Haskell equivalent to Scala's groupBy

Question

Scala has a function groupBy on lists that accepts a function for extracting keys from list items, and returns another list where the items are tuples consisting of

Answer 1

This solution will break and group by on (f x), regardless wether it is sorted or not

f = (`mod` (2::Int))

list = [1,3,4,6,8,9] :: [Int]


myGroupBy :: Eq t => (b -> t) -> [b] -> [(t, [b])]

myGroupBy f (z:zs) = reverse $ foldl (g f) [(f z,[z])] zs
  where
    -- folding function                        
    g f ((tx, xs):previous) y = if (tx == ty)
                           then (tx, y:xs):previous
                           else (ty, [y]):(tx, reverse xs):previous
        where ty = f y                        

main = print $ myGroupBy f list

result: [(1,[1,3]),(0,[4,6,8]),(1,[9])]