[GHC] #9328: MINIMAL pragma should supprt negation

18 Jul 2014


      #9328: MINIMAL pragma should supprt negation
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        Reporter:  augustss          |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.8.3
        Keywords:                    |  Differential Revisions:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  None/Unknown      |  Unknown/Multiple
       Test Case:                    |              Difficulty:  Unknown
        Blocking:                    |              Blocked By:
                                     |         Related Tickets:
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 Consider this class declaration
 {{{
 class Conv a where
     to :: Integer -> a
     from :: a -> Integer
     default to :: (Generic a) => Integer -> a
     to i = ...
     default from :: (Generic a) => a -> Integer
     from a = ...
 }}}
 The class provides default methods for the two methods using generics.  An
 instance declaration for this type is likely to want to use the default
 for both methods or for none.  So I'd like a MINIMAL pragma that can
 express this.  E.g.
 {{{
     {-# MINIMAL (to, from) | (!to, !from) #-}
 }}}
 I've used ! for negation (following the lead set by the MINIMAL pragma
 using a non-Haskell OR operator), so this says: either implement 'to' and
 'from' or don't implement neither 'to' nor 'from'.

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/9328
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