VHDL Issue Number:      0116
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Wording with regard to the termination of loop
                        statements is ambiguous.
Related Issues:         None.
Relevant LRM Sections:  8.8
Key Words and Phrases:  Loop statement
Current Status:         ISAC-Approved
1076-1993 Disposition:  Closed (All Issues Completely Addressed)
Disposition Rationale:  section 8.9 of VHDL93 corrects this issue.
Superseded By:          N/A
-----------------------
Date Submitted:         1989/06/21
Author of Submission:   Ken Scott
Author's Affiliation:   Vantage Analysis Systems, Inc.
Author's Post Address:  42840 Christy St., Suite 201
                        Fremont, CA   94538
Author's Phone Number:  (415) 659-0901
Author's Fax Number:    
Author's Net Address:   N/A
-----------------------
Date Analyzed:          1991/04/01
Author of Analysis:     chuck swart (cswart@mentor.com)
Revision Number:        $Revision: 1.7 $
Date Last Revised:      $Date: 1995/05/13 19:34:42 $


Description of Problem
----------------------
In the third paragraph of chapter 8.8, there is a statement which is a correct
statement, but is misleading:

       "Execution of the loop statement is complete when the loop is left as a
        consequence of the execution of a next statement, an exit statement, or
        a return statement."

In the context of the sentence preceding it, it appears to still be talking
about loops without iteration schemes.  Such loops can NEVER be left as a
result of the execution of a 'next' statement.

If is it attempting to describe all the ways in which to leave a loop, it
should also mention the completion of the iteration scheme for the loop as an
option.

All in all, the sentence is open to many different interpretations.  It would
be wise to make it more specific.


Proposed Resolution
-------------------
Break paragraph 3 into two separate paragraphs.  So that the reader is not
mislead into thinking that the 2nd sentence relates in any way to the first.


VASG-ISAC Analysis & Rationale
------------------------------
The paragraph in question reads:

       "A loop statement without an iteration scheme specifies repeated
        execution of the sequence of statements. Execution of the loop
        statement is complete when the loop is left as a consequence of the
        execution of a next statement, an exit statement, or a return
        statement."

This paragraph was adopted from the Ada LRM.  The first statement is taken
literally from Ada.  The corresponding second sentence in the Ada LRM reads:

       "Execution of the loop statement is complete when the loop is left as a
        consequence of the execution of an exit statement, or as a consequence
        of some other transfer of control (see 5.1)."

The annotated Ada LRM does not make any note of the fact that the second
sentence is not directly related to the first.

It is not clear just why the second sentence was changed from the original Ada.
Perhaps it was an attempt to clarify the sentence by explicitly listing those
statements which cause transfer of control.

In any event, the author raises two points, both of which are correct.

    1.  The two sentences of the third paragraph of chapter 8.8 are only
        loosely connected.  The second sentence contains references to concepts
        which do not apply to iteration schemes.  Therefore, it makes sense to
        break paragraph three into two separate paragraphs.

    2.  The second sentence is not complete as stated.  (It appears that the
        corresponding sentence in Ada is also incomplete.)  Indeed, a loop
        statement can be left as a result of completion of the iteration
        scheme.


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
Adopt the proposed solution. Read paragraph three of section 8.8 as if each of
the two sentences were a separate paragraph.


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
Re-word the sentence starting

       "Execution of the loop statement is complete...."

to reflect the fact that execution is complete either if the statement contains
an iteration scheme and the iteration scheme completes, or when the loop is
left as a consequence of the execution of a next statement, an exit statement,
or a return statement.