Re: [Caml-list] Functors and classes

[brogoff <brogoff@speakeasy.net>]

Ha, I just tried whipping up an example and I couldn't write a Thing.

Oh well, classes/polymorphic variants and the type abstraction features of
functors are hard to use together!

On Tue, 3 Aug 2004, brogoff wrote:

> On Tue, 3 Aug 2004, John Prevost wrote:
> > Oops.  A couple of errors in that last thing I wrote that weren't the
> > problem I was trying to illustrate (that's what I get for trying fixes
> > inline before giving up):
> >
> > module type ThingPoolT =
> >   sig
> >     type connection
> >     val connect : unit -> connection
> >     val test : connection -> int
> >   end
> >
> > module ThingPool (Thing : ThingIntf) :
> >     (ThingPoolT with type connection = Thing.connection) =
> >   struct
> >     type connection = Thing.connection
> >     let connect () = Thing.connect ()
> >     let test x = x#a + 1
> >   end
>
> Certainly if you're going to call method a it needs to be exposed. If I
> understand the problems you're trying to demonstrate, the following would be
> my stab at it. Untested code, caveat emptor, blah blah blah...
>
> module type ThingIntf =
>   sig
>     type 'a connection =
>         'a constraint 'a = < a: int; connect : unit -> 'a connection; ..>
>     val connect : unit -> 'a connection
>   end;;
>
> module type ThingPoolT =
>   sig
>     include ThingIntf
>     val test : 'a connection -> int
>   end;;
>
> module ThingPool (Thing : ThingIntf) : ThingPoolT =
>   struct
>     include Thing
>
>     let connect () =
>       let conn = Thing.connect () in
>       ignore (conn#a);
>       conn
>
>     let (test : _ connection -> int) = fun x -> x#a + 1
>   end;;
>
> -- Brian
>
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