[GHC] #8630: Kind inference fails to account for associated types

27 Dec 2013

      #8630: Kind inference fails to account for associated types
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       Reporter:  goldfire                 |             Owner:
           Type:  bug                      |            Status:  new
       Priority:  normal                   |         Milestone:
      Component:  Compiler (Type checker)  |           Version:  7.7
       Keywords:                           |  Operating System:
   Architecture:  Unknown/Multiple         |  Unknown/Multiple
     Difficulty:  Unknown                  |   Type of failure:
     Blocked By:                           |  None/Unknown
Related Tickets:                           |         Test Case:
                                           |          Blocking:
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 Consider this:

 {{{
 {-# LANGUAGE PolyKinds, TypeFamilies, FlexibleInstances #-}

 class C a where
   type F a

 instance C a where
   type F a = a -> a
 }}}

 HEAD gives me

 {{{
     Expected a type, but ‛a’ has kind ‛k’
     In the type ‛a -> a’
     In the type instance declaration for ‛F’
     In the instance declaration for ‛C a’
 }}}

 The problem is that the use of `(->)` in the RHS of the definition for the
 `F a` instance constrains `a` to be of kind `*`, but GHC does not
 propagate this information back to the instance head. This decision is in
 conflict with the behavior for ''class'' declarations (as opposed to
 ''instance'' declarations), where a type variable's use in the definition
 informs its kind in the head.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8630
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