oneShot (was Re: FoldrW/buildW issues)

David Feuer david.feuer at
Tue Oct 7 07:05:13 UTC 2014

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)

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)

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 make sure it doesn't go wrong when used sanely.


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