splitWhenPrev: A function somewhat similar to groupBy, but instead of using the first element of a group to determine when to start a new group, it compares adjacent values

SplitWhenPrev.hs

{-# LANGUAGE LambdaCase #-}
import Data.List (unfoldr)


both :: (a -> b) -> (a,a) -> (b,b)
both f (x,y) = (f x, f y)

withPrevious :: [a] -> [(a, Maybe a)]
withPrevious xs = zip xs (Nothing : map Just xs)


splitWhenPrev :: (a -> a -> Bool) -> [a] -> [[a]]
splitWhenPrev p = unfoldr f
  where f [] = Nothing
        f seed = Just 
               . both (map fst)
               . break (\case (a, Just b) -> p b a
                              _           -> False)
               . withPrevious 
               $ seed

{- 
-- Usage example (split when monotonicity is violated):
ghci> splitWhenPrev (>=) [1, 2, 3, 5, 4, 7, 6, 8, 9]
[[1,2,3,5],[4,7],[6,8,9]]
-}