It doesn't really matter whether we suggest ExplicitForAll, ScopedTypeVariables or RankNTypes. The code in question is plainly wrong and should be a parse error. I skimmed the parser code and I see no easy way of doing this without breaking the fix for #3155 at the same time. What I would try, I think, is to find a different solution to #3155. Janek Dnia czwartek, 18 sierpnia 2016, Richard Eisenberg napisał:

I tend to agree with Oleg that suggesting `ScopedTypeVariables` may be more helpful to users, even though `ExplicitForAll` is more principled.

Richard

...

On Aug 11, 2016, at 2:45 PM, Oleg Grenrus wrote:

FWIW. Often when I encounter that error, I want `ScopedTypeVariables`, yet my code doesn’t always has the scoped type variable used. So even GHC could parse further and propose it to me, there isn’t anything from to do it :(

I don’t know if many use /just/ `ExplicitForAll`...

- Oleg

...

On 11 Aug 2016, at 20:30, Karolina Drobnik mailto:karolinadrobnik@gmail.com> wrote:

Hello everyone,

I am working on my first ticket (#11669, linked below) and I have some doubts after a little bit of hacking.

There was a hint that an error message should be changed from the one suggesting RankNTypes to ExplicitForall. In my opinion it would be quite confusing for the user, especially where the type is ill-formed. A plain parse error should be shown here.

It is clear that it should be done in such a way after turning on one of the extensions, but what about the situation where proposed fix (suggesting RankNTypes/ExplicitForall) won't work? We should be able to distinguish ill-formed type from the correct one, even before the extension activation. To be honest - I don't know how to do it.

Additionally, I am not sure if we can assume that an user wants to use arbitrary rank (which implies ExplicitForall) or just a forall keyword. I am for the second one, but it is just my assumption.

And the last minor thing - a type formed in this way also rises an error suggesting using RankNTypes (as we know that wouldn't solve the problem):

f :: a. -> Int f = undefined

Maybe we could treat it as a typo (simple parse error) and propose an extension activation only when forall was parsed earlier? That could be tricky.

I'd appreciate some thoughts on this issue because I felt a bit lost after digging around the parser.

Best regards,

Karolina --------------------------------------------------------- #11669 - https://ghc.haskell.org/trac/ghc/ticket/11669 https://ghc.haskell.org/trac/ghc/ticket/11669 _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org mailto:ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs

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