[Caml-list] Hashtbl.keys

[oliver@first.in-berlin.de (Oliver Bandel)]

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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