Action Specification

An action specification (AS) is a named sequence of instructions. This sequence of instructions will be invoked by the system whenever the values of any source variables, specified explicitly in a list, are changed. The term ``changed'' is causally defined. It may mean the value of a variable is different from the previous one, or the value of a variable is overwritten (by the user or by the system) though the value may be the same as the previous value. EDEN takes the latter definition. This definition of ``changed'' is used throughout this handbook unless otherwise specified.

An example of an action definition is:

proc print_sum : a, b, c { writeln("a+b+c=", a+b+c); }

This action, named as print_sum, will invoked by the system whenever the values of the source variables a, b or c (listed after the colon) are changed. The action will print a string and the sum of the values of these variables on the screen due to the side-effect of the pre-defined function writeln.

The action execution manager (AEM) is responsible for invoking the action. This sub-system is called the action execution management sub-system.

Block Diagram of the Action Execution Management Sub-system

The arrows indicate the possible data flow in the system. Shaded arrows indicate the data flow due to the side-effects of action execution. From this diagram, we see that the actions are invoked by the AEM. AEM loads the procedure of the appropriate action from the AS and then executes it. The execution may cause a write to a (or some) RWV, or (re)define FV's or AS's (using some tricks). Of course, the action may also access I/O devices (not shown in the diagram).