VHDL Issue Number:      0200
Classification:         Language Deficiencies and Modeling Problems
Language Version:       VHDL-87
Summary:                'SUCC, 'PRED, 'LEFTOF, and 'RIGHTOF are not defined on
                        objects.
Related Issues:         None.
Relevant LRM Sections:  14.1
Key Words and Phrases:  'SUCC, 'PRED, 'LEFTOF, 'RIGHTOF, predefined attributes
Current Status:         Submitted
1076-1993 Disposition:  Bugs Fixed, Enhancements Outstanding (No ISAC Issues)
Disposition Rationale:  Non-ISAC issues outstanding.
Superseded By:          N/A
-----------------------
Date Submitted:         1991/03/16
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: 1.9 $
Date Last Revised:      $Date: 1995/08/04 01:45:13 $


Description of Problem
----------------------
The predefined attributes 'SUCC, 'PRED, 'LEFTOF, and 'RIGHTOF are not
applicable to objects, only to types.  This restriction prevents their use to
iterate on the elements of unconstrained interface objects.  Such restrictions
make the following procedure illegal:

        procedure WhyNot(
          variable outbuf : out string;
                      in1 : in  string;
                      in2 : in  string
        ) is
          variable ox : INTEGER;
        begin
          ox := outbuf'LEFT;
          for i in in1'RANGE loop
            outbuf(ox) := in1(i);
            ox := outbuf'RIGHTOF(ox);   -- illegal
          end loop;
          for i in in2'RANGE loop
            outbuf(ox) := in2(i);
            ox := outbuf'RIGHTOF(ox);   -- illegal
          end loop;
          loop
            ox := outbuf'RIGHTOF(ox);   -- illegal
            outbuf(ox) := "_";
            exit when ox = outbuf'RIGHT;
          end loop;
        end WhyNot;

Note that this example could be done by using aliases for the parameters.

Note further that 'LEFT, 'RIGHT, 'HIGH, 'LOW, 'RANGE, and 'REVERSE_RANGE are
defined both on types and on objects.


Proposed Resolution
-------------------
Allow 'SUCC, 'PRED, 'LEFTOF, 'RIGHTOF to apply to array types and objects,
analogously to 'LEFT, 'RIGHT, 'HIGH, 'LOW, 'RANGE, and 'REVERSE_RANGE.  The
suggested entries for Section 14.1 are as follows.  Thise should immediately
follow A'LOW [(N)].

        A'SUCC (X[,N])
                Kind:           Function
                Prefix:         Any prefix A that is appropriate for an array
                                object, or an alias thereof, that denotes a
                                constrained array subtype.
                Parameter:      A locally static expression of type *universal_
                                integer*, the value of which must be positive
                                and must not exceed the dimensionality of A.
                                If omitted, it defaults to 1.
                Result Type:    Type of the Nth index range of A.
                Result:         T'SUCC(X), where T is the type of the Nth index
                                range of A.
                Restriction:    An error occurs if X equals T'BASE'HIGH.

        A'PRED (X[,N])
                Kind:           Function
                Prefix:         Any prefix A that is appropriate for an array
                                object, or an alias thereof, that denotes a
                                constrained array subtype.
                Parameter:      A locally static expression of type *universal_
                                integer*, the value of which must be positive
                                and must not exceed the dimensionality of A.
                                If omitted, it defaults to 1.
                Result Type:    Type of the Nth index range of A.
                Result:         T'PRED(X), where T is the type of the Nth index
                                range of A.
                Restriction:    An error occurs if X equals T'BASE'LOW.

        A'LEFTOF (X[,N])
                Kind:           Function
                Prefix:         Any prefix A that is appropriate for an array
                                object, or an alias thereof, that denotes a
                                constrained array subtype.
                Parameter:      A locally static expression of type *universal_
                                integer*, the value of which must be positive
                                and must not exceed the dimensionality of A.
                                If omitted, it defaults to 1.
                Result Type:    Type of the Nth index range of A.
                Result:         T'LEFTOF(X), where T is the type of the Nth
                                index range of A.
                Restriction:    An error occurs if X equals T'BASE'LEFT.

        A'RIGHTOF (X[,N])
                Kind:           Function
                Prefix:         Any prefix A that is appropriate for an array
                                object, or an alias thereof, that denotes a
                                constrained array subtype.
                Parameter:      A locally static expression of type *universal_
                                integer*, the value of which must be positive
                                and must not exceed the dimensionality of A.
                                If omitted, it defaults to 1.
                Result Type:    Type of the Nth index range of A.
                Result:         T'RIGHTOF(X), where T is the type of the Nth
                                index range of A.
                Restriction:    An error occurs if X equals T'BASE'RIGHT.


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


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


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