add_set_function


`add_set_function`
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`add_set_function`

add_set_function — sets an expression to be evaluated when a given symbol changes value.

Synopsis

add_set_function (symbol, s_exp)

Arguments

symbol: A symbol.
s_exp: Any Gamma or Lisp expression.

Returns

t

Description

This function binds an expression to be evaluated whenever the value of the symbol changes. This expression is available globally, so if the value of the symbol changes during a change in scope, the expression will be evaluated. All changes in that sub-scope will trigger new evaluations of the expression. This can be used to automatically maintain consistency between the program and a DataHub instance, or to implement forward chaining in calculation rules. The expression will not be evaluated if the new value is eq to the previous value.

When a set expression (the s_exp) is being evaluated the special variables this, value and previous are all bound:

this The symbol whose value has changed.
value The current value of this as a result of the change.
previous The value of this immediately prior to the change.

Example

Gamma> b = 5;
5
Gamma> add_set_function(#b,#princ("changed\n"));
(princ "changed\n")
Gamma> b = 4;
changed
4
Gamma>

The following code automatically sounds an alarm whenever a computed_tank_level rises above 10000 and silences the alarm whenever it drops below 10000. The DataHub instance is automatically updated to maintain the same value as the Gamma task.

function send_to_datahub  (point)
{
	write_point(point, value);
}

function check_alarm (value)
{
	if (value == 1)
		sound_alarm();
	else
		silence_alarm();

	send_to_datahub(this);
}

function check_tank_level (depth)
{
	if (depth > 10000)
		high_alarm = 1;
	else
		high_alarm = 0;

	send_to_datahub(this);
}

add_set_function(#high_alarm, #check_alarm(high_alarm));
add_set_function(#computed_tank_level,#check_tank_level(computed_tank_level));