How-to: Explicit typing

In certain situations, you may want to use the same name for different system properties or actions.

For example, let's create two properties named sum: one of them will calculate the sum of the order line, and the second will calculate the sum of the entire order:

sum = DATA NUMERIC[10,2] (OrderDetail);
sum = GROUP SUM sum(OrderDetail od) BY order(od);

Accordingly, the first property gets one parameter of the OrderDetail class as input while the second gets one parameter of the Order class.

But if we create, for example, a constraint with one parameter and then try to refer to the sum property without explicitly specifying the class of this parameter, an error will occur:

All such references require an explicitly specified class:

CONSTRAINT sum(Order o) < 0 MESSAGE 'The order amount must be positive';

The class of a parameter introduced inside an expression is specified in the same way — at the parameter's first use. For example, a typical task is to find an object by the value of its property (here the name property is declared on several classes):

currency (STRING[10] name) = GROUP MAX Currency currency IF name(currency) = name;

The class of a parameter cannot be specified with the AS operator: in GROUP MAX currency AS Currency IF name(currency) = name the currency parameter remains untyped — AS creates a property that returns the value only if it belongs to the class, but it does not change the class of the parameter itself, so the name(currency) reference produces the same error.