[GHC] #11319: ImpredicativeTypes cause trouble (affects deriving of Traversable)

30 Dec 2015

      #11319: ImpredicativeTypes cause trouble (affects deriving of Traversable)
-------------------------------------+-------------------------------------
           Reporter:  Iceland_jack   |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.11
  (Type checker)                     |
           Keywords:                 |  Operating System:  Linux
  ImpredicativeTypes                 |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I don't have the latest version of GHC, trying to derive `Functor A` and
 `Foldable A` is fine but when I derive `Traversable A` in the attachment
 Error.hs:

 {{{#!hs
 {-# LANGUAGE DeriveFunctor, DeriveFoldable, DeriveTraversable,
 ImpredicativeTypes #-}

 import Data.Functor     (Functor)
 import Data.Foldable    (Foldable)
 import Data.Traversable (Traversable)

 data A a = A
   deriving (Functor, Foldable, Traversable)
 }}}

 GHC barks at me (verbose log attached):

 {{{#!hs
 /tmp/Error.hs:8:32: error:
     • Couldn't match type ‘forall a1. A a1’ with ‘A b’
       Expected type: f (A b)
         Actual type: f (forall a. A a)
     • In the expression: pure A
       In an equation for ‘traverse’: traverse f A = pure A
       When typechecking the code for ‘traverse’
         in a derived instance for ‘Traversable A’:
         To see the code I am typechecking, use -ddump-deriv
       In the instance declaration for ‘Traversable A’
     • Relevant bindings include
         f :: a -> f b (bound at /tmp/Error.hs:8:32)
         traverse :: (a -> f b) -> A a -> f (A b)
           (bound at /tmp/Error.hs:8:32)
 }}}

 With `-ddump-deriv` we get this (unqualified) instance:

 {{{#!hs
   instance Traversable A where
     traverse f_a2Le A = pure A
 }}}

 which by itself causes the same problem in the attachment Error2.hs:

 {{{#!hs
 {-# LANGUAGE DeriveFunctor, DeriveFoldable, ImpredicativeTypes #-}

 import Data.Functor     (Functor)
 import Data.Foldable    (Foldable)
 import Data.Traversable (Traversable)

 data A a = A
   deriving (Functor, Foldable)

 instance Traversable A where
   traverse f A = pure A
 }}}

 Works fine in GHC-7.10.2 and GHC-7.10.0.20150316 and GHC-7.4 (with some
 additional imports), is this an `ImpredicativeTypes` regression?

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11319
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