Haskell equivalent to Scala's groupBy

You can write the function yourself rather easily, but you need to place an Ord or Hashable constraint on the result of the classifier function if you want an efficient solution. Example:

import Control.Arrow ((&&&))
import Data.List
import Data.Function

myGroupBy :: (Ord b) => (a -> b) -> [a] -> [(b, [a])]
myGroupBy f = map (f . head &&& id)
                   . groupBy ((==) `on` f)
                   . sortBy (compare `on` f)

> myGroupBy (`mod` 2) [1..9]
[(0,[2,4,6,8]),(1,[1,3,5,7,9])]      

You can also use a hash map like Data.HashMap.Strict instead of sorting for expected linear time.

Specifically, the following should work:

scalaGroupBy f = groupBy ((==) `on` f) . sortBy (comparing f)

modulo that this doesn't get you the result of f in each group, but if you really need it you can always post-process with

map (\xs -> (f (head xs), xs)) . scalaGroupBy f

This isn't a function in the List library.

You can write it as the composition of sortBy and groupBy.