Re: [Caml-list] 'a Set?

["Frédéric Gava" <frederic.gava@wanadoo.fr>]

Hi

> Mike Hamburg writes:
>  > Is there any clean way to make a type 'a set, corresponding to Set.Make
>  > of a module with type t='a and compare=Pervasives.compare?  I'm trying
>  > to make a module which uses sets of arbitrary types of objects, and I
>  > don't want to have to make it a functor.
>
> This is a recurrent question on this list.
>
>  > Is there a clean way to do this without removing the code from set.ml
>  > and modifying it?
>
> Unfortunately, no.
> Note that when duplicating the code from set.ml, you can either keep a
> functorized code, with an additional type parameter:

I think this problem comes from the stdlib of OCaml. We have:
1) For the Hashtable
 type ('a, 'b) t
and
val length : ('a, 'b) t -> int
and
module type S = sig
 type key
 type 'a t
  .... end
 2) For the Map
module type S = sig
  type key
  type +'a t
end
3) For the Set
module type S = sig
  type elt
  type t
val cardinal : t -> int
end

This is hard to understand why the signatures of those modules are so
differents. Why there is (for example) not for the set, this signature:
module type S = sig
    type 'a elt
    type 'a t
    ...
  end ?
Furthermore there is no lenght/cardinal function for the Map. Note that
WeakHashtble have the function val count : t -> int, so three different
names for the same things. When I teach Ocaml, many students are lost with
this differences.

Best regards,
Frédéric Gava