[GHC] #8963: GeneralizedNewtypeDeriving unusable on associated types

6 Apr 2014

      #8963: GeneralizedNewtypeDeriving unusable on associated types
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       Reporter:  mojojojo          |             Owner:
           Type:  bug               |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.3
       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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 This code:
 {{{
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}

 class C c where
   data F c r

 instance C Int where
   newtype F Int r = F (IO r) deriving (Functor)
 }}}

 fails to compile with the following message:
 {{{
     Derived instance `Functor (F Int)'
       requires illegal partial application of data type family F
     In the newtype instance declaration for `F'
 }}}

 This might be related to https://ghc.haskell.org/trac/ghc/ticket/4185, but
 then it is marked as fixed in GHC 7.6.2, and I experience my error on
 7.6.3.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8963
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