apple/pkl · error · VmReferenceAccessError

externalMember

Error message

externalMember

What it means

Raised when a referenced property exists but is declared `external` (abstract/external in Pkl semantics — implemented by the runtime, not defined in Pkl). External properties cannot be referenced through the Reference property-type machinery.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/runtime/VmReference.java:260

    if (clazz.getPClass().isSubclassOf(baseModule) && property.equals("output")) {
      throw new VmReferenceAccessError(
          new PType.Class(baseModule), VmReferenceAccessErrorType.EXTERNAL_CLASS);
    }

    // dot access on `Reference<D, Null>` gives `Reference<D, Null>`
    if (clazz.getPClass().getInfo() == PClassInfo.Null) {
      result.add(clazz);
      return;
    }

    var prop = clazz.getPClass().getAllProperties().get(property);
    if (prop == null) {
      throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.CANNOT_FIND_MEMBER);
    }

    // restriction: cannot reference external properties
    if (prop.isExternal()) {
      throw new VmReferenceAccessError(type, VmReferenceAccessErrorType.EXTERNAL_MEMBER);
    }

    normalizeTypes(
        prop.getType(), result, clazz, new PType.Class(clazz.getPClass().getModuleClass()));
  }

  private static PClassInfo<?> getClassInfo(Object value) {
    if (value instanceof VmValue vmValue) return vmValue.getVmClass().getPClassInfo();
    if (value instanceof String) return PClassInfo.String;
    if (value instanceof Boolean) return PClassInfo.Boolean;
    if (value instanceof Long) return PClassInfo.Int;
    if (value instanceof Double) return PClassInfo.Float;
    throw new PklBugException("Not a Pkl value: " + value.getClass());
  }

  @SuppressWarnings("DuplicatedCode")
  private static void getCandidateSubscriptType(PType type, Object key, Set<PType> result) {
    if (type == PType.UNKNOWN) {

View on GitHub (pinned to f3efcbfc9b)

Solutions

1. Do not reference the external property directly; use a concrete override or a non-external property.
2. Override the property with a concrete definition in your subclass, then reference your property.
3. Restructure so the needed value is computed into a normal property.
4. Consult the class docs to find the concrete equivalent property.

Example fix

// before
x = someObj.someExternalProp
// after (in subclass)
someExternalProp: Int = 42
x = this.someExternalProp

// Pkl: skip external properties when referencing
prop = obj.getClass().getAllProperties()[name]
if (prop != null && !prop.isExternal()) { /* safe to reference */ }

// Pkl
function isReferenceable(obj: Typed, name: String): Boolean {
  val p = obj.getClass().getAllProperties()[name]
  return p != null && !p.isExternal
}

try {
  eval(src);
} catch (EvalException e) {
  if (e.getMessage().contains("externalMember")) {
    // override the external property concretely or avoid referencing it
  }
}

When it happens

Trigger: Referencing a property marked external (e.g. certain base-module built-ins) via property-reference access.

Common situations: Referencing built-in external properties of base classes; subclassing library classes and referencing inherited external declarations.

Related errors

• cannotFindMember
• defaultMember
• defaultMember/cannotFindMember
• cannotFindMember
• cannotFindPropertyInObjectNoHint

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/31916c977372e5e4. Report an issue: GitHub.