What causes "methodNotDefined4" in apple/pkl?

Verify all four argument classes from the error against the stdlib signature.

apple/pkl · error · VmException

methodNotDefined4

methodNotDefined4

Error message

methodNotDefined4

What it means

ExternalMethod4Node implements four-argument stdlib methods. When the runtime dispatch finds no type-specialized path for the receiver and the four argument types, the @Fallback throws "methodNotDefined4" naming the method and all four argument classes. No overload accepts that combination.

Source

Thrown at pkl-core/src/main/java/org/pkl/core/stdlib/ExternalMethod4Node.java:45

@NodeChild(value = "arg4Node", type = ExpressionNode.class)
public abstract class ExternalMethod4Node extends ExternalMethodNode {
  protected abstract ExpressionNode getArg1Node();

  protected abstract ExpressionNode getArg2Node();

  protected abstract ExpressionNode getArg3Node();

  protected abstract ExpressionNode getArg4Node();

  @Fallback
  @TruffleBoundary
  protected Object fallback(
      @SuppressWarnings("unused") Object receiver,
      Object arg1,
      Object arg2,
      Object arg3,
      Object arg4) {
    throw exceptionBuilder()
        .evalError(
            "methodNotDefined4",
            getQualifiedMemberName(),
            VmUtils.getClass(arg1),
            VmUtils.getClass(arg2),
            VmUtils.getClass(arg3),
            VmUtils.getClass(arg4))
        .withProgramValue("Argument 1", arg1)
        .withProgramValue("Argument 2", arg2)
        .withProgramValue("Argument 3", arg3)
        .withProgramValue("Argument 4", arg4)
        .build();
  }

  public interface Factory {
    ExternalMethod4Node create(
        ExpressionNode receiverNode,
        ExpressionNode arg1Node,

View on GitHub (pinned to f3efcbfc9b)

Solutions

Verify all four argument classes from the error against the stdlib signature.
Correct or convert each mismatched argument explicitly.
If a helper produces the arguments, trace its output types first.
Check Pkl stdlib changelog for signature changes across versions.

Example fix

// before
str.replaceAll(regex, replacement, /* regexFlags */ true)
// after
str.replaceAll(regex, replacement, 0) // Int flags, not Boolean

Defensive patterns

Strategy: type-guard

Validate before calling

// Pkl
assert(isInt(flags), "flags argument must be an Int")

Type guard

function isInt(x) = x is Int

Prevention

Read all four reported argument classes in the error before changing code
Document and test helper functions that build stdlib argument tuples
Re-check trailing optional arguments after refactors

When it happens

Trigger: Calling a four-argument stdlib method with any argument of an unsupported type — often the later positional arguments (flags/defaults) given wrong types, e.g. passing a Boolean where an Int is expected.

Common situations: Wrong arity/type from a helper function returning a differently-shaped tuple; misunderstanding which overload the trailing arguments select; nulls from optional properties.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

AI-assisted analysis of apple/pkl@f3efcbfc9b (2026-09-08). Data as JSON: /api/errors/5361366077a7065c. Report an issue: GitHub.