> Sent by:     owner-caml-list@pauillac.inria.fr

> To: caml-list@inria.fr
> cc:
> Subject:     Re: [Caml-list] mutability analysis too strict?



> "Ohad Rodeh" <ORODEH@il.ibm.com> writes:

> > First of all, sorry my mail was sent twice, this was due to mail
delivery
> > problems
> > from my site.
> >
> > Perhaps I was not specific enough about what I wanted to acheive. What
I
> > need
> > is a repository that has the following interface:
> >
> > module type Repos = sig
> >   val put : 'a -> 'b -> unit
> >   val get : 'a -> 'b
> > end
> >
> > An  implementation that looks like this:
> >
> > module S : Repos = struct
> >    let h = Hashtbl.create 10
> >    let put key data = Hashtbl.add h key data
> >    let get key = Hashtbl.find h key
> > end
> >
> > Does not work. Compilation error:
> >
> > Signature mismatch:
> > Modules do not match:
> >   sig
> >     val h : ('_a, '_b) Hashtbl.t
> >     val put : '_a -> '_b -> unit
> >     val get : '_a -> '_b
> >   end
> > is not included in
> >   Repos
> > Values do not match:
> >   val put : '_a -> '_b -> unit
> > is not included in
> >   val put : 'a -> 'b -> unit
> >
> > I tried also using the Map and Set modules, but they don't really allow
> > building
> > a repository of immutable values either. Is there any deep reason for
this
> > behavior? Could you expound on this line:
> >
> > > A monomorphic, mutable
> > > structure that contains polymorphic data is sound, but cannot be
> > expressed
> > > in ML's type system where universal quantification must be prenex.
> >
> > Ohad.



Remi VANICAT <vanicat@labri.u-bordeaux.fr>@pauillac.inria.fr on 10/12/2001
11:28:42

> it's dangerous. Imagine you can do :
>
> put 2 "xd"
>
> what will be the type of :
>
> get 2
>
> how the type inference algorithm can know that it is a string ? it
> can't because you can do something like :
>
> if x < y then put 2 "xd"
> else put 2 5
>
> and then, the type of get 2 wil be string or int depending on the fact
> that x was less than y or not.



Well, that's a strong argument ...

How do you propose building a repository of polymorphic values then?

     Ohad.


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