[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 RyanGlScott):

 One way we could help mitigate the effect of this is by doing an
 additional optimization for derived `Foldable` instances. First, we check
 each constructor, and if it does not contain an occurrence of the last
 type parameter, we filter out that constructor. Then we generate the
 `foldMap` cases as normal for fhe constructors that remain, and if any
 constructors were filtered out, we generate a catch-all `foldMap _ _ =
 mempty` case at the end.

 This should at least help in the case when all the data constructors of a
 phantom type are in scope, but its role has been annotated as something
 other than phantom.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:11>
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