CHANGE operator

The CHANGE operator creates an action that changes properties.

Syntax

[CHANGE] propertyId(expr1, ..., exprN) <- valueExpr [WHERE whereExpr]

Description

The CHANGE operator creates an action that writes the value of valueExpr into the property at the arguments expr1, ..., exprN for every set of arguments where whereExpr is not NULL. The argument list may introduce new local parameters; such parameters correspond to objects being iterated and are not parameters of the created action.

Parameters

• propertyId

  ID of the property whose value is changed. The property must be mutable.

• expr1, ..., exprN

  A list of expressions or typed parameters defining arguments of the property being changed. When using typed parameters, you can both reference already declared parameters and declare new local parameters; when using expressions, new local parameters cannot be added. A new parameter is declared in this list as a separate argument (discount(Customer c) <- 15), while an argument expression can use only parameters declared already — parameters of the action or of an enclosing operator, or typed parameters declared to the left in the same list (discount(customer(o)) <- 15): writing into a computed key such as byId(LONG(Item i)) <- ... is rejected when the module is loaded, and such a parameter is introduced outside — for example, by the FOR operator (see the last example). The number of items in this list must equal the number of parameters of the property being changed.

• valueExpr

  Expression whose value is written into the property.

• whereExpr

  Expression whose value is the condition under which the value is written. If not specified, it is considered equal to TRUE.

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;
}

CLASS Item;
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);
}

// an argument expression over the action's parameter: the discount for the order's customer
CLASS Order;
customer = DATA Customer (Order);
setOrderDiscount (Order o)  {
    discount(customer(o)) <- 15;
}

// the key is an expression over a new parameter: it cannot be declared in the argument list, so i is introduced by the FOR operator
inById = DATA LOCAL BOOLEAN (LONG);
fillInById  {
    FOR in(Item i) DO inById(LONG(i)) <- TRUE;
}