[GHC] #7633: Checkable "minimal complete definitions"

[GHC]

#7633: Checkable "minimal complete definitions"
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    Reporter:  shachaf           |       Owner:                  
        Type:  feature request   |      Status:  patch           
    Priority:  normal            |   Milestone:  7.8.1           
   Component:  Compiler          |     Version:  7.6.1           
    Keywords:                    |          Os:  Unknown/Multiple
Architecture:  Unknown/Multiple  |     Failure:  None/Unknown    
  Difficulty:  Unknown           |    Testcase:                  
   Blockedby:                    |    Blocking:                  
     Related:  6028              |  
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Comment(by dmwit):

 This might be a bit of feature creep, but I thought I would try to archive
 a bit of #haskell discussion: it might be worth thinking about how to do
 cross-class MINIMAL pragmas. For example, imagine a world where we had
 something like this:

 {{{
 {-# LANGUAGE DefaultSignatures #-}
 class Functor f where
     fmap :: (a -> b) -> (f a -> f b)
     default fmap :: Traversable f => (a -> b) -> (f a -> f b)
     fmap = getId . traverse (Id . f)

 class Functor f => Traversable f where
     traverse :: Applicative f => (a -> f b) -> t a -> f (t b)
     traverse f = sequenceA . fmap f

     sequenceA :: Applicative f => t (f a) -> f (t a)
     sequenceA = traverse id
 }}}

 I might like to be able to say that either "fmap and sequenceA" or
 "traverse" are okay minimal definitions for the two classes together.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7633#comment:20>
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