Re: Ada conditional compilation and program variants

[Jacob Sparre Andersen <jacob@jacob-sparre.dk>]

Niklas Holsti wrote:

> An interesting question is this: if package declaration or body B uses
> package A, and package A has a definition, how far can a compiler
> check that B uses A in legal ways if the compiler is allowed to look
> only at the definition of A, but not at the (or a) declaration of A?
>
> If the compiler could check legality using only the package
> definitions of server packages, it would make it easier to ensure that
> all program variants are legal, by two separate and non-combinatorial
> steps:
>
> - checking each variant package declaration against the (invariant)
>   package definition, and
>
> - checking each variant package body against the (invariant) package
>   definitions of all other packages (those used by this body).
>
> If checking the legality of how B uses A requires looking at the
> declaration of A (and not only at the definition of A), there is again
> a risk of combinatorial explosion in checking the legality of all
> (complete) program variants when there are variant package
> declarations.

The only counter-examples I've been able to come up with so far are
run-time errors, such as going beyond the range of a numerical type.

What about enumeration types?  Should it be allowed to declare names for
some, but not all values of an enumeration type?  One could of course
declare a function returning a value of the type in the package
definition, and say that it can be implemented in the package
declaration as an enumeration value. - Can't do that for characters
though, so maybe it is a bad idea.

Greetings,

Jacob
-- 
"Genes don't matter. It's all physics."