[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:3 nomeata]:
 > >  That looks wonderful. Is it certain to be changed to a foldl' in
 cases where it doesn't fuse?
 >
 > Hopefully not. `foldl'` would force the accumulator, which we do *not*
 want here (otherwise the `undefined` would be forced, or `last [undefined,
 1]` would not work).

 Yes, you're right. I got mixed up a bit.

 > I didn’t do further testing with that idea, it just crossed my mind. It
 maybe the that this implementation is only good when fusing works – would
 you mind trying to find out?

 I don't have anything beyond 7.8.3, and on 7.8.3 your code doesn't fuse.
 Could you maybe try it?

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9339#comment:4>
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