E4059

Cannot define method for builtin types or types that come from other packages.

MoonBit only allows defining methods or implementing traits for types that are defined in the current package.

Erroneous example

In package a:

pub type A Int

In package b:

fn f(self : @a.A) -> Int { // Error: Cannot define method f for type @a.A from package a
  0
}

Defining methods for builtin types, or types in standard library, is another frequent case of this error:

fn f(self : Int) -> Int { // Error: Cannot define method f for builtin type Int
  0
}

Suggestion

You can either move the type definition to the current package:

pub type A Int

fn f(self : A) -> Int {
  0
}

Or use new type to wrap the builtin type or type from other package:

pub type WrapA @a.A

fn f(self : WrapA) -> Int {
  ignore(self._) // Use `._` to access the wrapped value if @a.A is public
  0
}