I seem to have this function as "replaceBy": replace :: Eq a => [a] -> a -> [a] -> [a] replace sl@(_ : _) r = replaceBy $ \ l@(hd : tl) -> case stripPrefix sl l of Nothing -> (hd, tl) Just rt -> (r, rt) replaceBy :: ([a] -> (b, [a])) -> [a] -> [b] replaceBy splt = unfoldr (\ l -> if null l then Nothing else Just (splt l)) Cheers Christian Am 13.12.2010 18:17, schrieb Lennart Augustsson:
I would like to propose the following function for inclusion in Data.List
chop :: (a -> (b, [a]) -> [a] -> [b] chop _ [] = [] chop f as = b : chop f as' where (b, as') = f as
It's commonly occuring recursion pattern. Typically chop is called with some function that will consume an initial prefix of the list and produce a value and the rest of the list.
The function is clearly related to unfoldr, but I find it more convenient to use in a lot of cases.
Some examples -------------
-- From Data.List group :: (Eq a) => [a] -> [[a]] group = chop (\ xs@(x:_) -> span (==x) xs)
-- From Data.List words :: String -> [String] words = filter (not . null) . chop (span (not . isSpace) . dropWhile isSpace)
-- From Data.List lines :: String -> [String] lines = chop ((id *** dropNL) . span (/= '\n')) where dropNL ('\n':s) = s; dropNL s = s
-- From Data.List tails :: [a] -> [[a]] tails = (++ [[]]) . chop (\ xs@(_:xs') -> (xs, xs'))
-- From Data.List map f = chop (\ (x:xs) -> (f x, xs))
-- Split a list into a list of list with length n. splitEveryN n = chop (splitAt n)
-- Simple Haskell tokenizer tokenize = chop (head . lex)
History -------
I first encountered this function around 1981 when I was talking to Sören Holmström about this recursion pattern and he said that he had also observed it and he called the function chopList. Ever since then I've used chopList a lot, but unfortunately I always have to make my own definition of this common function.
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