Re: Proposal to add mapAccumL function to vector package

I thought about it a teeny bit more This should be trivially definable using mapM or equivalent using state t Have you tried doing that simple high level definition? I think that works with vector fusion quite nicely. ... ohhh. I see. There’s two vectors of inputs. I’ll have to think about this more. On Thu, Nov 8, 2018 at 8:14 AM Carter Schonwald wrote:

Hey John : I’m happy to help you contribute to to vector.

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 )

2) writing it in stream fusion form / as map accum l for streams will actually be simpler I think

3) put a ticket on our GitHub. I’ve a huge backlog (life and stuff got me a bit slow), but this sounds like a super reasonable feature request

On Thu, Nov 8, 2018 at 6:31 AM John Ky 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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