module Algorithms.List.BasicOperations.Span where
import Control.Arrow (first)
import Data.Functor.Foldable
span is a Prelude function and its spec is a following:
-- @
-- span :: (a -> Bool) -> [a] -> ([a], [a])
-- span p xs = (takeWhile p xs, dropWhile p xs)
-- @
We can implement span as an instance of paramorphism.
-- | >>> spanPara even [2, 4, 7, 8]
-- ([2,4],[7,8])
spanPara :: (a -> Bool) -> [a] -> ([a], [a])
spanPara p = para phi
where
phi = \ case
Nil -> ([], [])
Cons a (as, b) -> if p a then first (a :) b else ([], a : as)