Advanced Perl Programming

Example: Monitoring Variables

tie makes it really convenient to monitor a variable. In this section, we will develop a module called Monitor.pm that prints out a message on STDERR whenever a variable of your choice is accessed. ^[34]

use Monitor;
monitor(\$x, 'x');
monitor(\%y, 'y');

Whenever $x or %y is changed, this module prints out something like this on STDERR:

Wrote   : $x ... 10
Read    : $x ... 10
Died    : $x
Wrote   : $y{a} ... 1
Cleared : %y

This module is useful while debugging, where it is not clear at what point a certain variable is changing, especially when it changes indirectly through a reference. This module can be enhanced to support watch expressions such as print 'ahhh' when $array[5] > 10. Given Perl’s support for eval, this is a reasonably simple task.

monitor takes a variable by reference and a name to be used when it prints out its messages. The first parameter is used to do a tie on the variable. tie has the unfortunate property that it hides the original value held by the variable. (The value is restored upon untie.) Clearly, we don’t want Heisenberg’s Uncertainty Principle to creep in here—our act of monitoring should not affect the user’s view of that variable. For this reason, we store away the original value as an attribute of the tied object and have FETCH and STORE use this copy. Finally, when we are not interested in the variable any more, we use unmonitor, which calls untie internally.

Monitor, shown in Example 9.3, delegates responsibility to a nested ...