[GHC] #9017: Confusing error message with PolyKinds

[GHC]

#9017: Confusing error message with PolyKinds
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       Reporter:  edsko             |             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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 Consider the following example code:

 {{{
 import Control.Arrow

 foo :: a b (m b)
 foo = arr return
 }}}

 if we typecheck this we rightly get two errors about missing instances
 (Arrow a, Monad m). However, if we enable `PolyKinds` we get the following
 error message:

 {{{
 T.hs:4:7:
     Kind incompatibility when matching types:
       a0 :: * -> * -> *
       a :: k1 -> k -> *
     Expected type: a b (m b)
       Actual type: a0 b0 (m0 b0)
     Relevant bindings include foo :: a b (m b) (bound at T.hs:4:1)
     In the expression: arr return
     In an equation for ‘foo’: foo = arr return
 }}}

 I can ''sort of'' see where this is coming from if I think really hard :),
 so perhaps it's not a bug per se, but it's definitely confusing; and it's
 a pity that a type error (forgotten type qualifier) is reported as a kind
 error.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9017>
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