Brief: extensions

Extending classes

The EXTEND CLASS statement inherits an already declared class from further parent classes and adds new static objects to it.

Structurally this moves the relations between classes into a separate module: the base module declares the class, and a module depending on it adds a parent, changing nothing in the base one. The mechanism is described in class extension.

CLASS Box : Shape;
CLASS Quadrilateral;
EXTEND CLASS Box : Quadrilateral;   // adding inheritance

EXTEND CLASS ShapeType {            // adding a static object
    circle 'Circle'
}

Extending properties and actions

An abstract property or action is a declared extension point: the base module sets the contract with the ABSTRACT operator — the parameter classes, the way an implementation is chosen, and the result class, which a property always has and an action only when the operator names one — and other modules add implementations with the += statement for properties and the ACTION+ statement for actions. The kinds of selection and the options are covered in Brief: properties.

Structurally this is the deferred assembly of an ordinary operator: for a property, of a selection operator; for an action, of a branching or sequence operator. The implementations are kept in an ordered list, and a new one is added at its start or at its end.

Hence the main modularity technique: the base module declares the extension point, the modules depending on it add behaviour, and no reverse dependency appears.

Extending forms

The EXTEND FORM statement extends a form declared in another module — with the same blocks as the FORM declaration (see Brief: forms): objects, the properties and actions shown for them, filters, orders. Separate extension blocks change the elements already on the form; the form's design is likewise set from outside.

An added element can be placed before or after a specific element of the form, or at the start or at the end; for objects this position sets their place in the order of object groups, which a property's display group and the object group a filter applies to depend on. The mechanism is described in form extension.

EXTEND FORM items
    PROPERTIES(i) NEWSESSION DELETE     // a delete button
    OBJECTS g = ItemGroup BEFORE i      // the item group before the item
    PROPERTIES(g) READONLY name
    FILTERS itemGroup(i) = g
;

There is no separate extension logic for the navigator and the form design: these constructs are extensible by definition (extensions).