OCaml `List.group_by`

list-group-by.ml

let group_by (f : 'a -> 'b) (ll : 'a list) : ('b, 'a list) Hashtbl.t =
  List.fold_left
    (fun acc e ->
       let grp = f e in
       let grp_mems = try Hashtbl.find acc grp with Not_found -> [] in
       Hashtbl.replace acc grp (e :: grp_mems);
       acc)
    (Hashtbl.create 100)
    ll;;

(* Not working (yet) *)
let group_by (f : 'a -> 'b) (ll : 'a list) : ('b, 'a list) Hashtbl.t =
  List.fold
    ll
    ~init:(Hashtbl.create ~size:100)
    ~f:
    (fun acc e ->
       let grp = f e in
       let grp_mems = try Hashtbl.find acc grp with Not_found -> [] in
       Hashtbl.replace acc grp (e :: grp_mems))