This would be fine by me - it's a "user-experience" question. It would slightly threaten the notion that GHC is, by default, a Haskell-2010 compiler; that is, it accepts H-2010 programs and rejects non-H2010 programs. But I think it would be an acceptable bending of this principle, if people wanted it. Maybe ask ghc-users? Simon From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Krzysztof Gogolewski Sent: 12 January 2014 22:56 To: ghc-devs@haskell.org Subject: Enable TypeHoles by default? 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