[GHC] #10260: last uses too much space with optimizations disabled

[GHC]

#10260: last uses too much space with optimizations disabled
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        Reporter:  rwbarton          |                   Owner:  ekmett
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Core Libraries    |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
 Type of failure:  Runtime           |  Unknown/Multiple
  performance bug                    |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Description changed by simonpj:

Old description:

> I know we don't worry too much about performance when optimizations are
> disabled, but `last [1..n]` using O(n) space when the naive definition
> would run in O(1) space is a bit too much I think.
>
> Can we split out the new definition to a separate function and use a rule
> to rewrite `last` to that function when optimizations are enabled? I'm
> not expert enough on the use of rules and inlining phases etc. to write
> the patch myself, but I think it should be simple.

New description:

 I know we don't worry too much about performance when optimizations are
 disabled, but `last [1..n]` using O(n) space when the naive definition
 would run in O(1) space is a bit too much I think.

 Can we split out the new definition to a separate function and use a rule
 to rewrite `last` to that function when optimizations are enabled? I'm not
 expert enough on the use of rules and inlining phases etc. to write the
 patch myself, but I think it should be simple.

 This showed up when investigating #10247

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10260#comment:1>
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