apple/pkl · error

cannotApplyFixedModifier

cannotApplyFixedModifier

Error message

cannotApplyFixedModifier

What it means

Pkl throws cannotApplyFixedModifier when an overriding property adds the `fixed` modifier that the overridden superclass property does not have. UnresolvedPropertyNode.checkOverride detects the mismatch, locates the `fixed` token inside the property header, and raises the error against that exact span. Applying `fixed` during an override would retroactively restrict a property the superclass declared mutable, so it is forbidden.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/member/UnresolvedPropertyNode.java:112

    }
    var isFixed = VmModifier.isFixed(modifiers);
    if (superProperty.isFixed() == isFixed) {
      return;
    }
    CompilerDirectives.transferToInterpreter();
    if (superProperty.isFixed()) {
      throw exceptionBuilder()
          .withSourceSection(headerSection)
          .evalError(
              "missingFixedModifier",
              name,
              superClass.getQualifiedName(),
              sourceSection.getCharacters())
          .build();
    }
    var source = headerSection.getCharacters().toString();
    var fixedModifierIdx = source.indexOf("fixed");
    throw exceptionBuilder()
        .withSourceSection(
            headerSection
                .getSource()
                .createSection(headerSection.getCharIndex() + fixedModifierIdx, 5))
        .evalError("cannotApplyFixedModifier", name, superClass.getQualifiedName())
        .build();
  }

  private void checkConst(VmClass clazz) {
    var superClass = clazz.getSuperclass();
    if (superClass == null) {
      return;
    }
    var superProperty = superClass.getProperty(name);
    if (superProperty == null) {
      return;
    }
    var isConst = VmModifier.isConst(modifiers);

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Remove the `fixed` modifier from the overriding property.
2. If immutability is required, add `fixed` in the superclass declaration instead, then mirror it in overrides.
3. Keep override modifiers identical to the superclass property's modifiers.
4. Update the code generator to copy modifiers from the base rather than always emitting `fixed`.

Example fix

// before
class Server extends Base {  // Base.name is not fixed
  fixed name: String = "mine"
}

// after
class Server extends Base {
  name: String = "mine"
}

// Before adding `fixed` to an override, verify the superclass property is not non-fixed:
// Base declares: `name: String` (no fixed) -> override must not add `fixed`.

When it happens

Trigger: Writing `fixed` on an override whose superclass property is not fixed; tightening a property to fixed in a subclass to prevent later amendment; adding fixed modifiers in bulk during a refactor without checking the base class.

Common situations: Attempting to lock down inherited properties for safety and discovering Pkl forbids narrowing modifiers on override; generated code that stamps `fixed` on all properties; migrating a standalone class into a subclass hierarchy.

Understand the failure class

Background: Conflicting config options: "cannot be used together" — configuration validation errors across open-source libraries — this error's family across 162 libraries.

Related errors

• missingFixedModifier
• missingConstModifier
• cannotExtendExternalClass
• noImplementationForAbstractMethod
• noImplementationForAbstractMethods

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/3ab662d9785dba4a. Report an issue: GitHub.