E4061

Cannot define method of foreign trait for foreign type.

Foreign here means that the type or trait is not defined in the current package. MoonBit follows the orphan rule, which means that you can only:

特征	类型	Allowed?
Native	Native	Yes
Foreign	Native	Yes
Native	Foreign	Yes
Foreign	Foreign	No

See the Access control of methods and trait implementations section of the MoonBit documentation for more information.

错误示例

Suppose you have type A and trait B defined in package a in module username/hello:

a/a.mbt:

pub(all) type A Int
pub(open) trait B {
  to_int(Self) -> Int
}

a/moon.pkg.json:

{}

Now, if you want to implement trait @a.B for type @a.A:

b/moon.pkg.json:

{
  "import": [
    "username/hello/a"
  ]
}

b/b.mbt:

impl @a.B for @a.A with to_int(self : @a.A) -> Int {
  //                    ^~~~~~
  // Error: Cannot define method to_int of foreign trait @a.B for foreign type @a.A
  self._
}

MoonBit will report an error.

建议

You can move either the trait or type into the current package where the implementation is in. If that is not possible, you can create a new type over the foreign type and implement the trait for the new type:

b/b.mbt:

type WrapA @a.A

impl @a.B for WrapA with to_int(self : WrapA) -> Int {
  ...
}