On Mon, Dec 7, 2020, at 22:47, Richard Eisenberg wrote:
A plea for NamedWildCards: This feature allows type variables beginning with an underscore to be treated as a component in a partial type signature. Critically (for this plea), a type signature with a named wild card will not be accepted without -XPartialTypeSignatures. I do *not* advocate for -XPartialTypeSignatures! So, having -XNamedWildCards on simply means that we get an informative error message when the user writes a type variable beginning with an underscore. The only downside is that programs such as id :: _a -> _a are no longer accepted. But does anyone write type variables with leading underscores? If we think this is at all common, then I would change my mind here. Why have it on by default? Because it's not easily discoverable, and the error messages really are quite nice.
Aha, so what you're saying is that with NamedWildCards but not PartialTypeSignatures, GHC will tell you what `_a` was solved for, but it won't accept the resulting program? I didn't realize that NamedWildCards did anything in the absence of PartialTypeSignatures, but I agree that this is better. I was already voting for NamedWildCards, but now I'm inclined to additionally vote against PartialTypeSignatures.