Partitionby.hs

partitionBy :: Eq b => (a -> b) -> [a] -> [[a]]
partitionBy f [] = []
partitionBy f [x] = [[x]]
partitionBy f l = partitionBy' f (head l) (tail l)

partitionBy' _ x [xs] = [x : [xs]]
partitionBy' f x xs@(x':_)
  | (f x) == (f x') = (x : (head generator)) : (tail generator) -- the recursion is delayed here by lazily creating a generator
  | otherwise = [x] : generator
  where
    generator = partitionBy' f (head xs) (tail xs)

-- Tests

{-
infiniteSeq = (repeat 2)
l = [1,6,3,13] ++ infiniteSeq ++ [18,8]

let c = paritionBy (\x -> x `mod`5) l

> take 2 c
[[1,6],[3,13]]

> take 7 $ (c !! 2) -- the infinite sequence in between
[2,2,2,2,2,2,2]

-}