Suggesting RankNTypes for ill-formed types

[Jan Stolarek]

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 <oleg.grenrus at iki.fi> 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 <karolinadrobnik at gmail.com
> >> <mailto:karolinadrobnik at 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>
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> >
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--- 
Politechnika Łódzka 
Lodz University of Technology 

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