apple/pkl · error · VmException

objectAmendmentCannotHaveParameters

objectAmendmentCannotHaveParameters

Error message

Expected a function that returns an object, but got an object.

What it means

When amending an object produced by a function call (e.g. in `new` or an object amendment position), Pkl expects either a plain object or a function returning an object. An amendment targeting a Dynamic object that declares parameters is rejected: parameterized object literals cannot be amended this way.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorObjectLiteralNode.java:237

  @TruffleBoundary
  protected void fallback(Object parent) {
    var value = parent;
    // blame the non-null type (e.g. blame `Duration` instead of `Null` in the case of `Duration?`)
    if (value instanceof VmNull vmNull) {
      value = getNullDefaultValue(vmNull);
    }
    VmUtils.checkIsInstantiable(
        value instanceof VmClass vmClass ? vmClass : VmUtils.getClass(value), getParentNode());

    throw exceptionBuilder().unreachableCode().build();
  }

  @Idempotent
  protected boolean checkObjectCannotHaveParameters() {
    if (parametersDescriptor == null) return true;

    CompilerDirectives.transferToInterpreter();
    throw exceptionBuilder()
        .evalError("objectAmendmentCannotHaveParameters")
        .withLocation(getParentNode())
        .build();
  }

  @Idempotent
  protected boolean checkListingCannotHaveParameters() {
    if (parametersDescriptor == null) return true;

    CompilerDirectives.transferToInterpreter();
    throw exceptionBuilder()
        .evalError("listingAmendmentCannotHaveParameters")
        .withLocation(getParentNode())
        .build();
  }

  @Idempotent
  protected boolean checkMappingCannotHaveParameters() {

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Remove the parameter list from the amendment; amend the object directly (`obj { ... }`).
2. If parameters are intended, call the function: `obj(params) { ... }` so the target is a function returning an object.
3. Verify the target expression's type — a Dynamic object cannot accept amendment parameters.

Example fix

// before
base { (x) ->
  y = x
}
// after
base {
  y = 42
}

When it happens

Trigger: Applying an amendment (`foo { ... }`) to an expression that is a Dynamic object (not a function returning one), where the amendment carries a parameters list (`(x) -> ...`), via `checkObjectCannotHaveParameters` in GeneratorObjectLiteralNode.

Common situations: Confusing `new Foo { ... }` / `Foo { ... }` amendment syntax with function-call syntax; adding `(param) ->` to an amendment of a plain object instead of a function; migrating code where the target changed from a function to an object.

Understand the failure class

Background: "must be a positive integer", "cannot be empty", "invalid argument": how invalid-argument errors work across open-source libraries — this error's family across 33 libraries.

Related errors

• cannotAnalyzeBecauseSyntaxError
• invalidTripleDotSyntax
• forWhenBodyCannotHaveParameters
• invalidTypeName
• unseparatedObjectMembers

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/a04718523e859f44. Report an issue: GitHub.