VHDL Issue Number:      0172
Classification:         Language Definition Problem
Language Version:       VHDL-87
Summary:                Rules for non-local access to signals and variables in
                        functions are too restrictive.
Related Issues:         None.
Relevant LRM Sections:  2.2
Key Words and Phrases:  Non-local signal and variable access within functions,
                        nested procedures, pure functions, side effects
Current Status:         Submitted
1076-1993 Disposition:  Superseded (ISAC Issues Outstanding)
Disposition Rationale:  N/A
Superseded By:          1035
-----------------------
Date Submitted:         1991/03/15
Author of Submission:   Paul Menchini
Author's Affiliation:   CAD Language Systems, Inc.
Author's Post Address:  P.O. Box 13036
                        Research Triangle Park, NC  27709-3036
Author's Phone Number:  (919) 361-1913
Author's Fax Number:    
Author's Net Address:   mench@clsi.com
-----------------------
Date Analyzed:          TBD
Author of Analysis:     TBD
Revision Number:        $Revision: 1.10 $
Date Last Revised:      $Date: 1995/05/13 21:53:48 $


Description of Problem
----------------------
Consider the following VHDL fragment:

        procedure P1 (...) is
          variable V: ...;

          procedure P2 (...) is
          begin
            -- manipulates V
          end P2;
        begin
          P2 (...);
        end P1;

        function F (...) return ... is
        begin
          P1 (...);
          return ...;
        end F;
          :
          :
        ... F(...)..

This fragment is illegal because of the restrictions of paragraph 7 of Section
2.2 (the one on page 2-5 that starts with "If a function subprogram is a
parent....").  However, this fragment meets the spirit and the intent of the
restriction, in that neither the function, nor any subprogram invoked by the
funcition, refers to a signal or variable whose lifetime is not a subset of the
function invocation's lifetime.  Therefore, the function call is still "pure."


Proposed Resolution
-------------------
Relax the restrictions of paragraph 7 of Section 2.2 to allow the above
fragment while still preserving the purity of functions.


VASG-ISAC Analysis & Rationale
------------------------------
TBD


VASG-ISAC Recommendation for IEEE Std 1076-1987
-----------------------------------------------
TBD


VASG-ISAC Recommendation for Future Revisions
---------------------------------------------
TBD