group_by.ml

open Core_kernel.Std

let group_by l f =
  List.fold_left l ~init:[] ~f:(fun groups x ->
    let y = f x in
    let group =
      match List.Assoc.find groups y with
      | Some xs -> x :: xs
      | None    -> [x]
    in
    List.Assoc.add groups y group
  )

  group_by [1; 2; 3; 4; 5; 6; 7; 8; 9; 10] (fun x -> x mod 2);;
- : (int, int list) Core.Std.List.Assoc.t =
[(0, [10; 8; 6; 4; 2]); (1, [9; 7; 5; 3; 1])]
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