Re: Suggesting RankNTypes for ill-formed types

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 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 http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs