On Wed, 21 Apr 2004, Oliver Bandel wrote:

>       Hashtbl.keys <hash>  returns a list of all keys of the
>       hash "hash", that exists and therefore have values bound
>       to it. If a key has several bindings, it will occur only
>       once in this list.

For that purpose, it is possible instead of a (key, data) Hashtbl.t to use
a (key, data list ref) Hashtbl.t:

let add tbl key data =
  let r =
    try Hashtbl.find tbl key
    with Not_found ->
      let r = ref [] in
      Hashtbl.add tbl key r;
      r in
  r := data :: !r

let fold f tbl init =
  Hashtbl.fold (fun key r accu -> f key (List.hd !r) accu) tbl init

let keys tbl =
  fold (fun key _ accu -> key :: accu) tbl []


Martin

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