add_exception_function, add_echo_function

Arguments

Returns

t

Description

When a Gamma or Lisp program is run in conjunction with the real-time datahub, process points may change at any time, causing point change events to occur. A point change event is referred to as an exception. It is possible to bind any Gamma or Lisp expression to a symbol to be evaluated when an exception occurs. If a program can both write a point on the datahub and react to exceptions on that point, it is possible that the datahub will "echo" a point written by the program itself. If this is not handled, an infinite loop between the program and the datahub could occur. The datahub tags point echoes so that a different function can be called in the program when that echo arrives back at its origin. Only the originating task will see a point exception as an echo. All other tasks will see a normal exception.

When an exception handler (the s_exp argument) is being evaluated the special variables this, value and previous are all bound:

Example

Gamma> add_exception_function(#temp, #princ("temp change\n"));
(princ "temp change\n")
Gamma> add_echo_function(#temp,nil);
nil
Gamma> next_event();
temp change
(t)
Gamma> read_point(#temp);
30
Gamma> write_point(#temp,25);
t
Gamma> next_event();
(nil)
Gamma> 
		

See Also

register_point, when_echo_fns, when_exception_fns, remove_echo_function, remove_exception_function