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ghc --version: The Glorious Glasgow Haskell Compilation System, version 6.9.20080825

A.hs:

{-# LANGUAGE TypeFamilies #-}

module A
  where

data H a = A a | B a deriving Eq

class PL p

class PE a where
  type P a :: *

instance PE [a] where
  type P [a] = H a

instance PL (H a)

foo :: PE a => [P a] -> [P a] -> (([P a], [P a]),([P a],[P a]))
foo pa pb = ((pa, pa), (pb, pb))

-- bar :: (PE a) => [P a] -> [P a] -> Either ([P a], [P a]) [P a]
bar pa pb
  = let _ = foo pa pb
    in case True of
         True -> Right pa
         False -> Left (pa, pb)

Load the above module into GHCi.

*A> :t bar
bar :: (PE a) => [P a] -> [P a] -> Either ([P a], [P a]) [P a]

However, if we uncomment bar's type signature (which is exactly the same as the one printed by GHCi), we get a type error:

[1 of 1] Compiling A                ( A.hs, interpreted )

A.hs:23:16:
    Couldn't match expected type `P a' against inferred type `P a1'
      Expected type: [P a]
      Inferred type: [P a1]
    In the first argument of `foo', namely `pa'
    In the expression: foo pa pb
Failed, modules loaded: none.

I'm not all that familiar with type families, but I'm pretty sure that when GHCi prints a type, we ought to be able to specify that type and the module ought to compile...

Trac metadata

Trac field	Value
Version	6.9
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system	Unknown
Architecture	Unknown