[GHC] #13520: instance Alternative ZipList

[GHC]

#13520: instance Alternative ZipList
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        Reporter:  Iceland_jack      |                Owner:  (none)
            Type:  feature request   |               Status:  closed
        Priority:  normal            |            Milestone:  8.4.1
       Component:  libraries/base    |              Version:  8.0.1
      Resolution:  fixed             |             Keywords:  newcomer
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3416,
       Wiki Page:                    |  Phab:D4638
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Changes (by dfeuer):

 * differential:  Phab:D3416 => Phab:D3416, Phab:D4638


Comment:

 Zemyla, in an amazing coincidence of timing, I happened to write something
 very similar to your first instance in Phab:D4638 just now. I think it
 makes sense to use `foldr` to fold over the first argument. Unfortunately,
 the `build` is a bit trickier, because it will end up copying the second
 list if it doesn't fuse away. I'm not sure if it's possible to work around
 that issue with `RULES`. Appends are generally handled by `augment`, but
 that mechanism isn't powerful enough for what we're doing here because it
 leaves no room to use an accumulated count in the tail. As for the hand-
 unboxing, I'd only go for that if GHC proves incapable of unboxing on its
 own; it's really quite good at that most of the time.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13520#comment:10>
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