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{-# 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.

$ ghci --version
The Glorious Glasgow Haskell Compilation System, version 8.0.0.20160122