Proposal to add mapAccumL function to vector package

[Carter Schonwald]

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 <newhoggy at 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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