Re: Generic-Package Elaboration Question / Possible GNAT Bug.

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

Shark8 <onewingedshark@gmail.com> writes:

> I was working on some simple little array manipulation functions when
> I encountered an interesting behavior in the compiler; it said that
> trying to instantiate a function of the form:
>    Generic
>       Type Element is Private;
>       Type Index_Type is (<>);
>       Type Array_Type is Array (Index_Type Range <>) of Element;
...
> But the question that came to my mind after reading the ARM section on
> the error (sorry, I forgot to write it down) is, why is it not static?

Generic formals are not static, because that would violate the generic
contract model.  That is, the compiler should be able to determine the
legality of the generic body without knowing anything about any
instantiations.

In your case statement, what if the range of Index_Type were 1..1?
Then 'First = 'Last, so there's a duplicate in your case statement
(which would be illegal in the instance, but we're not supposed to
know about the instance).

In general, the design of Ada is that if a generic body is legal,
then every possible instance is legal (so the compiler need not
check the instances).  The design of C++ templates is very different
in this regard!

> Is it because though the elaboration-parameters *may* be known [and
> static] at compile-time that some instantiation might NOT be
> [guaranteed] to be known at compile-time? (i.e. because the compiler
> cannot guarantee that you won't throw 1..N where N is something pulled
> from the user-input?)

Yeah, that's another example.

By the way, I suggest:

    type Index_Type is range <>;

Unconstrained arrays don't make a whole lot of sense when the index
type is enumeration or modular.

And if you add some assertions, to require that Index_Type'First >
Index_Type'Base'First, and Data'First = Index_Type'First, then
you can simplify your code -- you won't need the case statement.

Well, maybe you also need to assert that Data'Last < Index_Type'Base'Last.

Consider using Ada 2012 preconditions and/or predicates.

- Bob