apple/pkl · error · VmException

listingAmendmentCannotHaveParameters

listingAmendmentCannotHaveParameters

Error message

Expected a function that returns a `Listing`, but got a `Listing`.

What it means

A Listing amendment must be a plain object literal — it cannot introduce function parameters, because the amended value is a `Listing` produced by an expression, not a parameterized object constructor. `checkIsValidListingAmendment` throws this when `parametersDescriptor` is non-null, i.e. the amendment literal declares a parameter list.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/expression/literal/SpecializedObjectLiteralNode.java:158

        long index = (long) key;
        if (index < 0) {
          // defer handling of negative index to checkMaxListingMemberIndex() (gives more uniform
          // error messages)
          maxIndex = Long.MAX_VALUE;
          break;
        } else if (index > maxIndex) {
          maxIndex = index;
        }
      } else if (memberName != Identifier.DEFAULT) {
        throw exceptionBuilder()
            .evalError("objectCannotHaveProperty", BaseModule.getListingClass())
            .withSourceSection(member.getHeaderSection())
            .build();
      }
    }

    if (parametersDescriptor != null) {
      throw exceptionBuilder()
          .evalError("listingAmendmentCannotHaveParameters")
          .withSourceSection(getParentNode().getSourceSection())
          .build();
    }

    maxListingMemberIndex = maxIndex;
    return true;
  }

  @SuppressWarnings("SameReturnValue")
  @TruffleBoundary
  @Idempotent
  protected final boolean checkIsValidMappingAmendment() {
    if (checkedIsValidMappingAmendment) return true;

    for (var member : EconomicMaps.getValues(members)) {
      if (member.isLocal()) continue;

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Remove the parameter list from the Listing amendment and inline the values
2. If parameterization is required, amend a class whose property returns the Listing, parameterizing at that level instead
3. Compute the elements in a separate expression/function and assign the result rather than parameterizing the amendment

Example fix

// before
items { (x) { [0] = x } }
// after
items { [0] = 42 }

// Listing amendments take no parameter list: write items { [0] = v } not items { (x) { ... } }

When it happens

Trigger: Writing `items { (x) { ... } }`-style or `amends (a) -> ...` parameterized amendment targeting a Listing; applying a function-shaped object literal to a Listing parent.

Common situations: Copy-pasting a parameterized `amends` pattern from a class-typed object onto a Listing; misunderstanding that only class-amending literals support parameters.

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

• listingAmendmentCannotHaveParameters
• wrongFunctionAmendmentParameterCount
• mappingAmendmentCannotHaveParameters
• elementIndexOutOfRange
• defaultMember/cannotFindMember

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/b178bf70fb2523ac. Report an issue: GitHub.