Proposal: Strict scanl, scanl1 and mapAccumL
lemming at henning-thielemann.de
Tue Nov 13 11:38:47 CET 2012
On Tue, 13 Nov 2012, Takano Akio wrote:
> 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.
Can this be achieved with Strategies as well?
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