consecutive.hs

import Data.List (intercalate)

consecutivesStrings :: [Int] -> String
consecutivesStrings = intercalate "," . map f . groupBy' (\a b -> a + 1 == b)
  where
    f [x] = show x
    f xs  = "[" ++ show (head xs) ++ "-" ++ show (last xs) ++ "]"

groupBy' :: (a -> a -> Bool) -> [a] -> [[a]]
groupBy' p (x : xs) = go [x] xs
    where
      go (x : xs) (y : zs) | p x y = go (y : x : xs) zs
      go g (y : zs) = reverse g : go [y] zs
      go g [] = [reverse g]