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From: eachus@goldfinger.mitre.org (Robert I. Eachus)
Subject: Re: Language lawyer needed
In-Reply-To: davidg@oracorp.com's message of Thu, 1 Apr 1993 21:57:53 GMT
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Date: Fri, 2 Apr 1993 16:40:13 GMT
Date: 1993-04-02T16:40:13+00:00
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In article <1993Apr1.215753.3147@oracorp.com> davidg@oracorp.com (David Guaspari) writes:

  > Consider the following pathological program:

      procedure Q(x : integer) is  
      begin
	declare
	  x : integer;   -- (1)
	begin
	  Q(x => 0);     -- (2)
	end;
      end Q;

  > My question concerns the legality of Q (forgetting about the fact that
  > its execution won't terminate).  One way to apply chapter 8 in the
  > reference manual goes as follows:

  >   The declaration of x in (1) hides the declaration of x as a formal
  >   parameter of Q.

  So the name of the parameter is no longer directly visible, but is
visible only by selection.  Okay.

  >   Therefore, by the visibility rules, the occurrence of x in (2) has
  >   exactly one possible meaning, namely that given by the declaration
  >   in (1). 

  >   However, the declaration in (1) is not visible by selection at the
  >   point at which x occurs in (2).

  >   Therefore the call in (2) is illegal.

  Nope.  8.3(6 & 11) say:

  Visibility is either by selection or direct.  A declaration is
visible by selection at places that are defined as follows.

  (e)  For a parameter specification of a given subprogram declaration
or entry declaration:  at the place of the formal parameter (before
the compound delimiter =>) in a named parameter association of a
corresponding subprogram or entry call.

   So not only is the parameter X visible by selection, it is the only
X visible so there is no problem.  The terminology here is a little
misleading.  The or in 8.3(6) is not a choice, it really stands for
"there are locations where visibility is determined using the rules
for selection, other rules apply everywhere else."  The first sentence
of paragraph 8.3(14) is the offender here, it is technically correct
but misleading: "Where it is not visible by selection, a visible
declaration is said to be _directly_ visible."  It says that
visibility by selection preempts direct visibility, but seems to say
that it applies on a name by name basis, rather than to the process of
visibility as a whole.



--

					Robert I. Eachus

with Standard_Disclaimer;
use  Standard_Disclaimer;
function Message (Text: in Clever_Ideas) return Better_Ideas is...