oneShot (was Re: FoldrW/buildW issues)

[David Feuer]

Yes, and it does a very good job in many cases. In other cases, it's
not as good.

On Tue, Oct 7, 2014 at 7:59 AM, Sophie Taylor <sophie at traumapony.org> wrote:
> 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
>> _______________________________________________
>> ghc-devs mailing list
>> ghc-devs at haskell.org
>> http://www.haskell.org/mailman/listinfo/ghc-devs
>
>