[GHC] #13463: Deriving Functor / Foldable / Traversable for co-pattern type

[GHC]

#13463: Deriving Functor / Foldable / Traversable for co-pattern type
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  low               |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:  wontfix           |             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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Changes (by dfeuer):

 * priority:  normal => low
 * status:  new => closed
 * resolution:   => wontfix


Comment:

 Valid `Functor` instances are always uniquely determined. Valid
 `Traversable` instances usually aren't, but are they're tightly
 constrained.

 `Foldable` is pretty wild; it could shuffle the stream however it liked.
 In this case, you'd have to come up with a general convention for ordering
 such things, which seems impractical.

 The biggest problem here, probably, is that the deriving mechanism only
 works with one type at a time (indeed, really one constructor at a time).
 It doesn't dig into other mentioned types at all, and it seems likely
 quite hard to do so in a consistent way. GADTs only compound the problem.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13463#comment:2>
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