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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.

Trac metadata

Trac field	Value
Version	7.8.1-rc1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	GHCi
Test case
Differential revisions
BlockedBy
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Blocking
CC	hvr
Operating system
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