Re: Proposal: Strict scanl, scanl1 and mapAccumL

On Tue, 13 Nov 2012, Takano Akio wrote:

On Mon, Nov 12, 2012 at 12:02:28PM +0100, Bas van Dijk wrote:

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On 12 November 2012 11:34, Bas van Dijk wrote:

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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?