Proposal: add uncons (and unsnoc) to Data.List

[Oliver Charles]

+1 for uncons, I've wanted that a few times.


On Sat, Jul 19, 2014 at 11:47 PM, David Feuer <david.feuer at gmail.com> wrote:

> Alexander Berntsen indicates that he has a branch all ready to go. Henning
> Thienemann seems to prefer the term viewL. Alexander says that Edward Kmett
> says that uncons/unsnoc is the more common term. Personally, I find uncons
> and unsnoc to be more intuitive names. Details:
>
> uncons :: [a] -> Maybe (a, [a])
> uncons [] = Nothing
> uncons (a:as) = Just (a,as)
>
> -- Henning's implementation of "viewR", renamed, looks like
>
> unsnoc :: [a] -> Maybe ([a], a)
> unsnoc = foldr (\x -> Just . forcePair . maybe ([],x) (mapFst (x:)))
> Nothing
>
> I wonder if it would be possible to tease that apart a bit to get the
> maybe out of the loop, which I think might be prettier. The really tricky
> part of unsnoc is making it lazy enough—messing around with it a bit
> suggested that it's very easy to mess up the pair lifting. The tough rule
> seems to be this mouthful, if I'm not mistaken:
>
> unsnoc (xs ++ (y : _|_)) = Just (xs ++ _|_, _|_)
>
> David
>
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