[GHC] #13058: Use coercions to implement fmapDefault and foldMapDefault

[GHC]

#13058: Use coercions to implement fmapDefault and foldMapDefault
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        Reporter:  dfeuer            |                Owner:
            Type:  task              |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Core Libraries    |              Version:  8.0.1
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Sounds reasonable to me.

 We already have most of the machinery needed to do this, since `(#.)` is
 [http://git.haskell.org/ghc.git/blob/8d63ca981f689463655766c252160d3fec160264:/libraries/base/Data/Functor/Utils.hs#l78
 already used in base], located within the internal `Data.Functor.Utils`
 module. David, would you care to add `(.#)` as well and refactor
 `fmapDefault` and `foldMapDefault` as you propose?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13058#comment:1>
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