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foldMap, foldMap' for PrimArray #186

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foldMap, foldMap' for PrimArray #186

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ff7f68a
foldMap, foldMap'
chessai Jul 15, 2018
6948536
update changelog
chessai Jul 16, 2018
853b996
Merge branch 'master' of github.com:haskell/primitive into foldable-p…
chessai Sep 17, 2018
af41b39
fix ordering of mappend in foldMapPrimArray
chessai Sep 17, 2018
0bb9dd6
have 4 foldMaps; two right, two left, two lazy, two strict, red fish,…
chessai Sep 17, 2018
8aaa9d1
bump quickcheck-classes for travis happiness
chessai Sep 17, 2018
96eb0cc
change fold{dir}MapPrimArray to foldMap{dir}PrimArray, and mention al…
chessai Sep 18, 2018
534a20c
provide an example to demonstrate association and accumulation, corre…
chessai Sep 18, 2018
6f0c84e
simplify examples for foldMapRPrimArray foldMapLPrimArray
chessai Sep 18, 2018
d4eb3d0
make strict foldMaps on PrimArray force function value at each step
chessai Sep 18, 2018
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12 changes: 12 additions & 0 deletions Data/Primitive/PrimArray.hs
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Expand Up @@ -66,6 +66,8 @@ module Data.Primitive.PrimArray
, foldlPrimArray
, foldlPrimArray'
, foldlPrimArrayM'
, foldMapPrimArray
, foldMapPrimArray'
-- * Effectful Folding
, traversePrimArray_
, itraversePrimArray_
Expand Down Expand Up @@ -467,6 +469,16 @@ sizeofPrimArray :: forall a. Prim a => PrimArray a -> Int
{-# INLINE sizeofPrimArray #-}
sizeofPrimArray (PrimArray arr#) = I# (quotInt# (sizeofByteArray# arr#) (sizeOf# (undefined :: a)))

-- | Lazily map each element of the primitive array to a monoid, and combine the results.
foldMapPrimArray :: forall a m. (Prim a, Monoid m) => (a -> m) -> PrimArray a -> m
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The documentation is insufficient. It should explain how the combination associates. In fact, I think we should almost surely offer more than one version: associating to the left, associating to the right, and (for certain tree-like monoids) associating in a balanced fashion are all potentially useful.

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Tree-like or in some cases numerical. Balanced association can reduce floating point errors somewhat, for example.

{-# INLINE foldMapPrimArray #-}
foldMapPrimArray f = foldrPrimArray (\a acc -> acc `mappend` f a) mempty
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-- | Strictly map each element of the primitive array to a monoid, and combine the results.
foldMapPrimArray' :: forall a m. (Prim a, Monoid m) => (a -> m) -> PrimArray a -> m
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The documentation is insufficient. It should explain which way the combination associates. Also, it's notably not (necessarily) strict in the function results. The documentation should reflect this and there should be a comment explaining that decision.

{-# INLINE foldMapPrimArray' #-}
foldMapPrimArray' f = foldlPrimArray' (\ !acc x -> acc `mappend` f x) mempty

-- | Lazy right-associated fold over the elements of a 'PrimArray'.
{-# INLINE foldrPrimArray #-}
foldrPrimArray :: forall a b. Prim a => (a -> b -> b) -> b -> PrimArray a -> b
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4 changes: 3 additions & 1 deletion changelog.md
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@@ -1,6 +1,8 @@
## Changes in version 0.6.4.1

* Add instances for the following newtypes from `base`:
* Add `foldMapPrimArray` and `foldMapPrimArray'`
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This needs to be updated now that you've added more functions.


* Add `Prim` instances for the following newtypes from `base`:
`Const`, `Identity`, `Down`, `Dual`, `Sum`, `Product`,
`First`, `Last`, `Min`, `Max`

Expand Down