infinite lists, lazy evaluation and length

Question

Haskell noob here: I\'m still trying to understand the mechanics of the language, so if my question is plain stupid, forgive me and point me to some link which I can learn from

Answer 1

Another trick you can do (which I've seen in Data.Text, but am surprised is not in Prelude for lists in general) is to make length short-circuit as soon as possible by returning an Ordering rather than a Bool.

compareLength :: [a] -> Int -> Ordering
compareLength [] n = compare 0 n
compareLength _ 0 = GT
compareLength (x : xs) n = compareLength xs (n - 1)

Then you can use it in chunks.

chunks :: Int -> [a] -> [[a]]
chunks n xs = case compareLength xs n of
                   LT -> [xs]
                   _  -> let (ch, rest) = splitAt n xs in ch:chunks n rest

And this works fine.

*Main> take 4 $ chunks 3 [1..]
[[1,2,3],[4,5,6],[7,8,9],[10,11,12]]

For this particular case, other implementations might be more idiomatic, but hopefully this is a nice trick to know.