Q: parametric types in simple functors?

[STARYNKEVITCH Basile <Basile.Starynkevitch@cea.fr>]

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The subject title is poor. I have a concrete simple question.

(All CAML code is indented to the right here)
Consider

   type 'a symbol = { sy_name: string; sy_hash: int; mutable sy_prop: 'a option };;

This is essentially a symbol, memoizing its hash code for fast
access. Some structures are naturally hashed map, whose key are
symbols (eg bindings). We make symbols with

  let get_symbol n =
    try Hashtbl.find symdict n 
    with Not_found ->
     let s = { sy_name=n; sy_hash=Hashtbl.hash n; sy_prop=None } in
      Hashtbl.add symdict n s;
      s
  ;;


The property field of symbols could be used the way you like it.

I want to define a binding as an hashtable mapping symbols to (say)
integers - or even any type 'b. But I can't code:

  module SymbolHash = Hashtbl.Make(
     struct 
       type t = symbol
       let equal s1 s2 = s1 == s2
       let hash {sy_hash=h} = h
     end)
;;

Because the type constructor t has no argument in the
Hashtbl.HashedType signature.

Any comments or suggestions (could define a class abstract_symbol
with a subclass symbol holding the properties. But I don't want
classes because I don't need inheritance).


The intent is that the symbol properties would be defined in another
module. I don't want to fix the 'a in symbol_t here, yet I want to
make an hashtable whose keys are symbols.

In other words, 

  why the stdlib doesn't give parametrized types for many modules
  (Hashtbl, Set...)

  when should I use a type constructor with an argument, or a functor?


Any comments?


N.B. Any opinions expressed here are only mine, and not of my organization.
N.B. Les opinions exprimees ici me sont personnelles et n engagent pas le CEA.

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