Hello,

I propose to enable -XTypeHoles in GHC by default.

Unlike other -X* flags, holes do not really change meaning of the program, they only change error messages. Instead of "_x not in scope", we effectively get "_x not in scope, its expected type is a -> a". You get it only if you precede the identifier not in scope with underscore, so in some sense you declare the intention of using holes.

Two possible issues:

(a) If you use -fdefer-type-errors, then a program might compile, while previously it did not. However, we should facilitate compiling with defer-type-errors, so I don't think this is a disadvantage.

(b) The identifier _ becomes both a pattern and a hole by default, which might confuse new users.

Reply: I have never seen anyone ask why code such as "Just _ -> _" does not work.

IMO the productivity boost by having holes by default outweighs those two objections. I am open to hearing any other possible issues others might find.

The change is trivial implementation-wise; add Opt_TypeHoles to the list in languageExtensions Nothing in DynFlags.

-KG