[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:  dfeuer
            Type:  task              |               Status:  patch
        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):  Phab:D2916
       Wiki Page:                    |
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Comment (by Ben Gamari <ben@…>):

 In [changeset:"5f91ac89a38eb128d374a04c741bbd81c41fed37/ghc" 5f91ac8/ghc]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="5f91ac89a38eb128d374a04c741bbd81c41fed37"
 Coerce for fmapDefault and foldMapDefault

 Define `fmapDefault = coerce traverse` and `foldMapDefault = coerce
 traverse`.  This ensures that we won't get unnecessary allocation and
 indirection when the arguments don't inline.

 Fixes #13058

 Reviewers: ekmett, RyanGlScott, austin, hvr, bgamari

 Reviewed By: RyanGlScott

 Subscribers: simonpj, RyanGlScott, thomie

 Differential Revision: https://phabricator.haskell.org/D2916

 GHC Trac Issues: #13058
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13058#comment:6>
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