The functions with return value are standard functions that occur in
expressions. On the basis of their evaluation, they gain the value that is
then implied into the expression evaluation.
Control functions (with a void return value) are functions that
may not occur in an expression, because they do not gain the value. Their
evaluation is characterized by the execution of the specific operation, on
the basis of the function parameters. Such functions may be used only in a
script, not in the expression, e.g. for object of
I/O tag type.
All up-to-now implemented control functions are used only in the
context of ESL script within the frame of Active
The control function in ESL script, are written like other functions.
This is the
example of notation of the control function %HI_VisiblePicture, that hides
or shows the picture according to one value of its one parameter.
%HI_VisiblePicture(@FALSE) ; hides the picture
%HI_VisiblePicture(@TRUE) ; shows the picture
The incorrect using of the control function within the frame of the