Function using foldr is too eager
groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]
groupBy' f = foldr step []
    where step x [] = [[x]]
          step x (xs:xss)
              | x `f` head xs = (x:xs):xss
              | otherwise     = [x]:xs:xss

groupBy' f = foldr step []
  where step x xss = let (ys, yss) = step' x xss in (x:ys):yss
        step' x [] = ([], [])
        step' x (xs:xss) | f x (head xs) = (xs, xss)
                         | otherwise     = ([], xs:xss)

*Main> groupBy' (<) [1, 2, 3, 2, 3, 4, 1, undefined]
[[1,2,3],[2,3,4],[1*** Exception: Prelude.undefined

groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]
groupBy' f [] = []
groupBy' f [x] = [[x]]
groupBy' f (x : xs)
  | x `f` head xs = (x : head l) : tail l
  | otherwise = [x] : l
  where
    l = groupBy' f xs

groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]
groupBy' f [] = []
groupBy' f (x : xs) = hd : tl
  where
    (hd, tl) = go x xs
    go x [] = ([x], [])
    go x xs@(x' : xs')
      | x `f` x' = (x : hd', tl')
      | otherwise = ([x], hd' : tl')
      where
        (hd', tl') = go x' xs'