Question

I find myself often using a pattern where I transform a list with a function that consumes 1..n elements from the list and produces some result out of those. E.g.

process :: [a] -> [b]
process [] = []
process xs = first : rest
    where (first, xs') = consume xs
          rest         = process xs'

Where the consume function consumes a variable number of items from the list and returns a result and the remaining list items. Can I use some standard higher order function here instead of the explicit recursion?

Answer 1

The function you need is similar to unfoldr. If you would bring your consume into the following form:

consume' :: [a] -> Maybe (b,[a])

Where consume' returns Nothing in case of an empty list, or Just ... otherwise. This is a small wrapper, that captures that pattern:

wrap :: ([a] -> (b,[a])) -> [a] -> Maybe (b,[a])
wrap f [] = Nothing
wrap f xs = Just $ f xs

Then you can use consume with unfoldr with the original definition of consume :: [a] -> (b,[a]) like this:

unfoldr (wrap consume)