Re: [GHC] #8865: Cannot derive well-kinded instance of form ‘Category

11 Mar 2014

      #8865: Cannot derive well-kinded instance of form ‘Category
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        Reporter:  adinapoli                  |            Owner:
            Type:  bug                        |           Status:  merge
        Priority:  normal                     |        Milestone:  7.8.1
       Component:  Compiler                   |          Version:
      Resolution:  fixed                      |  7.8.1-rc2
Operating System:  MacOS X                    |         Keywords:
 Type of failure:  GHC rejects valid program  |     Architecture:  x86_64
       Test Case:                             |  (amd64)
        Blocking:                             |       Difficulty:  Unknown
                                              |       Blocked By:
                                              |  Related Tickets:
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Comment (by Lemming):

 Just encountered this problem myself. It can be reproduced with the simple
 module:
 {{{
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}

 import Control.Arrow (Arrow)
 import Control.Category (Category)

 newtype Fails a b = Fails (a -> b)
    deriving (Category, Arrow)

 newtype Works a b = Works (a -> b)
    deriving (Arrow)

 instance Category Works where
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8865#comment:11
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