[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):

 That proposed `foldMap` implementation has different strictness properties
 than the current one that `deriving` Foldable uses. That is, this program:

 {{{#!hs
 {-# LANGUAGE DeriveFoldable #-}

 data Phantom a = Z | S (Phantom a)
   deriving Foldable

 main :: IO ()
 main = print $ foldMap (const ()) (undefined :: Phantom Int)
 }}}

 will throw an exception, whereas this program:

 {{{#!hs
 data Phantom a = Z | S (Phantom a)
 instance Foldable Phantom where
   foldMap _ _ = mempty

 main :: IO ()
 main = print $ foldMap (const ()) (undefined :: Phantom Int)
 }}}

 will output `()`.

 I like the intent of this proposal, however. Can the strictness problem be
 fixed by changing it to `foldMap _ !_ = mempty`/`foldr _ z !_ = z`?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13328#comment:1>
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