What causes "wrongFunctionArgumentCount" in apple/pkl?

Call the function with exactly one argument

wrongFunctionArgumentCount

wrongFunctionArgumentCount

Error message

wrongFunctionArgumentCount

What it means

`wrongFunctionArgumentCount` is thrown by IdentityMixinNode when the generated mixin identity function is invoked with the wrong number of arguments. The node expects a receiver plus exactly 1 argument (frame arguments length 3); any other count is a call-site arity error. The message states the expected count (1) versus the actual count.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/ast/type/IdentityMixinNode.java:59

    this.argumentTypeNode = argumentTypeNode;
  }

  @Override
  public SourceSection getSourceSection() {
    return sourceSection;
  }

  @Override
  public String getName() {
    return qualifiedName;
  }

  @Override
  protected Object executeImpl(VirtualFrame frame) {
    var arguments = frame.getArguments();
    if (arguments.length != 3) {
      CompilerDirectives.transferToInterpreter();
      throw exceptionBuilder()
          .evalError("wrongFunctionArgumentCount", 1, arguments.length - 2, "")
          .withSourceSection(sourceSection)
          .build();
    }
    var argument = arguments[2];
    if (argumentTypeNode != null) {
      return argumentTypeNode.execute(frame, argument);
    }
    return argument;
  }
}

View on GitHub (pinned to f3efcbfc9b)

Solutions

Call the function with exactly one argument
Remove extra arguments or supply the missing one at the call site
Check the function's declared parameter list and update all call sites

Example fix

// before
mixinIdentity()
// after
mixinIdentity(value)

Defensive patterns

Strategy: validation

Validate before calling

// check arity before invoking
if (typeof fn === "function" && fn.length !== 1) throw new Error("expected exactly 1 argument");

Prevention

Match call-site arguments to the declared parameter count
Re-grep call sites after changing a function signature

When it happens

Trigger: Calling a Pkl function that compiles to this mixin identity function with 0 or more than 1 argument, e.g. `foo()` or `foo(a, b)` instead of `foo(a)`.

Common situations: Calling a property-like function with parentheses but wrong arity; copy-paste call sites after a signature change; misreading a zero-argument property as a function.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/7b4786ae6cb7ca99. Report an issue: GitHub.