[GHC] #7436: Derived Foldable and Traversable instances become extremely inefficient due to eta-expansion

[GHC]

#7436: Derived Foldable and Traversable instances become extremely inefficient due
to eta-expansion
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    Reporter:  shachaf           |       Owner:                         
        Type:  bug               |      Status:  patch                  
    Priority:  normal            |   Milestone:                         
   Component:  Compiler          |     Version:  7.6.1                  
    Keywords:                    |          Os:  Unknown/Multiple       
Architecture:  Unknown/Multiple  |     Failure:  Runtime performance bug
  Difficulty:  Unknown           |    Testcase:                         
   Blockedby:                    |    Blocking:                         
     Related:                    |  
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Comment(by twanvl):

 I wrote the original deriving code for Functor/Foldable/Traversable.
 Someone else later cleaned it up a bit.

 The strange code that is generated are lambdas applied to arguments, which
 is something that GHC should be able to optimize away easily. To avoid the
 eta-expansion, the generated code is always a function. So you end up with
 code like
 {{{
 data Cons a = Cons (a,a)
 fmap f (Cons x) = (\x -> case x of (x1,x2) -> (f x1 , f x2)) x
 }}}
 I could add this example to a note, if you insist.

 There are no library changes in the patch, and the derived code is
 functionally equivalent to what was there before.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7436#comment:12>
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