Implement a polymorphic method

[gregghz]

I've created a macro that takes a type parameter (referencing a trait) and attempts to create an implementation of that trait.

That is, calling: make[TestTrait] should produce a class that implements TestTrait. However, so far, I am unable to implement polymorphic methods. TestTrait has this abstract method: def poly[A](x: List[A]): A and I want the implementation created via the macro to just call x.head.

My attempt at this is here: https://gist.github.com/gregghz/6b74e50c1600c30322403449ce978a8c

The code produced looks how I'd expect/want with a class defined and an override for poly that looks like this:

def poly[A](x: scala.collection.immutable.List[A]): A = x.head

However, the compilation fails with an error saying that my class needs to be abstract because poly is not defined. The error says that my override does not match because their type parameters differ even though the compiler output shows them side by side and they look identical:

[error]   |(Note that
[error]   | parameter List[A] in def poly[A](x: List[A]): A in trait TestTrait in package macros does not match
[error]   | parameter List[A] in def poly[A](x: List[A]): A in class ImplT
[error]   | their type parameters differ)

Can anyone point me in the right direction here?

[bishabosha]

The issue is that type parameters are unique for each method, so should not be shared. In your example, you are reusing the type for x from the original poly method, this references the old A type parameter from the original poly, rather than the new A you defined for your new poly method.

Currently in Quotes there is substituteTypes which can substitute references to symbols by types, however I do not think this will work for type parameters of methods - they do not use symbols.

[gregghz]

This all makes sense. FWIW, the copying approach was used because in my real use case, I'm listing all methods on the trait and creating stub implementations. That is, I don't know up front the number of and types of the parameters.

How can I reference the A created in the new poly method?

      val methodType = PolyType(List("A"))(
        _ => List(TypeBounds(TypeRepr.of[Nothing], TypeRepr.of[Any])),
        _ =>
          MethodType(List("x"))(
            mt => List(mt.param(0)),
            _ => abstractDef.returnTpt.tpe
          )
      )

Oddly, this ^^ just gets stuck in an infinite loop of some sort. Normally the whole thing compiles in less than 5 seconds. When I use that implementation, it never finishes compiling (at least, I've waiting as long as 5 minutes).