Re: Crosspost: Help wanted from comp.compilers

["Robert I. Eachus" <rieachus@attbi.com>]

Hyman Rosen wrote:

> I'm starting to remember an earlier discussion on this subject, (that
> I don't think I was involved in). If I recall correctly, it's valid to
> define "exist in the environment" as "source code is readable" and to
> define "compile" as "check that the code is legal". Then object file
> generation doesn't have to be tied to the requirements that you are
> talking about, so the issue can be sidestepped (while still remaining
> true to both the letter and spirit of the Ada requirements).

NO! Wrong. You can define a simple text copy as putting a unit in the 
environment.  In effect GNAT among others does this for package specs. 
But there is no way to get around the ordering requirements, and there 
should not be.  Some people get confused by the situation with generics. 
  Modifying a generic body may seem like it should invaldate instances 
of the generic, but that is why the generic contract model is there.

An implementation can however make the "compile" process a simple 
check-in and defer code generation until link time.  I think Rational 
may still do that.  But the whole dependency checking thing is one of 
the major strengths of Ada.  Trying to short-cut it has been discovered 
time and again to not be worth the effort--whether or not it can be made 
to work.

Similarly there was a lot of screaming about "elaboration checks" and 
access-before-elaboration issues in early Ada.  The elaboration rules in 
Ada 83 (and Ada 95) were adopted fairly late in the standardization 
process.  But once compiler vendors got used to them, the issue 
disappeared from most Ada programmer's event horizons.

This is in part why I say that at least 80% of the Reference Manual 
deals with what happens when you try to shoot yourself in the foot.  For 
most programmers who understand basic software engineering principles, 
all they need to know about overload resolution is that it works.  All 
they need to know about semantic dependence is that sometimes when they 
call gnatmake (or whatever compiler) it will recompile some units they 
didn't touch.  And they may never see Program_Error raised by an 
elaboration check, because almost all of the work is done in the main 
program, not in the sequence of statements of a package body.  (In fact, 
I wonder how many package bodies even have a sequence of statements...)

-- 

                                          Robert I. Eachus

�In an ally, considerations of house, clan, planet, race are 
insignificant beside two prime questions, which are: 1. Can he shoot? 2. 
Will he aim at your enemy?� -- from the Laiden novels by Sharon Lee and 
Steve Miller.