[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):

 > The question about `fmap` strictness looks like a red herring to me.
 There's no real difference between the empty case definition and the
 `coerce` definition.

 Sure, I just wanted to note the "conflict".

 > The illegitimate `fmap` is of very little interest to me. Someone who
 cares about preserving their map count or whatever just won't be able to
 use `Functor` deriving for types explicitly mentioning their bogus one.

 That's also fine, just make sure we note this if this gets implemented.
 Luckily, `Functor`/`Foldable`/`Traversable` happen to be pretty rigid,
 law-abiding classes, so this restriction won't really hurt anyone in
 practice.

 What is your opinion on the `RoleAnnotations` example above? This is the
 only thing that genuinely concerns me, since I think having the strictness
 of a derived `Foldable` instance change depending on whether a type
 parameter is phantom or non-phantom is quite unintuitive. I'm not sure of
 a good workaround, however.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:7>
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