On 29 April 2010 15:48, matthew coolbeth
I understand that higher-order functions are incredibly powerful, and that you can do essentially anything you might ever want while using only 'map' and 'foldl'.
Hello In this case neither map nor foldl are adequate as both provide an 'elementary' consumption pattern i.e. one item is consumed at each step. mapAccumL can be seeded with a state tracking the previous element, though as others have said you might want to define your own recursion scheme: replace2 :: (a -> a -> a) -> a -> [a] -> [a] replace2 f2 initial xs = snd $ mapAccumL fn initial xs where fn a x = (x, f2 a x) -- some example replaceCharAfterA :: String -> String replaceCharAfterA = replace2 fn 'Z' -- seed 'state' with 'Z' (wont match) where fn 'A' _ = '*' fn _ b = b demo1 = replaceCharAfterA "abcABC"
demo1 "abcA*C"
Best wishes Stephen