Can extension macro be used to extend a type different from the one the macro is applied to?

Using Swift
1

I have a scenario where I'd like to apply extension macro to type Foo but to generate extension for type Bar. For example, the code

@MyMacro
struct Foo {
    ...
}

should be expanded to

struct Foo {
    ...
}

extension Array<Foo> {
    ...
}

My quick experiments show the code compiles, but with a mysterious warning, which seems to indicate the above usage isn't supported by design.

Extending a protocol composition is not supported; extending 'Foo' instead

I wonder if someone can clarify if this usage is supported or not?

My Scenario

Suppose I have a model like the following:

struct Model {
   var foos: [Foo]
   var bars: [Bar]
}

I like to organize CRUD methods in layers:

  • Foo has create/modify/etc. methods
  • Array<Foo> has corresponding create/modify/etc. methods
  • Model has corresponding create/modify/etc. methods for Foo.

The CRUD code in Array and Model are very similar for different properties (e.g. Foo, Bar). It becomes a burden to write them manually when adding new properties to Model. So I'd like to use macro to generate CRUD methods for them. That leads to the above question.

UPDATE:

I find the following in SE-0402, so the compiler should output error, instead of warning, in this case.

The generated extensions of the macro must only extend the type the macro is attached to.

I personally think it would be very useful to support generating extension to other types in extension macro.

1 Like
2

I've read before that extending arbitrary types is specifically disallowed to enable the compiler to be conservative about when it expands macros.

Perhaps it could support your specific use case - extending a generic type with the attached type as a generic parameter.

3

I have extremely similar requirement in code base. I just want to add a computed property which does optional type casting of another class into the Type I am attaching the Macro to. :slightly_frowning_face: Is there any other way to achieve this. It would be so helpful for my case.