I actually find the OCaml hash table implementation to be very lacking and
have thus written my own, which probably isn't as efficient.  Nevertheless
you can find it at http://www.isi.edu/~hdaume/hashtable.ml.  It requires
garray.ml(i) found at the same location.  It doesn't have a keys function,
but such a thing is easy to write:

> let get_keys = fold (fun l k _ -> k :: l) []

You could probably do it marginally more efficiently by looking at the
actual implementaiton.

The reason I wrote this not-incredibly hash table implementation is
essentially because I needed a map function for hash tables and there was
no way to implement that on top of the current HashTbl interface.  I would
be more than happy to go back to the standard interface if it supported
maps.

--
Hal Daume III

 "Computer science is no more about computers    | hdaume@isi.edu
  than astronomy is about telescopes." -Dijkstra | www.isi.edu/~hdaume

On Tue, 18 Feb 2003, Oliver Bandel wrote:

> Hello,
> 
> the Hastbl-module is a fine thing. To have functions like
> Hastbl.find_all is convenient for a lot of problems, which would
> be not so easy to solve, when only hastbl.find would be in
> that library.
> 
> Well... and Hashtbl.iter seems to work on *all* antries in the
> *whole* hash-table, that is: A Hastbl.find_all  for each of the
> keys in that table.
> At least it is, how I understand the decription in the manual.
> 
> Well, but Hashtbl.iter is used for unit-functions.
> But I need bool-functions, and maybe other tyopes
> too.
> 
> So, if I want to apply a function with return-type different then
> unit, I have to use a second Hash-table to remember, which
> keys I have inside my main hastbl (using hashtbl.replace).
> 
> Or I may have to use Hastbl.replace instead of Hastbl.add
> in my main Hashtbl, where my new data-entry is a list of
> the old entry with the new entry appended.
> 
> But that all would be easier, if there would be a function
> Hashtbl.keys, which give back all keys of a hashtbl.
> 
> 
> So, Hashtbl.keys would have the following type:
> 
> ('a, 'b) t -> key 'a list
> 
> 
> With that function, I could use things like
> 
> ##################################################
> let myhash = Hashtbl.create 100000;;
> 
> Hashtbl.add myhash "k1" "hellO";;
> Hashtbl.add myhash "k2" "xxx";;
> Hashtbl.add myhash "k3" "iuzwiuezriuw";;
> 
> Hashtbl.add myhash "k1" "la la la";;
> Hashtbl.add myhash "k1" "shoo bi doo";;
> 
> Hashtbl.find_all (Hashtbl.keys myhash) 
> 
> (*
>    But the later function could also be named
>    Hashtbl.values
>    and this one may makes sense in the Hashtbl-module
>    too, but a Hashtbl.keys would be much more necessary!
> 
>    let all_values_of_a_hash hash = Hashtbl.find_all(Hashtbl.keys hash)
> 
>    But I think, the implementation could be more efficient, when
>    doing the latter function directly in the underwood of the
>    Hashtbl-module itself.
> 
> *)
> ##################################################
> 
> 
> Wouldn't it a good idea to add such a
> function "get all keys of a hashtable"
> into the hashtbl-module? (And a "get all values..." to?)
> 
> Would that be hard work, or can it be added easily?
> 
> 
> What workaround makes sense until such a funtion will
> be available?
> Should I use a second hash to save all keys
> of the main-hash?
> Or what is a good way to solve this problem?
> 
> 
> Ciao,
>    Oliver
> 
> 
> P.S.: For a possible Ocaml-advocacy discussion I want to drop
>       that note here:
> 
>       I can't believe that programming such problems in perl
>       should be easier than in Ocaml... in perl there is a function
>       "keys" as well as a function "values" for working on hash-tables.
> 
> 
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