TAL Programmer's Guide

Using Procedure Options
Using Procedures
11–12 096254 Tandem Computers Incorporated
Passing Parameters Conditionally
As of the D20 release, you can pass a parameter or parameter pair to a VARIABLE or
EXTENSIBLE procedure based on a condition at execution time by using the
$OPTIONAL function. $OPTIONAL is evaluated each time the encompassing CALL
statement is executed:
If the conditional expression is true, the actual parameter is passed.
If the conditional expression is false, the actual parameter is not passed.
In the following example, parameter pair S:I is passed because I equals 1, which is less
than 9. Parameter J is not passed because J equals 1, which is not greater than 2.
PROC p1 (str:len, b) EXTENSIBLE;
STRING .str;
INT len;
INT b;
BEGIN
!Lots of code
END;
PROC p2;
BEGIN
STRING .s[0:79];
INT i:= 1;
INT j:= 1;
CALL p1 ($OPTIONAL (i < 9, s:i), !Pass S:I if I < 9.
$OPTIONAL (j > 2, j) ); !Pass J if J > 2.
END;
You can use $OPTIONAL and $PARAM when one procedure provides a front-end
interface for another procedure that does the actual work:
PROC p1 (i, j) EXTENSIBLE;
INT .i;
INT .j;
BEGIN
!Lots of code
END;
PROC p2 (p, q) EXTENSIBLE;
INT .p;
INT .q;
BEGIN
!Lots of code
CALL p1 ($OPTIONAL ($PARAM (p), p ),
$OPTIONAL ($PARAM (q), q ));
!Lots of code
END;