[GHC] #8889: GHCI reports nasty type signatures

13 Mar 2014

      #8889: GHCI reports nasty type signatures
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       Reporter:  MikeIzbicki     |             Owner:
           Type:  bug             |            Status:  new
       Priority:  normal          |         Milestone:
      Component:  GHCi            |           Version:  7.8.1-rc1
       Keywords:                  |  Operating System:  Linux
   Architecture:  x86_64 (amd64)  |   Type of failure:  Other
     Difficulty:  Unknown         |         Test Case:
     Blocked By:                  |          Blocking:
Related Tickets:                  |
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 Load a file that contains:

 {{{

 {-# LANGUAGE TypeFamilies
            , ConstraintKinds
            , MultiParamTypeClasses
            , UndecidableInstances
            , FlexibleInstances
   #-}

 import GHC.Prim
 import Prelude hiding (Functor(..))

 class Functor f where
     type C_fmap_a f a :: Constraint
     type C_fmap_a f a = ()
     type C_fmap_b f b :: Constraint
     type C_fmap_b f b = ()
     fmap :: (C_fmap_a f a, C_fmap_b f b) => (a -> b) -> f a -> f b

     fmap1 :: (ValidFunctor f a, ValidFunctor f b) => (a -> b) -> f a -> f
 b
     fmap2 :: (ValidFunctor' f a, ValidFunctor' f b) => (a -> b) -> f a ->
 f b

 type ValidFunctor f a =
     ( Functor f
     , C_fmap_a f a
     , C_fmap_b f a
     )

 class ValidFunctor f a => ValidFunctor' f a
 instance ValidFunctor f a => ValidFunctor' f a

 }}}

 Then check the following types in ghci
 {{{
 ghci> :t fmap
 fmap
   :: (t, t1, Functor f, C_fmap_b f b ~ t1, C_fmap_a f a ~ t) =>
      (a -> b) -> f a -> f b

 ghci> :t fmap1
 fmap1
   :: (t, t1, t2, t3, Functor f, C_fmap_b f b ~ t3, C_fmap_b f a ~ t1,
       C_fmap_a f b ~ t2, C_fmap_a f a ~ t) =>
      (a -> b) -> f a -> f b

 ghci> :t fmap2
 fmap2
   :: (t, t1, t2, t3, Functor f, C_fmap_b f b ~ t3, C_fmap_b f a ~ t1,
       C_fmap_a f b ~ t2, C_fmap_a f a ~ t) =>
      (a -> b) -> f a -> f b
 }}}

 These types are much nastier looking than they need to be.  There are two
 problems:

 1) Bogus types t,t1,t2,t3 are introduced when they don't need to be.  This
 is confuses the type signatures quite a bit.

 2) The type alias ValidFunctor is being desugared in the type signature
 for fmap1.  This makes type aliases for constraints rather pointless.

 Also, I tried to solve problem two by adding an extra class, and hoping
 the class would be displayed instead, but this still doesn't work.  I
 assume this is actually intended behavior though.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8889
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