComment getDocComment() {
-    assert children != null;
     return (DocComment) children.get(0);
   }
 
   @SuppressWarnings("unchecked")
   public List<Annotation> getAnnotations() {
-    assert children != null;
     return (List<Annotation>) children.subList(1, modifiersOffset);
   }
 
   @SuppressWarnings("unchecked")
   public List<Modifier> getModifiers() {
-    assert children != null;
     return (List<Modifier>) children.subList(modifiersOffset, nameOffset);
   }
 
   public Keyword getTypealiasKeyword() {
-    assert children != null;
     var ret = (Keyword) children.get(nameOffset);
     assert ret != null;
     return ret;
   }
 
   public Identifier getName() {
-    assert children != null;
     var ret = (Identifier) children.get(nameOffset + 1);
     assert ret != null;
     return ret;
   }
 
   public @Nullable TypeParameterList getTypeParameterList() {
-    assert children != null;
     return (TypeParameterList) children.get(nameOffset + 2);
   }
 
   public Type getType() {
-    assert children != null;
     var ret = (Type) children.get(nameOffset + 3);
     assert ret != null;
     return ret;
@@ -82,7 +75,6 @@ public final class TypeAlias extends AbstractNode {
   @SuppressWarnings("DuplicatedCode")
   public Span getHeaderSpan() {
     Span start = null;
-    assert children != null;
     for (var i = modifiersOffset; i < children.size(); i++) {
       var child = children.get(i);
       if (child != null) {

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeAnnotation.java

@@ -31,7 +31,6 @@ public final class TypeAnnotation extends AbstractNode {
   }
 
   public Type getType() {
-    assert children != null;
     var ret = (Type) children.get(0);
     assert ret != null;
     return ret;

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeArgumentList.java

@@ -27,7 +27,6 @@ public final class TypeArgumentList extends AbstractNode {
 
   @SuppressWarnings("unchecked")
   public List<Type> getTypes() {
-    assert children != null;
     return (List<Type>) children;
   }
 

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeParameter.java

@@ -39,7 +39,6 @@ public final class TypeParameter extends AbstractNode {
   }
 
   public Identifier getIdentifier() {
-    assert children != null;
     var ret = (Identifier) children.get(0);
     assert ret != null;
     return ret;

pkl-parser/src/main/java/org/pkl/parser/syntax/TypeParameterList.java

@@ -32,7 +32,6 @@ public final class TypeParameterList extends AbstractNode {
 
   @SuppressWarnings("unchecked")
   public List<TypeParameter> getParameters() {
-    assert children != null;
     return (List<TypeParameter>) children;
   }
 }