On Sat, 2007-02-24 at 15:58 +0100, Andreas Rossberg wrote:
> "skaller" <skaller@users.sourceforge.net>:
> >
> > It seems like a module functor allows both anonymous
> > signatures (structural) and also anonymous argument
> > modules (structural), yet you cannot have
> > anonymous functor applications: you have to bind the application to
> > a module name.
> 
> Not at all. For instance, given
> 
>   module Id(X : sig type t end) = X
>   module A = struct type t = int end
> 
> it is perfectly legal to write:
> 
>   module A' = Id(Id(Id(A)))
> 
> Obviosuly, the inner applications of Id are all "anonymous".
> 
> Likewise, you can say
> 
>   (3 : Id(Id(A)).t)
> 
> Also purely anonymous applications.

I had no idea you could do that!


> You can more or less do that already, as long as you introduce a suitable 
> global module to host the integer type:
> 
>   module Int = struct type t = int let compare = compare end
> 
>   signature A = sig ... val foo : u -> Set.Make(Int).t -> unit ... end
>   signature B = sig ... val bar : v -> w -> Set.Make(Int).t ... end
> 
> Admittedly, the type looks a bit ugly, and it would be even nicer if Int was 
> in the stdlib. But that are merely a questions of library design.

[..]

> Due to the "weak syntactic notion of module equivalence" I was mentioning 
> earlier you have to make sure that all these type expressions really refer 
> to the same Int module. This is a limitation of OCaml's module type system, 
> and may be what sometimes gives the impression of "nominal" typing. 

Yes .. I think I see. Two such modules 'Int' would have the same type,
but not the same 'identity'. In a similar way that

module Int' = struct type t = int let compare = mycompare end

has the same type as Int .. but it's a different module because
the value of 'compare' member is different.

I think what you're saying also explains some of the weird
sharing constraints sometimes seen.

[When I tried to implement modular functors in Felix I couldn't
figure out what was going on and had to give up .. typeclasses
were easier, but it's discouraging to find you can only have
one total order defined for integers.. :]


-- 
John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net