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

We can also use the SQL-like then group by syntax in list comprehension, which requires TransformListComp language extension.

Since Scala groupBy returns a Map, we can call fromDistinctAscList to convert the list comprehension to a Map.

$ stack repl --package containers

Prelude> :set -XTransformListComp
Prelude> import Data.Map.Strict ( fromDistinctAscList, Map )
Prelude Data.Map.Strict> import GHC.Exts ( groupWith, the )
Prelude Data.Map.Strict GHC.Exts> :{
Prelude Data.Map.Strict GHC.Exts| scalaGroupBy f l =
Prelude Data.Map.Strict GHC.Exts|   fromDistinctAscList
Prelude Data.Map.Strict GHC.Exts|     [ (the key, value)
Prelude Data.Map.Strict GHC.Exts|     | value <- l
Prelude Data.Map.Strict GHC.Exts|     , let key = f value
Prelude Data.Map.Strict GHC.Exts|     , then group by key using groupWith
Prelude Data.Map.Strict GHC.Exts|     ]
Prelude Data.Map.Strict GHC.Exts| :}
Prelude Data.Map.Strict GHC.Exts> :type scalaGroupBy
scalaGroupBy :: Ord b => (t -> b) -> [t] -> Map b [t]
Prelude Data.Map.Strict GHC.Exts> scalaGroupBy (`mod` 2) [1, 2, 3, 4, 5, 6, 7, 8, 9]
fromList [(0,[2,4,6,8]),(1,[1,3,5,7,9])]

The only difference from Scala groupBy is that the above implementation returns a sorted map instead of a hash map. For implementation that returns a hash map, see my other answer at https://stackoverflow.com/a/64204797/955091.