Re: Generic procedures and their parameters

[Robert A Duff <bobduff@shell01.TheWorld.com>]

Maciej Sobczak <no.spam@no.spam.com> writes:

> But then, you would need to have even such fundamental things like
> Integer belong to Ordered'Class. It's a shaky ground.

Why is that shaky?  Integer IS ordered.

> > Regarding Array_Type being implied, it is not. You can have any number of
> > array types based on exactly same combination of index and element types.
> > These types will be different types:
> > type I is ...;
> > type E is ...;
> > type Array1 is array (I range <>) of E;
> > type Array2 is array (I range <>) of E;
> 
> > The types Array1 and Array2 are distinct types.
> 
> And now I see. I'm still having troubles with the fact that
> "granularity" of type system in Ada can be much smaller than what can be
> achieved with structural differences only.

It's a good thing that Ada allows Array1 and Array2 above to be
different types.  But sometimes, structural typing would be better.  In
fact, Ada has moved a bit in that direction -- anonymous access types
use structural typing, more or less.

> > The next step would be getting "<" from the element type,
> > which would eliminate a need to have Sort generic.
> 
> No, I disagree. There is a place for generic subprograms, otherwise we'd
> have everything in a single hierarchy with Object at the top. It's not a
> coincidence that "other languages" drift towards supporting generics,
> even if they already had such single-rooted hierarchies.

A hierarchy with Object at the top would be a Good Thing, IMHO.  Trees
are more aesthetically pleasing than forests.  But you're right that you
still need some sort of generic facility, so you can have strongly typed
containers.  But I think perhaps we should have generic types
(i.e. parameterized types) rather than generic packages.  That's what
discriminants do, but they're severely limited.  It would make sense to
have a discriminant that is a type, so you could say:

    My_Sequence: Sequence(Element => Integer) := (1, 2, 3);

- Bob