Property change (CHANGE)

The property change operator creates an action that writes the value of an expression (source) into a property (destination) for every set of arguments where a third expression (condition) is not NULL. The condition may be omitted; in that case it is considered to always hold.

The source and the condition share the same arguments as the destination property. If the source evaluates to NULL for a set of arguments matched by the condition, NULL is written for that set, which erases the previously stored value.

Changeable properties

The destination property must be a mutable property. Mutable properties are:

• data properties, including local data properties;
• properties created by the selection operator — the write is distributed over the conditions in the order they are listed: each result property receives the part of the write for which its condition holds, and the untaken remainder passes to the following conditions. For the polymorphic form in which the condition is the result property itself, the condition of the write is the result's ability to accept the write as a mutable property, not its current value: a non-mutable result (for example, a constant) accepts nothing. So for a selection property with a mutable result and a constant as the default value, the write goes to the mutable result even while the current value comes from the constant;
• composition of mutable properties — the write is routed through the composition to the underlying mutable property at the resolved arguments.

info

In addition to the above, mutable properties are also properties created using the extremum operator and logical operators (which are basically varieties of the selection operator).

Language

To declare an action that implements property change, use the CHANGE operator.

Examples

// set a 15 percent discount for all customers who have an order amount over 100
CLASS Customer;
discount = DATA NUMERIC[5,2] (Customer);
totalOrders = DATA NUMERIC[14,2] (Customer);
setDiscount  {
    discount(Customer c) <- 15 WHERE totalOrders(c) > 100;
}

discount = DATA NUMERIC[5,2] (Customer, Item);
in = DATA BOOLEAN (Item);
// change the discount for selected products for a customer
setDiscount (Customer c)  {
    discount(c, Item i) <- 15 WHERE in(i);
}

// copy property g to property f
f = DATA INTEGER (INTEGER);
g = DATA INTEGER (INTEGER);
copyFG  {
    f(a) <- g(a);
}