Proposal to add mapAccumL function to vector package

[John Ky]

Hello,

I'd like to add the mapAccumL function to the vector package.

Specifically the Data.Vector.Storable module, but it would also be useful
other vector modules.

This is my attempt at an implementation:

{-# LANGUAGE ScopedTypeVariables #-}

mapAccumL :: forall a b c. (Storable b, Storable c)
=> (a -> b -> (a, c))
-> a
-> DVS.Vector b
-> (a, DVS.Vector c)
mapAccumL f a vb = DVS.createT $ do
vc <- DVSM.unsafeNew (DVS.length vb)
a' <- go 0 a vc
return (a', vc)
where go :: Int -> a -> DVS.MVector s c -> ST s a
go i a0 vc = if i < DVS.length vb
then do
let (a1, c1) = f a0 (DVS.unsafeIndex vb i)
DVSM.unsafeWrite vc i c1
go (i + 1) a1 vc
else return a0
{-# INLINE mapAccumL #-}

The implementation should obey the following law:

import qualified Data.List as L
import qualified Data.Vector.Storable as DVS

(DVS.toList <$> DVS.mapAccumL f a (DVS.fromList bs)) === L.mapAccumL f a bs

Cheers,

-John
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