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Functorized stdlib ???
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Date: -- (:)
From: Jacques GARRIGUE <garrigue@k...>
Subject: Re: Functorized stdlib ???

>>>>>  <bravier@dassault-avion.fr> writes:

 > Here is a suggestion about the standard ocaml library Hashtbl.

 > This module is very useful but it happens to uses the polymorphic
 > equality ( = ) : 'a -> 'a -> bool.

 > Unfortunately, there are cases where you want a hash-table with another
 > equality predicate, for example you might want physical equality ( == ).

 > This is precisely what functors are for !

I have a doubt about the need for modifying specifically Hashtbl for
that.  In fact, there is already a Map module for that. The fact you
have to define a C function (to be efficient) limits the variants you
may have with your functor anyway.

But the general parameterizing problem you expose is interesting.

 > It seems to me that functors and polymorphism might/should go together well.

 > At any rate I would like a way to conceive modules and functors that
 > do not forbid polymorphism because, if you look at the short example :

 > module type ORDER =
 > sig
 >    type 'a t
 >    val ( <= ) : 'a t -> 'a t -> bool
 > end

 > It is really conter-intuitive since I have to parameterize the type t
 > with 'a before any use of type t.

 > What I first wrote (which does not work in the end) was :

 > module type ORDER_first =
 > sig
 >    type t
 >    val ( <= ) : 'a t -> 'a t -> bool
 > end

 > this is what I meant but OrderPoly was not of module type ORDER_first,
 > that's why I finally had to choose type 'a t.

 > Unfortunately this is no real solution since a predicate over
 > couples, say (fun (x1,y1) (x2, y2) -> y1 <= y2) : 'a * 'b -> 'a * 'b -> bool
 > will not fit in module type ORDER whose type 'a t has only one parameter.

 > Well, well, if somebody has clues ...

A (partial solution) would be to use classes, and in my opinion this
is more natural. You generally use only few hashtables/maps of the
same type in the same program, and having to define a module for each
type is a pain. With a class, you can either give the parameter when
creating an object, or define specialized classes by inheritance (as
you would do with a functor). The most interesting point is that you
don't need to prefix your method names by a strange module name: this
is taken from the object.

Here is the interface for map (imperative style : there is an internal
state and modifications return unit, but one could do it functional
style):

class ('a, 'b) map ('a -> 'a -> int) =
  method clear : unit
  method add : 'a -> 'b -> unit
  method find : 'a -> 'b
  method remove : 'a -> unit
  method iter : ('a -> 'b -> unit) -> unit
end

The parameter is the comparison function which you usually give to the 
functor.

Either use it directly

  # let m = new map compare;;
  m : ('_a,'_b) map = <object>

or create a specialized class, integer maps for instance

  # class 'a imap () =
      inherit (int,'a) map (-)
    end

There is still the problem that, at the moment, all polymorphic
variables in methods have to be bound at the class level.
Here this means that we cannot define fold:

  method fold : ('c -> 'a -> 'b -> 'c) -> 'c -> 'c

is not allowed : 'c should be bound at the class level, but then this
is meaningless. You can still simulate it with iter, so this is not
that bad.

Remark also that since functors bind type constructors rather then
types themselves, the map class cannot be defined using the functor,
and the source code has to be modified.

 > By the way, this technique of using paramerized types can also be used
 > to modify the Set module of stdlib to get a polymorphic set type
 > (just change type t into type 'obj t and type elt into type 'obj elt !)

Again I can define a set class, and avoid the problem with multiple
parameters. (I shall distribute this class library soon.)

 > This does not seem much but it enables the user to share code (as I do not
 > know whether applying functors duplicates code or not) and most of all, to
 > use only one module (no need to create modules all over the code ...)

Code is not duplicated, so the reason is mostly making program clearer 
by avoiding the multiplication of modules. We have the same concern.

 > Please, feel free to be controversial :-)

I think I've been :-)

In fact there is here a choice to be done between using
object-oriented features or not. In my opinion object-oriented style
is easier to understand, but modules and functors may be more
"functional". Not only Map and Set, but also Genlex, Hashtbl, Queue,
Stack and Stream can be turned OO with some profit.

In the current version Objective Caml allows both styles, but only
provides support for the first one: there are no predefined classes.
A concern is efficiency, but well implemented there is no reason that
it should be slower than Smalltalk, which is widely used.

On this point, it would be nice to have a clearer view of what
Objective Caml is going to be, if somebody knows.

	Jacques Garrigue