On Sat, Jul 19, 2014 at 6:47 PM, David Feuer
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)
+1 for uncons. Yes, uncons is a view, but I see no benefit in naming the function after that fact— given that "uncons" is the standard/popular name, and that we don't typically name any other views "viewFoo".
-- 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 ++ _|_, _|_)
unsnoc :: [a] -> Maybe ([a],a) unsnoc [] = Nothing unsnoc (x:xs) = Just $ go x xs where go x [] = ([], x) go x (y:ys) = let ~(zs,z) = go y ys in (x:zs, z) I've never had any reason to use it. But since the implementation is tricky, I'm +1 if other folks want it. -- Live well, ~wren