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.