VHDL Issue Number:      1054
Classification:         Language Definition Problem
Language Version:       VHDL-93
Summary:                Short-circuit operators don't when called as functions
Related Issues:         Supersedes 0252
Relevant LRM Sections:  7.2, 7.2.1, 7.3.3
Key Words and Phrases:  Short-circuit operators, expressions, function calls
Current Status:         ISAC-Approved
1076-1993 Disposition:  N/A
Disposition Rationale:  N/A
Superseded By:          N/A
-----------------------
Date Submitted:         1991/03/12
Author of Submission:   Paul Menchini
Author's Affiliation:   Menchini & Associates
Author's Post Address:  2 Davis Drive
                        P.O. Box 13036
                        Research Triangle Park, NC  27709-3036
Author's Phone Number:  919.990.9506
Author's Fax Number:    919.990.9507
Author's Net Address:   mench@mench.com
-----------------------
Date Analyzed:          8 June 1995
Author of Analysis:     John Willis (jwillis@acm.org)
Revision Number:        $Revision: 1.8 $
Date Last Revised:      $Date: 1995/06/20 23:04:03 $


Description of Problem
----------------------
Consider the following two expressions:

    1.  a and b

    2.  "and" (a, b)

These two expressions do not have identical semantics if the operator in
expression 1 and the operator denoted by the prefix in expression 2 denote
the same short-circuit operator.

Specifically, in expression 1, if the value of a is sufficient to determine
the entire expression's value, b is never evaluated.  In contrast, in
expression 2, b is always evaluated.

The LRM is clear on this point.  The first sentence of the last paragraph of
Section 7.3.3 states (in part):

        Evaluation of a function call includes evaluation of the actual
        parameter expressions specified in the call ....

No exception is made for function names that denote short-circuit operators.


Proposed Resolution
-------------------
Rework the language of the final paragraph of Section 7.3.3 to allow short-
circuit operators called as functions to evaluate their second argument only
as necessary to determine the value to be returned by the call.


VASG-ISAC Analysis & Rationale
------------------------------
In Section 2.3.1 of the LRM, Note 2 speaks only of user-defined operators:

"A user-defined operator that has the same designator as a short-circuit 
 operator (this is, that overloads the short-circuit operator) is not 
 invoked in a short-circuit manner.  Specifically, calls to the user-defined
 operator always evaluate both arguments prior to the execution of the 
 function".
 
Thus user defined operators are never short circuited.
 
Except for user-defined operators, Section 2.3.1, Note 2 appears to be 
completely consistent with the last paragraph of Section 7.3.3, which 
states:

"Evaluation of a function call includes evaluation of the actual parameter
 expressions specified in the call and evaluation of the default expression
 expressions associated with the formal parameters of the function that do
 not have actual parameters associated with them."
 
Note that LRM Section 2.3.1 Note 3 establishes the duality of operator
and function call notations:

"Functions that overload operator symbols may also be called using function
 call notation rather than operator notation.  This is also true of the
 predefined operators themselves".
 
For example:

	variable a, b: bit;
	
	a and b
	
	is just another notation for
	
	"and" (a, b)

However according to the LRM, the two notations have distinct semantics
(one call is short-circuit, one is not).

Having distinct semantics for predefined operator (short circuited) and 
function call notation (never short circuited) is well-defined by the '93
LRM however this seems like a distinction which violates the duality which
Section 2.3.1 Note 3 tries to establish.

VASG-ISAC Recommendation for IEEE Std 1076-1993
-----------------------------------------------

In order to maintain the duality of operator and function call notations
for predefined operators, LRM Section 7.3.3 should be modified as follows:

"Function calls which denote a predefined, short-circuit operator must 
 be converted into the equivalent operator form before evaluation.  
 Evaluation of remaining function calls includes both an evaluation of 
 the actual parameter expressions specified in the call and evaluation 
 of the default expressions associated with formal parameters of the 
 function call which do not have actual parameters associated with them.  
 In both cases ... "
 
Note addition of the first sentance clarifying treatment of calls to
predefined, short-circuit operators and exclusion of such calls from
the second (originally first) sentance.