[GHC] #9545: Evaluate Takano Akio's foldrW/buildW fusion framework as a possible replacement for foldr/build

[GHC]

#9545: Evaluate Takano Akio's foldrW/buildW fusion framework as a possible
replacement for foldr/build
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              Reporter:  dfeuer      |            Owner:
                  Type:  task        |           Status:  closed
              Priority:  normal      |        Milestone:
             Component:              |          Version:  7.9
  libraries/base                     |         Keywords:
            Resolution:  wontfix     |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Project (more
  Unknown/Multiple                   |  than a week)
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dfeuer):

 Replying to [comment:7 quantheory]:

 > I did find the issues with scanl and reverse; in fact, even after fixing
 `reverse . scanl f z $ xs`, I still had issues with `reverse . scanl f z .
 reverse $ xs`. But I did eventually fix both cases with this change:
 >
 > https://github.com/quantheory/ww-
 fusion/commit/74618328c34572270154dbd93eb441f448760e0e

 I must admit I'm still nervous about that implementation, for the very
 simple and unscientific reason that your unwrapper has a `_` in it:

 {{{#!hs
 (\u -> Env $ \k b -> unwrap $ \e _ -> u e k b)
 }}}

 The fact that it makes me nervous does not mean that it's actually wrong,
 of course.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9545#comment:8>
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