oneShot (was Re: FoldrW/buildW issues)

[Sophie Taylor]

Wait, isn't call arity analysis meant to do this by itself now?

On 7 October 2014 17:05, David Feuer <david.feuer at gmail.com> wrote:

> Just for the heck of it, I tried out an implementation of scanl using
> Joachim Breitner's magical oneShot primitive. Using the test
>
> scanlA :: (b -> a -> b) -> b -> [a] -> [b]
> scanlA f a bs = build $ \c n ->
>     a `c`
>     foldr (\b g x -> let b' = f x b in (b' `c` g b'))
>           (const n)
>           bs
>           a
>
> scanlB :: (b -> a -> b) -> b -> [a] -> [b]
> scanlB f a bs = build $ \c n ->
>     a `c`
>     foldr (\b g -> oneShot (\x -> let b' = f x b in (b' `c` g b')))
>           (const n)
>           bs
>           a
>
> f :: Int -> Bool
> f 0 = True
> f 1 = False
> {-# NOINLINE f #-}
>
> barA = scanlA (+) 0 . filter f
> barB = foldlB (+) 0 . filter f
>
>
> with -O2 (NOT disabling Call Arity) the Core from barB is really,
> really beautiful: it's small, there are no lets or local lambdas, and
> everything is completely unboxed. This is much better than the result
> of barA, which has a local let, and which doesn't seem to manage to
> unbox anything. It looks to me like this could be a pretty good tool
> to have around. It certainly has its limits—it doesn't do anything
> nice with reverse . reverse  or  reverse . scanl f b . reverse, but it
> doesn't need to be perfect to be useful. More evaluation, of course,
> is necessary.to make sure it doesn't go wrong when used sanely.
>
> David
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