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IntSet foldl and foldr smell funny #666

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Tracked by #1059
treeowl opened this issue Jul 17, 2019 · 1 comment · May be fixed by #667
Open
Tracked by #1059

IntSet foldl and foldr smell funny #666

treeowl opened this issue Jul 17, 2019 · 1 comment · May be fixed by #667
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IntSet performance

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@treeowl
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treeowl commented Jul 17, 2019 •
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The implementations of foldl'Bits and foldr'Bits look efficient. The implementations of foldlBits and foldrBits, on the other hand, don't look efficient for their purposes. I'd expect foldlBits to look more like foldr'Bits, and I'd expect foldrBits to look more like foldl'Bits.
@treeowl
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treeowl commented Jul 17, 2019

Roughly speaking:

foldrBits !prefix f z = go
  where
        go 0 = z
        go bm = f (prefix + bi) $ go (bm `xor` bitmask)
          where
                !bitmask = lowestBitMask bm
                !bi = indexOfTheOnlyBit bitmask

treeowl added a commit to treeowl/containers that referenced this issue Jul 18, 2019
@treeowl
Improve foldl and foldr for IntSet
7278be6 
`foldrBits` was previously defined just like `foldr'Bits`, and
`foldlBits` was defined just like `foldl'Bits`. That is, each of them
built up an accumulator, lazily, across a whole leaf before producing
anything. Swap that around so the folds can process values promptly.

Fixes haskell#666
@treeowl treeowl linked a pull request Jul 18, 2019 that will close this issue
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Improve foldl and foldr for IntSet treeowl/containers
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@treeowl @sjakobi