New subject: [GHC] #8740: Deriving instance conditionally compiles (was: Code conditionally compiles)

5 Feb 2014

      #8740: Code conditionally compiles
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       Reporter:            |             Owner:
  thomaseding               |            Status:  new
           Type:  bug       |         Milestone:
       Priority:  normal    |           Version:  7.6.3
      Component:  Compiler  |  Operating System:  MacOS X
       Keywords:            |   Type of failure:  GHC rejects valid program
   Architecture:            |         Test Case:
  Unknown/Multiple          |          Blocking:
     Difficulty:  Unknown   |
     Blocked By:            |
Related Tickets:            |
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 {{{
 {-# LANGUAGE GADTs #-}
 {-# LANGUAGE StandaloneDeriving #-}

 data Abstract
 data Reified
 data Player

 data Elect p a where
     ElectRefAsTypeOf :: Int -> Elect Abstract a -> Elect Abstract a
     ElectHandle :: a -> Elect Reified a
     Controller :: Elect Abstract Player
     Owner :: Elect Abstract Player
     You :: Elect Abstract Player

 deriving instance (Eq a) => Eq (Elect p a)
 deriving instance (Ord a) => Ord (Elect p a)
 }}}

 As is, the above code fails to compile. But if I move `ElectRefAsTypeOf`
 to be the last constructor for the GADT, the code does compile. If I
 remove one of the `Elect Abstract Player` constructors, the code still
 won't compile even if the `ElectRefAsTypeOf` is moved.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8740
GHC http://www.haskell.org/ghc/
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[GHC] #8740: Code conditionally compiles

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