[GHC] #13153: Several Traversable instances have an extra fmap

[GHC]

#13153: Several Traversable instances have an extra fmap
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        Reporter:  dfeuer            |                Owner:  dfeuer
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.1
       Component:  Core Libraries    |              Version:  8.1
      Resolution:                    |             Keywords:
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):

 Ah, I see what you're getting at now.

 It's quite unfortunate that we have to go through such contortions to make
 applying the newtype constructor zero-cost. We certainly could change the
 `Traversable ZipList` implementation to what you suggest to avoid this
 quandary, but there's no getting around the fact that it's a hack.

 One thing I've contemplated for a while is adding an `unsafenewtype`
 deriving strategy that implements what Richard suggests in
 https://ghc.haskell.org/trac/ghc/ticket/9123#comment:27. That is, if you
 wrote:

 {{{#!hs
 {-# LANGUAGE DerivingStrategies #-}
 {-# LANGUAGE GeneralizedNewtypeDeriving #-}

 newtype ZipList a = ZipList [a]
   deriving newtype (Functor, Foldable)
   deriving unsafenewtype (Traversable)
 }}}

 Then the derived `Traversable ZipList` instance would be:

 {{{#!hs
 instance Traversable ZipList where
   traverse :: forall f a b. Applicative f => (a -> f b) -> ZipList a -> f
 (ZipList b)
   traverse = unsafeCoerce (traverse :: (a -> f b) -> [a] -> f [b])
 }}}

 Granted, this is a separate hack to get around the fact that we don't have
 higher-kinded roles yet, but it's (IMO) much nicer to use than having to
 manually inline the definition of `traverse` like you demonstrated above.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13153#comment:5>
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