sendmail, 4th Edition

Conditionals Can Nest

V8 sendmail allows conditionals to nest. As an example, consider the following expression:

$?x $?y both $| xonly $. $| $?y yonly $| none $. $.

This is just like the example in the previous section:

$?x text1 $| text2 $.

except that text1 and text2 are both conditionals:

text1 = $?y both $| xonly $.
text2 = $?y yonly $| none $.

The grouping when conditionals nest is from the outside in. In the following example, parentheses have been inserted to show the groupings (they are not a part of either expression):

($?x  (text1)  $|  (text2)  $. )
($?x  ($?y  both $| xonly $. )  $|  ($?y  yonly $| none $. )  $. )

Interpretation is from left to right. The logic of the second line is therefore this: if both $x and $y have values, the result is both. If $x has a value but $y lacks one, the result is xonly. If $x lacks a value but $y has one, the result is yonly. And if both lack values, the result is none.

The sendmail program does not enforce or check for balance in nested conditionals. Each $? should have a corresponding $. to balance it. If they do not balance, sendmail will not detect the problem. Instead, it might interpret the expression in a way that you did not intend.

The depth to which conditionals can be nested is limited only by our ability to easily comprehend the result. More than two deep is not recommended, and more than three deep is vigorously discouraged.