[GHC] #7542: GHC doesn't optimize (strict) composition with id

[GHC]

#7542: GHC doesn't optimize (strict) composition with id
-------------------------------------+-------------------------------------
        Reporter:  shachaf           |                Owner:  simonpj
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  7.6.1
      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:                    |
-------------------------------------+-------------------------------------

Comment (by chessai):

 Ah, you're right.

 look at this:

 {{{
 module Foo (foo) where

 data List a = Nil | Cons a (List a)

 mapList :: (a -> b) -> List a -> List b
 mapList f Nil = Nil
 mapList f (Cons a l) = Cons (f a) (mapList f l)

 (#) :: (b -> c) -> (a -> b) -> a -> c
 (#) f g = f `seq` g `seq` \x -> f (g x)

 foo :: (a -> b) -> List a -> List b
 foo f = mapList (id # f) xs
 }}}

 The core i get looks like:

 {{{
 mapList
 mapList
   = \ @ a_avr @ b_avs f_atJ ds_d10t ->
       case ds_d10t of {
         Nil -> Nil;
         Cons a1_atL l_atM -> Cons (f_atJ a1_atL) (mapList f_atJ l_atM)

 foo = \ @ a_avy @ b_avz f_atQ xs_atR -> mapList f_atQ xs_atR
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/7542#comment:27>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler