[GHC] #9621: New Data.Foldable methods

[GHC]

#9621: New Data.Foldable methods
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              Reporter:  hvr         |            Owner:  ekmett
                  Type:  task        |           Status:  new
              Priority:  normal      |        Milestone:  7.10.1
             Component:  Core        |          Version:
  Libraries                          |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:  #9586
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Changes (by hvr):

 * cc: dfeuer (added)
 * version:  7.8.3 =>


Old description:

> place-holder text; details to follow soon

New description:

 See http://thread.gmane.org/gmane.comp.lang.haskell.libraries/22828

 The current version of the proposal at time of writing:

 > Add the following to `Foldable`:
 >
 > - `length` — often stored as a separate field for //O(1)// access
 > - `null` — called very often and slightly faster than `foldr (\_ _ ->
 False) True` for some containers
 > - `toList` — may be the identity or nearly so, and this fact may not be
 statically available to the foldr/id rule
 > - `sum`, `product` — may be cached internally in tree nodes for fast
 update, giving //O(1)// access; may be calculated in parallel.
 > - `maximum`, `minimum` — //O(n)// for a search tree; //O(1)// for a
 finger tree.
 > - `elem` — //O(log n)// for search trees; likely better for hash tables
 and such.
 >
 > **Don't** add anything to `Traversable`, because `scanl1` and `scanr1`
 are not worth the extra dictionary weight.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9621#comment:2>
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