[GHC] #9356: scanl does not participate in stream fusion

[GHC]

#9356: scanl does not participate in stream fusion
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              Reporter:  dfeuer      |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:  Runtime     |       Blocked By:
  performance bug                    |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Changes (by dfeuer):

 * version:  7.8.2 => 7.8.3


Old description:

> nomeata came up with a fusing version of a strict `scanl'` in a comment
> on #9345. It's not entirely clear to me whether that's entirely safe, but
> a non-strict version of it for `scanl` would be, as both a good producer
> and good consumer, presumably made efficient using the same sorts of
> magic used in HEAD for `foldl`. The strict `scanl'` is obviously safe to
> write as a good consumer; only the production side remains to be proved.

New description:

 nomeata came up with a fusable version of a strict `scanl'` in a comment
 on #9345 that uses the same basic technique as the new `foldl`. I think we
 should use something like the following non-strict version to replace the
 current `scanl`:

 {{{#!haskell
 scanl :: (b -> a -> b) -> b -> [a] -> [b]
 scanl f a bs = build $ \c n ->
     a `c`
     foldr (\b g x -> let b' = f x b in (b' `c` g b'))
           (const n)
           bs
           a
 }}}

 While we're at it, we should make sure that `map g . scanl f a` and `scanl
 f a . map g` fuse properly.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9356#comment:1>
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