1). This might not actually be needed with stream fusion on ... though perhaps this sort of shared computation needs to be its own combinators because of the sharing meaning that the stream fusion on map and foldl might not work. (Have you tried doing let x = map ... in let y = foldl ... in something with x and y? Even eg just writing map accum l in terms of just that? It could very well fuse ... though I don’t think it can with the current vector fusion framework. Though I think one of the more exotic fusion frameworks Amos Robinson did a few years ago could handle that fusion. Sadly that one requires an ILP solver at compile time. But there’s some tricks I think we could do )

On Thu, Nov 8, 2018 at 6:31 AM John Ky <newhoggy@gmail.com> wrote:

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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