[GHC] #9355: scanr does not participate in stream fusion

[GHC]

#9355: scanr does not participate in stream fusion
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              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:  7.8.4
             Component:              |          Version:  7.8.3
  libraries/base                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Moderate (less
  Unknown/Multiple                   |  than a day)
       Type of failure:  Runtime     |       Blocked By:
  performance bug                    |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 Replying to [comment:2 nomeata]:
 > > (because the argument to build isn't allowed to inspect its own result
 as the implementation in Data.List does),
 >
 > I wouldn’t be surprised that returning `(# x, x:xs #)` is faster than
 returning `x:xs` and pattern matching on it. OTOH, the CPR optimization
 should change the code returning `x:xs` into (# x,  xs #) and move the
 consing to the caller.
 >
 > Can’t  you do that by hand, i.e.:
 SNIP
 > But I don’t expect there to be a measurable difference (and I didn’t
 check):


 I brilliantly read your code wrong and drew a bad conclusion. This looks
 much better than I initially thought (Look, ma, no `let`! ). I'm going to
 see how nofib likes it.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9355#comment:5>
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