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@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 | _______________________________________________ | ghc-devs mailing list | ghc-devs@haskell.org | http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs