INTERFACE usage

Defining interfaces

An interface is defined by a group of methods that apply on user-defined types, to define a common usage interface for several individual types.

A method declared in an interface must use the same parameter names, parameter types and return types as the method implementation it refers to.

The next code example defines a variable as an interface:

DEFINE v INTERFACE
    kind () RETURNS STRING,
    area () RETURNS FLOAT
END INTERFACE
Consider using a TYPE declaration to reuse an interface definition:
TYPE Shape INTERFACE
    kind () RETURNS STRING,
    area () RETURNS FLOAT
END INTERFACE

A variable defined with an interface can receive any type related to the interface.

Associating types to an interface

A user-defined type for which methods are defined is implicitly associated to any interface that defines a set of methods for this type.

For example, a type Rectangle gets associated to a method named area():
TYPE Rectangle RECORD
    height, width FLOAT
END RECORD

FUNCTION (r Rectangle) area () RETURNS FLOAT
    RETURN r.height * r.width
END FUNCTION
An interface defined with the area() method becomes implicitly an interface for the type Rectangle:
TYPE Shape INTERFACE
    area() RETURNS FLOAT
END INTERFACE
If you define a variable v with an INTERFACE structure listing the area() method, you can assign a variable defined as Rectangle to v, and invoke the method with v.area():
FUNCTION main()
    DEFINE r Rectangle = ( height:10, width:20 )
    DEFINE v Shape
    LET v = r
    DISPLAY v.area()
END FUNCTION

Implementation tips

For maximum flexibility, consider implementing the types and corresponding methods in individual modules, and implement the interface in another individual module.

In the parent module using the types/methods and interfaces, import each module with the IMPORT FGL instruction.