apple/pkl · error · VmException

cannotAmendFixedProperty

cannotAmendFixedProperty

Error message

Cannot amend fixed property `{0}`.

What it means

Pkl properties declared `fixed` (or `const`) cannot be amended or assigned in an object that amends their declaring class. `checkIsValidTypedAmendment` detects a member overriding a fixed/const class property and picks `cannotAmendFixedProperty` when the member is an amends-style declaration (header == body section), or `cannotAssignFixedProperty` otherwise.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/expression/literal/SpecializedObjectLiteralNode.java:98

      if (!parentClass.hasProperty(memberName)) {
        throw exceptionBuilder()
            .cannotFindProperty(parentClass.getPrototype(), memberName, false, false)
            .withSourceSection(member.getHeaderSection())
            .build();
      }
      var classProperty = parentClass.getProperty(memberName);
      if (classProperty != null && classProperty.isConstOrFixed()) {
        // tailor error message based on whether an amends declaration is used or not
        // (i.e. `friends {}` vs. `friends = new {}`)
        // an amends declaration's body section includes the header section, whereas normal property
        // assignment's body section is the section after the equal sign.
        var isAmendsDeclaration =
            member.getHeaderSection().getCharIndex() == member.getBodySection().getCharIndex();
        var errMsg = isAmendsDeclaration ? "cannotAmendFixedProperty" : "cannotAssignFixedProperty";
        if (classProperty.isConst()) {
          errMsg = isAmendsDeclaration ? "cannotAmendConstProperty" : "cannotAssignConstProperty";
        }
        throw exceptionBuilder()
            .evalError(errMsg, memberName)
            .withSourceSection(member.getHeaderSection())
            .build();
      }
    }

    if (parametersDescriptor != null) {
      throw exceptionBuilder()
          .evalError("objectAmendmentCannotHaveParameters")
          .withLocation(getParentNode())
          .build();
    }

    return true;
  }

  @SuppressWarnings("SameReturnValue")
  @TruffleBoundary

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Remove the override of the fixed property — its value is intentionally immutable
2. If you control the base class, change `fixed` to a plain (defaultable) property so amendments can override it
3. If you control the base class and the value must never change at all, use `const` semantics consciously — the error will persist either way for `const`

Example fix

// base
class Server { fixed port: Int = 8080 }
// before (amendment)
server { amends Server; port = 9090 }
// after: either delete the override, or in base:
class Server { port: Int = 8080 }

// Check the base class for fixed/const on the property you plan to override:
// class Server { fixed port: Int = 8080 }  // do not override 'port'

When it happens

Trigger: Amending a class whose property is declared `fixed port: Int = 8080` with `port = 9090` in an `amends` literal; the error fires from `SpecializedObjectLiteralNode.checkIsValidTypedAmendment` during amendment validation.

Common situations: Trying to override library defaults that the library author intentionally locked down with `fixed`; copying override blocks between environments where one property is fixed in a newer package version.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• wrongFunctionAmendmentParameterCount
• cannotFindPropertyInObject
• listingAmendmentCannotHaveParameters
• mappingAmendmentCannotHaveParameters
• elementIndexOutOfRange

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/0caa6400f291315c. Report an issue: GitHub.