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

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.