[GHC] #9587: Type checking with type functions introduces many type variables, which remain ambiguous. The code no longer type checks.

[GHC]

#9587: Type checking with type functions introduces many type variables, which
remain ambiguous. The code no longer type checks.
-------------------------------------+-------------------------------------
        Reporter:  oleg              |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler (Type    |                 Version:  7.8.3
  checker)                           |                Keywords:  type
      Resolution:                    |  family, ambiguity check
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:  #9607             |  Differential Revisions:
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Comment (by jstolarek):

 For the record, this code can now be compiled using injective type
 families:

 {{{#!hs
 {-# LANGUAGE NoMonomorphismRestriction #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE ScopedTypeVariables #-}

 module T9587 where

 type family Arr (repr :: * -> *) (a :: *) (b :: *) = (r :: *) | r -> repr
 a b

 class ESymantics repr where
     int :: Int  -> repr Int
     add :: repr Int  -> repr Int -> repr Int

     lam :: (repr a -> repr b) -> repr (Arr repr a b)
     app :: repr (Arr repr a b) -> repr a -> repr b

 te4 = let c3 = lam (\f -> lam (\x -> f `app` (f `app` (f `app` x))))
       in (c3 `app` (lam (\x -> x `add` int 14))) `app` (int 0)
 }}}

 Not sure if that addresses the original problem because:

   a) `Arr` might not be injective at all

   b) if I understand correctly [comment:10 comment 10] the actual
 injectivity annotation on `Arr` is `r repr -> a b` and that is not
 supported yet

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9587#comment:12>
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