[GHC] #9339: last is not a good consumer

[GHC]

#9339: last is not a good consumer
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              Reporter:  dfeuer      |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  normal      |         Milestone:
             Component:              |           Version:  7.8.3
  libraries/base                     |          Keywords:
            Resolution:              |  Operating System:  Unknown/Multiple
Differential Revisions:              |   Type of failure:  Runtime
          Architecture:              |  performance bug
  Unknown/Multiple                   |         Test Case:
            Difficulty:  Unknown     |          Blocking:
            Blocked By:              |
       Related Tickets:              |
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Comment (by dfeuer):

 Replying to [comment:1 nomeata]:
 > What general fusion work are you referring to? I only know about the
 improvements for fusing foldl, which doesn’t seem to apply here.

 I was under the impression (possibly bogus) that the `foldl` fix involved
 some new transformation that could have other effects.

 > Although maybe it could. How about this definition:
 >
 > {{{
 > myLast2 :: [a] -> a
 > myLast2 = foldl (\_ x -> x) undefined
 > }}}
 >
 > While `last` yields 800051648 bytes, and `myLast` yields 560084432
 bytes, this runs in 51648 bytes (i.e. it fuses completely).

 That looks wonderful. Is it certain to be changed to a `foldl'` in cases
 where it ''doesn't'' fuse?


 > Admitted, the resulting code looks almost stupidly efficient (at least
 if I write `10^7` as `10000` – I’m unjustifiably surprised that that is
 not constant-folded):

 Imagine the confusion if it were! Yes, `map f [m..n] !! p` ''could'' be
 optimized to O(1) whenever `m` has a known-sane type, but then all the
 newbies and half the oldies would get mixed up when little changes had
 radical performance impacts.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9339#comment:2>
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