Properties

A property is an element of the system that takes an object collection (parameters) and returns exactly one object (the return value).

The kind and the specifics of how to calculate each property are determined by the operator used to create the property.

Data and calculated properties

Properties are either data or calculated properties. The values of a data property are stored and are changed by actions. The values of a calculated property are obtained by the rule of the operator that created the property — usually from the values of other properties — and are stored only if the property is materialized. Some calculated properties can be changed too.

Absence of a value

Instead of an object, a property can return the special value NULL, which means there is no value: for example, a data property is NULL for every object collection for which nothing has been written to it.

Value and parameter classes

There is no class that all objects belong to: strings and numbers, for example, have no common ancestor. So, for the same parameter classes, the values of a property must belong to classes with a common ancestor: a property that returns a number for some values of a numeric parameter and a string for others, for instance, is rejected by the platform at startup.

The list of a property's parameter classes is called its signature. Parameter classes are set when the property is declared, and a class that is not set is inferred from the property's definition. The class of a parameter may also remain undetermined. The signature serves first of all to find a property. A parameter's class by itself does not always restrict the objects for which the property has a value: that depends on the operator that created the property. For example, the parameter classes of a data property must be specified, and if at least one object of a collection does not belong to the class of its parameter, the value is NULL.

The number of parameters of a property is fixed. A property may also take no parameters at all.

Language

A property is declared with the = statement, which sets the property's name, its parameters, and the operator that creates the property.

Examples

CLASS Item;

// a property associates each Item with a value kept in the database
price (Item i) = DATA NUMERIC[14,2] (Item);

// a property can be calculated from other properties
// for an item without a price, price(i) is NULL, so priceWithVAT(i) is NULL too
priceWithVAT (Item i) = price(i) * 1.20;

// a property may take no parameters
defaultVAT = DATA NUMERIC[6,2] ();