[GHC] #9674: Foldable doesn't have any laws

[GHC]

#9674: Foldable doesn't have any laws
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              Reporter:  dfeuer      |            Owner:  ekmett
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:  7.8.3
  Libraries                          |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  Documentation bug                  |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by ekmett):

 `foldMap f = fold . fmap f` assumes you have a `Functor` instance as well,
 so any such law would need that caveat.

 `fold = foldMap id` holds universally.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9674#comment:6>
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