Desugaring introduces

[Simon Peyton Jones]

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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