Enable TypeHoles by default?

[Dan Frumin]

Hi!

> On 13 Jan 2014, at 02:56, Krzysztof Gogolewski <krz.gogolewski at gmail.com> wrote:
> 
> 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.
> 

I do think that having _ both as a pattern and a hole might be confusing, I can see that. However that's more of a syntax issue, than an issue about default extensions IMO

> 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
> _______________________________________________
> ghc-devs mailing list
> ghc-devs at haskell.org
> http://www.haskell.org/mailman/listinfo/ghc-devs