gistfile1.ml

open Core.Std


let maybe_list this that =
  match this with
  | Some l ->
     Some (List.append l [that]);
  | None -> Some [that]
  
let group_by f l =
  let ht = Hashtbl.Poly.create () in
  List.fold
    ~f:(fun m el ->
        let res = f el and
            cur = Hashtbl.Poly.find ht el in
        Hashtbl.Poly.replace ht cur (maybe_list cur el);
        ht)
    ~init:ht l