[GHC] #9196: Higher-rank constraint treated as type instead

11 Jun 2014

      #9196: Higher-rank constraint treated as type instead
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       Reporter:  goldfire          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 When I say

 {{{
 {-# LANGUAGE RankNTypes #-}

 module Bug where

 class P z
 foo :: (forall b. P (a, b)) => a -> a
 foo x = x
 }}}

 I get

 {{{
 Bug.hs:7:9:
     Couldn't match expected type ‘a -> a’ with actual type ‘P (a, b0)’
     Relevant bindings include
       x :: forall b. P (a, b) (bound at Bug.hs:7:5)
       foo :: (forall b. P (a, b)) -> a -> a (bound at Bug.hs:7:1)
     In the expression: x
     In an equation for ‘foo’: foo x = x
 }}}

 I expected my code to be rejected, but not with that error message. It
 seems that GHC thinks `forall b. P (a, b))` has kind `*`, not kind
 `Constraint`.

 Encountered while experimenting with #9195.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9196
GHC http://www.haskell.org/ghc/
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