[GHC] #13328: Foldable, Functor, and Traversable deriving handle phantom types badly

[GHC]

#13328: Foldable, Functor, and Traversable deriving handle phantom types badly
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        Reporter:  dfeuer            |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  8.1
      Resolution:                    |             Keywords:  deriving-perf
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by dfeuer):

 No, you have to pick efficiency or strictness here. The bottom could show
 up anywhere down the line. I think derived instances should generally go
 for what someone's likely to write by hand, rather than trying to imitate
 the lousy results of the default. The default implementation of `null`
 will diverge on an infinite snoc-list, but that doesn't mean the derived
 implementation should. I think the same is true here. If the type is
 phantom, there's no possible reason to go there, so we shouldn't.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:2>
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