apple/pkl · error · VmException

methodMustBeConst

methodMustBeConst

Error message

Cannot call method `{0}` from here because it is not `const`.

What it means

Same const-ness rule as class methods but for object (definition) methods: when the call site requires a constant value, InvokeLexicalObjectMethodNode looks up the member on the owner and throws if its modifiers lack the `const` flag.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/expression/member/InvokeLexicalObjectMethodNode.java:43

/** A non-virtual call of an object method that is lexically scoped. */
public final class InvokeLexicalObjectMethodNode extends AbstractInvokeLexicalMethodNode {
  public InvokeLexicalObjectMethodNode(
      SourceSection sourceSection,
      Identifier methodName,
      int levelsUp,
      ExpressionNode[] argumentNodes,
      boolean needsConst,
      boolean argsRequireInference) {
    super(sourceSection, methodName, levelsUp, argumentNodes, needsConst, argsRequireInference);
  }

  @Override
  protected void doCheckConst(VmObjectLike owner) {
    var member = owner.getMember(methodName);
    assert member != null;
    if (!VmModifier.isConst(member.getModifiers())) {
      throw exceptionBuilder().evalError("methodMustBeConst", methodName).build();
    }
  }

  @Override
  protected Method getMethod(VmObjectLike owner) {
    var member = owner.getMember(methodName);
    assert member != null && member.isLocal();
    var method = (ObjectMethodNode) member.getMemberNode();
    assert method != null;
    return method;
  }
}

View on GitHub (pinned to f3efcbfc9b)

Solutions

Mark the object method `const` in its definition
Replace the call with a `const` property or literal
Reconsider whether the call site truly needs const and relax it

Example fix

// before
x {
  function helper() = 42
}
constY = x.helper() // const context
// after
x {
  const function helper() = 42
}

Defensive patterns

Strategy: validation

Validate before calling

// declare object methods const when they should be usable in const positions:
// const function helper() = 42

Prevention

Prefer `const` for all pure object methods
Keep const call sites limited to const methods and literals
Review library APIs for const modifiers before using them in constant properties

When it happens

Trigger: An object method invoked via lexical scope from a const-requiring position; doCheckConst finds the member and VmModifier.isConst(modifiers) is false.

Common situations: Calling a non-const object method while computing a const property or module-level constant; methods copied from non-const examples into const contexts.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/ae3533514ebf1ff2. Report an issue: GitHub.