apple/pkl · error · VmException

objectCannotHaveProperty

objectCannotHaveProperty

Error message

Object of type `{0}` cannot have a property (other than `default`).

What it means

Only `default` (and local) properties may be set on objects that aren't Dyanamic — e.g. typed class instances, List/Set/Map values, or functions. The generic fallback of GeneratorPropertyNode rejects any other property assignment with the object's type.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/expression/generator/GeneratorPropertyNode.java:90

  @SuppressWarnings("unused")
  @Specialization(guards = {"parent == getListingClass()", "checkIsValidListingProperty()"})
  protected void evalListingClass(VirtualFrame frame, VmClass parent, ObjectData data) {
    data.addProperty(frame, member, this);
  }

  @SuppressWarnings("unused")
  @Specialization(
      guards = {"isTypedObjectClass(parent)", "checkIsValidTypedProperty(parent, member)"})
  protected void evalTypedObjectClass(VirtualFrame frame, VmClass parent, ObjectData data) {
    data.addProperty(frame, member, this);
  }

  @Fallback
  @SuppressWarnings("unused")
  void fallback(Object parent, ObjectData data) {
    CompilerDirectives.transferToInterpreter();
    throw exceptionBuilder()
        .evalError(
            "objectCannotHaveProperty",
            parent instanceof VmClass ? parent : VmUtils.getClass(parent))
        .withSourceSection(member.getHeaderSection())
        .build();
  }

  @Idempotent
  protected boolean checkIsValidListingProperty() {
    if (member.isLocal() || member.getName() == Identifier.DEFAULT) return true;

    CompilerDirectives.transferToInterpreter();
    throw exceptionBuilder()
        .evalError("objectCannotHaveProperty", BaseModule.getListingClass())
        .withSourceSection(member.getHeaderSection())
        .build();
  }

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Remove the property or change it to `default` if overriding the object's default value is intended.
2. Mark the member `local` if it's an internal helper that must not appear on the object.
3. If the target should accept arbitrary properties, ensure it is a Dynamic (or the class declares the property).

Example fix

// before
myList { extra = 1 }
// after
local extra = 1
myList { ... }

// Only assign named properties to Dynamic/module objects
if (target is Dynamic || target is Module) {
  // named property assignment is safe
}

function acceptsProperties(v: Any): Boolean = v is Dynamic || v is Module

When it happens

Trigger: Amending a typed object or non-Dynamic value with a named property (`value { foo = 1 }`) where the parent type's fallback path is hit — parent is neither Dynamic, Listing, Mapping, nor Module. Raised in the public `fallback` node.

Common situations: Adding properties to a List or Map amendment; trying to override a property on a typed class instance that doesn't declare it; amending a base value like Int/String as if it were an object.

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

• classMustBeLocal
• typeAliasMustBeLocal
• nonLocalObjectPropertyCannotHaveTypeAnnotation
• externalMemberCannotHaveBody
• cannotAmendPropertyDefinition

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/16451ffa5f040b46. Report an issue: GitHub.