### Re: Proposal: Strict scanl, scanl1 and mapAccumL

Bas van Dijk <v.dijk.bas <at> gmail.com>

2012-12-04 22:56:10 GMT

On 13 November 2012 11:06, Takano Akio <aljee <at> hyper.cx> wrote:
> Hi,
>
> On Mon, Nov 12, 2012 at 12:02:28PM +0100, Bas van Dijk wrote:
>> On 12 November 2012 11:34, Bas van Dijk <v.dijk.bas <at> gmail.com> wrote:
>> > I just realized that mapAccumL' is not needed since the caller has the
>> > ability to force the accumlator. So please ignore that part of my
>> > proposal. This leaves just scanl' and scanl1' as orignally proposed by
>> > Niklas Hambüchen.
>>
>> Oops scrap that. After thinking about it more and testing it I realize
>> the caller really doesn't have control over the evaluation order in
>> the function passed to mapAccumL. So please consider my original
>> proposal again.
>
> scanl' and scanl1' can be naturally defined using their non-strict
> counterparts like:
>
> scanl' :: (b -> a -> b) -> b -> [a] -> [b]
> scanl' f z xs = headStrict $ scanl f z xs
>
> scanl1' :: (a -> a -> a) -> [a] -> [a]
> scanl1' f xs = headStrict $ scanl1 f xs
>
> headStrict :: [a] -> [a]
> headStrict = foldr (\x y -> seq x (x : y)) []
>
> So the `headStrict' function could be exported from Data.List instead. A
> nice thing about it is that it can also be used with other
> list-generating functions like `iterate' to make it strict.
>
> It's also possible to define mapAccumL' in terms of mapAccumL, but the
> corresponding forcing function would not look as nice as `headStrict'
> above, so I would think providing mapAccumL' is a good idea.
>
> Regards,
> Takano Akio
>
Thanks Takano, headStrict looks like a useful function.
I would like to drop the proposal for adding scanl', scanl1' and mapAccumL'.
Cheers,
Bas
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