Switch to ? for wildcard types

[odersky]

Refactor dotty codebase so that we now use ? instead of _ for wildcard types

[julienrf]

I don’t remember if that has been discussed somewhere, but should we use ? as well for the wildcard pattern, for symmetry?

x match {
  case Foo("bar", ?) => ...
  case q: Quux[?] => ...
}

Edit: I see that a similar idea was proposed in lampepfl/dotty-feature-requests#49

[odersky]

@julienrf

should we use ? as well for the wildcard pattern, for symmetry?

That's a good question. We have two uses of _, depending on location:

• in expressions or types (i.e. things that are produced), an underscore will be a placeholder for a missing parameter.
• in patterns or bindings (i.e. things that are consumed), an underscore represents an ignored parameter.

You can say in both places an underscore represents an unnamed parameter, but that meaning changes between producers and consumers.

Pattern cases are fundamentally lambdas, so using an underscore meaning "ignored" is natural. I was on the fence for a while what to do about type patterns. E.g

case xs: List[t] => xs.head: t

The case is logically a type lambda over t followed by a term lambda over List[t]. By that analogy, we should keep _ for ignored parts of type patterns. But then we would run into an ambiguity for parts of higher-kinded types, like this one:

case x: EitherT[List[_], A, B] => 

[liufengyun]

A purely subjective comment: when I see a question mark, I feel something is wrong and there is something to be done.