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

Since Scala groupBy returns an immutable HashMap, which does not require ordering, the corresponding Haskell implementation should return a HashMap as well.

import qualified Data.HashMap.Strict as M

scalaGroupBy :: (Eq k, Hashable k) => (v -> k) -> [v] -> M.HashMap k [v]
scalaGroupBy f l = M.fromListWith (++) [ (f a, [a]) | a <- l]