2.hs

import Data.List

groupByFirst :: (Eq a) => [(a, b)] -> [(a, [b])]
groupByFirst = map (trim . attach) . groupBy eq
  where
    eq (x, _) (y, _) = x == y
    attach xs = (x, xs)
      where (x, _):_ = xs
    trim (a, abs) = (a, map snd abs)

main :: IO ()
main = do
  print $ groupByFirst [(0, 1), (0, 2), (1, 3), (1, 4)]

-- [(0,[1,2]),(1,[3,4])]