Re: subprogram must not be deeper than access type

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

Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org> writes:

> The problem is that there is nothing special about the main-program
> procedure. Specifically, it could be withed and called from something
> else, or call itself recursively. In either case, its declarative
> region, with its declaration of the function, would be nested deeper
> than the declaration of the access type. Since this is a compile-time
> check, it rejects anything that might be deeper, even when it never is.

Right.  Also, when the main procedure returns, that's not the end of
the program.  Library-level tasks can keep running for a long time
after that.

It's really annoying that in Ada you can't write a small example program
(with a single compilation unit in a single file) that has multiple
library-level things.  If I ran the circus, there would be no library
subprograms.  The main procedure would be declared inside a library
PACKAGE.

> The basic rule with named access-to-subprogram types is that the actual
> subprogram must be declared in a library-level package.

Right.

Just to clarify: library-level means not nested in any procedures,
functions, or tasks.  If the (named) access type is declared at library
level, then for P'Access, P must be declared at library level.  Nesting
within any number of packages doesn't count, for accessibility levels.

Anonymous access types work differently.

GNAT has P'Unrestricted_Access, which allows you to violate the
accessibility rules, but that's not portable.

- Bob