obsolete vs. removed from the library

obsolete vs. removed from the library

[Bryan]

Dear Ada Fans (if you're not a fan of Ada, I don't care that you read this),

I'm confused again... is there a difference between "obsolete" compilation
units and units "removed" ("deleted" for you VMS fans) from the library?
Consider:

	package P is end P;

	with P;
	procedure Q is begin null; end;

	package P is end P;

	with P;
	procedure Q is begin null; end;

The second compilation of P makes Q obsolete.  It also makes the implicit
specification of Q (which is in the library) obsolete.  Does this mean
that the second compilation of the body of Q is invalid since it is
a secondary unit of an obsolete spec?

This is related to a question I sent out a while ago, but never received
any replies to.  Are there only two library states associated with a unit,

	1. not in the library; never successfully compiled.
	2. in the library; ready to elaborate

??

Or are there three library states assocaited with a unit?

	1 - 2 as above
	3. obsolete; not ready to elaborate, but was successfully
	   compiled once

doug
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Re: obsolete vs. removed from the library

[John B. Goodenough]

Doug Bryan asked a question using the following example:

	package P is end P;

	with P;
	procedure Q is begin null; end;

	package P is end P;

	with P;
	procedure Q is begin null; end;

He asserted that "the second compilation of P ... makes the implicit
specification of Q (which is in the library) obsolete."  He then went on to
ask "Does this mean that the second compilation of the body of Q is invalid
since it is a secondary unit of an obsolete spec?"

The problem with this question is that it's based on an invalid assumption,
namely, that there is an implicit specification for procedure Q that is the
library unit.  I can well understand why Doug may have this interpretation,
since it was my understanding of the RM at one time also, but when the
Language Maintenance Panel/Committee considered this issue (in AI-00199), it
concluded that there is no implicit specification for procedure Q that serves
as a library unit; instead, procedure Q's body is itself the library unit.
Specifically, AI-00199 says:

	A subprogram body  given in a  compilation unit (following  the
	context clause)  is  interpreted as  a  secondary unit  if  the
	program library already  contains a  subprogram declaration  or
	generic subprogram declaration with the same identifier; it  is
	otherwise interpreted as a library unit.

Now the consequences of this approved interpretation are that when P is
recompiled, library unit Q is made obsolete, so the second compilation of
procedure Q must be accepted; it is not a secondary unit of an obsolete
(implicit) specification.

"Obsolete" and "not in the library" are semantically equivalent, although a
given implementation may treat these states differently.  (In particular, an
implementation might "recompile" very quickly units that are obsolete, but
physically "in the library".)
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obsolete vs. removed from the library

[amn]

According to LRM 10.1:6  a subprogram body is treated as a secondary
unit if the program library already contains a library unit (= a sub-
program declaration) that is a subprogram with the same name.
This means that to recompile a library subprogram the declaration
or context clauses of which have been changed one has to compile
a genuine subprogram specification without the corresponding body
as a separate compilation unit.

LRM 10.3:5 states that obsolete compilation units must be recompiled
unless they are no longer needed.  I think this means you cannot
recompile a subprogram body if its specification is obsolete, as
the recompilation needs the specification.

I am not sure whether there is any need for having obsolete
compilation units in the library.  I think that 10.3:5 means that
obsolete compilation units cannot be utilized.  The only reason
to have obsolete compilation units in a program library could be
that the compilation library tools can decide which compilation
units need to be recompiled after an 'obsoleting' compilation.

Ari Mujunen
Helsinki University of Technology
(mcvax!penet!hut!amn, I suppose)