[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           |           Version:  8.1
  Libraries                          |
           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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 For example, we define

 {{{#!hs
 instance Traversable ZipList where
   traverse f (ZipList xs) = ZipList <$> traverse f xs
 }}}

 If the list is very short, the extra `fmap` could be bad. We can fix this
 by inlining the inner `traverse`. However, I suspect a better approach
 would be to add a method to `Traversable`:

 {{{#!hs
 mapTraverse :: Applicative f
             => (t b -> r) -> (a -> f b) -> t a -> f r
 mapTraverse p f xs = p <$> traverse f xs
 }}}

 but I need to work through whether this is enough power to solve enough
 problems.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13153>
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