[GHC] #11552: ScopedTypeVariables / InstanceSigs do not work in GHC 8.0

7 Feb 2016

      #11552: ScopedTypeVariables / InstanceSigs do not work in GHC 8.0
-------------------------------------+-------------------------------------
           Reporter:  bitemyapp      |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.0.1
          Component:  Compiler       |           Version:  8.0.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{
 {-# LANGUAGE InstanceSigs        #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module MyMaybeT where

 newtype MaybeT m a =
   MaybeT { runMaybeT :: m (Maybe a) }

 instance (Functor m) => Functor (MaybeT m) where
   fmap f (MaybeT ma) =
     MaybeT $ (fmap . fmap) f ma

 instance forall f . (Applicative f) => Applicative (MaybeT f) where
   pure :: a -> MaybeT f a
   pure x = MaybeT (pure (pure x))

   (<*>) :: forall a b . Applicative f => MaybeT f (a -> b) -> MaybeT f a
 -> MaybeT f b
   (MaybeT fab) <*> (MaybeT mma) =
    let fab' :: f (Maybe (a -> b))
        fab' = fab
    in MaybeT $ undefined
 }}}

 Works fine in 7.10.3, breaks in GHC 8.0:

 {{{
 Prelude> :l code/myMaybeT1.hs
 [1 of 1] Compiling MyMaybeT         ( code/myMaybeT1.hs, interpreted )

 code/myMaybeT1.hs:13:10: error:
     Malformed instance: forall f.
                         (Applicative f) => Applicative (MaybeT f)

 code/myMaybeT1.hs:14:3: error:
     The class method signature for ‘pure’ lacks an accompanying binding
       (The class method signature must be given where ‘pure’ is declared)

 code/myMaybeT1.hs:17:3: error:
     The class method signature for ‘<*>’ lacks an accompanying binding
       (The class method signature must be given where ‘<*>’ is declared)

 code/myMaybeT1.hs:17:37: error: Not in scope: type variable ‘f’

 code/myMaybeT1.hs:17:49: error: Not in scope: type variable ‘f’

 code/myMaybeT1.hs:17:70: error: Not in scope: type variable ‘f’

 code/myMaybeT1.hs:17:84: error: Not in scope: type variable ‘f’
 Failed, modules loaded: none.
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11552
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