Desugaring introduces

[Gabor Greif]

Yes, I am using 7.8.

I'll also try HEAD now...

... and it works! :-)

Thanks, I am happy now.

Cheers,

    Gabor

PS: Would it be worth adding this as a regression test?




On 2/23/15, Simon Peyton Jones <simonpj at microsoft.com> wrote:
> Gabor
>
> You don't say which version of GHC you are using.  I assume 7.8.
>
> Yes, you should really get the same behaviour with the surgared and
> desugared versions.
>
> Happily, with HEAD (and 7.6) it compiles fine without ImpredicativeTypes.
>
> Simon
>
> | -----Original Message-----
> | From: ghc-devs [mailto:ghc-devs-bounces at haskell.org] On Behalf Of Gabor
> | Greif
> | Sent: 21 February 2015 11:42
> | To: ghc-devs
> | Subject: Desugaring introduces
> |
> | Hi devs,
> |
> | before I file a bug, I'd like to double check on a strange desugaring
> | behaviour with RankNTypes and RebindableSyntax.
> |
> | Here is the snippet
> | {{{
> | {-# LANGUAGE RankNTypes, RebindableSyntax #-}
> | {-# LANGUAGE ImpredicativeTypes #-}
> |
> | import qualified Prelude as P
> |
> | (>>=) :: a -> ((forall b . b) -> c) -> c
> | a >>= f = f P.undefined
> | return a = a
> | fail s = P.undefined
> |
> | t1 = 'd' >>= (\_ -> 'k')
> |
> | t2 = do _ <- 'd'
> |         'k'
> |
> | main = P.putStrLn [t1, t2]
> | }}}
> |
> | Without ImpredicativeTypes I get this error:
> | {{{
> | rebindtest.hs:13:9:
> |     Cannot instantiate unification variable ‘t0’
> |     with a type involving foralls: forall b. b
> |       Perhaps you want ImpredicativeTypes
> |     In a stmt of a 'do' block: _ <- 'd'
> |     In the expression:
> |       do { _ <- 'd';
> |            'k' }
> |     In an equation for ‘t2’:
> |         t2
> |           = do { _ <- 'd';
> |                  'k' }
> | }}}
> |
> | t1 is supposed to be the desugaring of t2. Strangely t2 only compiles
> | with ImpredicativeTypes. Why? Isn't desugaring a purely syntactic
> | transformation (esp. with RebindableSyntax)?
> |
> | Any hints welcome!
> |
> | Cheers,
> |
> |     Gabor
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