On Sat, 4 Nov 2006, Bulat Ziganshin wrote:
Hello libraries,
similar to mapFst and Co, we can define:
-- |Map various parts of list mapHead f [] = [] mapHead f (x:xs) = f x : xs
mapTail f [] = [] mapTail f (x:xs) = x : map f xs
mapInit f [] = [] mapInit f xs = map f (init xs) : last xs
mapLast f [] = [] mapLast f xs = init xs : map f (last xs)
btw, two last operations may be a bit slow. it's possible to define initLast function that returns both parts and still lazy?
In order to make such operations more efficient I defined: {- | It holds @splitLast xs == (init xs, last xs)@, but 'splitLast' is more efficient if the last element is accessed after the initial ones, because it avoids memoizing list. -} splitLast :: [a] -> ([a], a) splitLast [] = error "splitLast: empty list" splitLast [x] = ([], x) splitLast (x:xs) = let (xs', lastx) = splitLast xs in (x:xs', lastx) propSplitLast :: Eq a => [a] -> Bool propSplitLast xs = splitLast xs == (init xs, last xs)