Branching (CASE, IF, MULTI)

The branching operator creates an action that determines for a set of conditions which condition is met, and calls the corresponding action.

All conditions are defined as properties and/or parameters. Accordingly, a condition is met if the value of the property or parameter by which it is set is not equal to NULL.

Conditions are checked in the order written; the first met condition selects the action to be called, and the remaining conditions are not checked. You can also specify an alternative action that is called only if none of the conditions is met.

Interruption, next iteration and exit signals raised by the called action are passed on to the surrounding action — the branching operator itself does not consume them.

Polymorphic form

This operator also allows defining a condition implicitly — the condition is that the arguments belong to the parameter classes of the action corresponding to that condition. We will call this the polymorphic form of the operator. The polymorphic form is the natural way to dispatch on the class of an argument.

Mutual exclusion of conditions

The branching operator lets you specify that all its conditions are mutually exclusive. If this option is set, and the conditions are not in fact mutually exclusive, the platform will throw the corresponding error.

The general form is non-exclusive by default and may be marked exclusive explicitly. The polymorphic form is mutually exclusive by default — implementations for disjoint argument classes do not overlap. The single form has exactly one condition and does not use mutual exclusion.

Implicit definition

The general and polymorphic forms support implicit definition through the technique of extensions. The single form has one fixed condition and does not support implicit definition.

Single form

The single form of the branching operator checks exactly one condition. If this condition is met, the specified action is called. It is also possible to specify an alternative action that is called if the condition is not met.

Language

To declare an action implementing general form of branching, the CASE operator is used. For the single form of branching, the IF operator is used, and for the polymorphic form the MULTI operator.

Examples

test = DATA INTEGER (INTEGER);
caseActionTest(a)  {
    CASE
        WHEN test(a) > 7 THEN MESSAGE '>7';
        WHEN test(a) > 6 THEN MESSAGE '>6';
        WHEN test(a) > 5 THEN MESSAGE '>5';
}

// Action that compares the value of the count property to 3 and displays a message to the user
moreThan3(obj)  {
    IF count(obj) > 3 THEN
        MESSAGE '>3';
    ELSE
        MESSAGE '<=3';
}

checkNullName (Store st) {
    IF NOT name(st) THEN
        MESSAGE 'Name is null';
}

CLASS Shape;

CLASS Square : Shape;
CLASS Circle : Shape;

message (Square s)  { MESSAGE 'Square'; }
message (Circle c)  { MESSAGE 'Circle'; }

message (Shape s) = MULTI message[Square](s), message[Circle](s);