[GHC] #9790: Produce coercion rules for derived Functor instances

[GHC]

#9790: Produce coercion rules for derived Functor instances
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              Reporter:  dfeuer      |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:  coercion
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:  Runtime     |  Related Tickets:
  performance bug                    |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by goldfire):

 Ah -- I missed the word "derive". But, unfortunately, this still doesn't
 pan out in all cases:

 {{{
 data BadFunctor a = MkBF a a
   deriving Show

 instance Functor BadFunctor where
   fmap f (MkBF x _) = MkBF (f x) (f x)

 data Wrapped f a = MkW (f a)
   deriving (Functor, Show)
 }}}

 `Wrapped` has a derived `Functor` instance, and yet `fmap id (MkW (MkBF
 True False))` is not the same as `id (MkW (MkBF True False))`.

 I do think your idea is sound as long as the derived `Functor` instance
 has no constraints.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9790#comment:3>
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