Re: some trivial questions?

[Robert Eachus <rieachus@comcast.net>]

On Saturday, November 11, 2017 at 6:32:14 AM UTC-5, AdaMagica wrote:
> But the with-clause is not about dependence, it's only about visibility.

No!  Come on guys, the with clause does affect visibility, but it has a lot of other effects.  The use clause is only about (direct) visibility.  

However, the important thing which I have not seen mentioned here, is that the parsing for with clauses is completely different from the parsing for use clauses.  Assuming that the compiler has the concept of a library, the with clause searches the LIBRARY for compilation units and adds them to the program.  The use clause searches NAMES visible at the point of the use clause, and if legal adds the names visible in the named unit to the list of visible names. If the use clause is in a context clause, the names become (potentially) visible in the declaration following the context clause.

One of my two line compiler killers back in the early sixties (before compiler validations) was:

with Text_IO; use ASCII, Text_IO;
procedure Main is begin Put(Nul); end;

Special casing ASCII could handle this, but there are many other instances where a use clause in a context clause doesn't need to reference the same context clause, for example on the body of a package.

In 90+% of cases, a "with and use" clause would work.  It was even considered for Ada 9X.  But the problem with it was that the rules for what it meant--other than making it syntactic sugar for "with Foo; use Foo;" would be almost impossible to understand.  Trying to explain to a user why with and use failed was seen as too big an unnecessary addition to the language. Adding qualified names for compilation units (Ada.Text_IO) was considered much more important.