From: STARYNKEVITCH Basile <Basile.Starynkevitch@cea.fr>
Date: Mon, 5 Jul 1999 12:57:08 +0200 (MET DST)
To: caml-list@inria.fr
Subject: Q: parametric types in simple functors?
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.
---------------------------------------------------------------------
Basile STARYNKEVITCH ---- Commissariat à l Energie Atomique
DTA/LETI/DEIN/SLA * CEA/Saclay b.528 (p111f) * 91191 GIF/YVETTE CEDEX * France
phone: 1,69.08.60.55; fax: 1.69.08.83.95 home: 1,46.65.45.53
email: Basile point Starynkevitch at cea point fr
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