Example 1: Simple INTERFACE usage

Defines an INTERFACE to handle shape objects.

This first module called rectangle.4gl defines the Rectangle type and the methods that apply to this type:

PUBLIC TYPE Rectangle RECORD
    height, width FLOAT
END RECORD

PUBLIC FUNCTION (r Rectangle) area() RETURNS FLOAT
    RETURN r.height * r.width
END FUNCTION
    
PUBLIC FUNCTION (r Rectangle) kind() RETURNS STRING
    RETURN "Rectangle"
END FUNCTION
    
PUBLIC FUNCTION (r Rectangle) setDimensions(w FLOAT, h FLOAT) RETURNS()
    LET r.width = w
    LET r.height = h
END FUNCTION

The circle.4gl module defines the Circle type and methods that apply to this type:

IMPORT util

PUBLIC TYPE Circle RECORD
    diameter FLOAT
END RECORD

PUBLIC FUNCTION (c Circle) area() RETURNS FLOAT
    RETURN util.Math.pi() * (c.diameter / 2) ** 2
END FUNCTION

PUBLIC FUNCTION (c Circle) kind() RETURNS STRING
    RETURN "Circle"
END FUNCTION

PUBLIC FUNCTION (c Circle) setDiameter(d FLOAT) RETURNS()
    LET c.diameter = d
END FUNCTION

The "shapes.4gl" module implements the Shape interface, to group the area() and kind() methods, which both apply to the Rectangle and Circle types. The totalArea() functions takes a dynamic array of Shape objects, knowing only about the behavior defined by the Shape interface.

In this example, the module implementing the Shape interface does not need to import the rectangle and circle modules: The interface definition is abstract and is independent from the types definitions: Only the method definitions matters. Other shape types could be implemented, such as Triangle, Trapeziod, without touching the Shapes interface.

Note also that the Rectangle.setDimensions() and Circle.setDiameter() methods are not part of the Shape interface: These methods are specific to each shape types and therefore cannot be part of a common interface.

PUBLIC TYPE Shape INTERFACE
    area() RETURNS FLOAT,
    kind() RETURNS STRING
END INTERFACE 
    
PUBLIC TYPE ShapesArray DYNAMIC ARRAY OF Shape

PUBLIC FUNCTION totalArea(shapes ShapesArray) RETURNS FLOAT
    DEFINE i INT
    DEFINE area FLOAT
    FOR i = 1 TO shapes.getLength()
        LET area = area + shapes[i].area()
    END FOR 
    RETURN area
END FUNCTION

The main module imports the rectangle, circle and shapes modules, defines rectangle and circle variables, that can be assigned to an dynamic array of shapes, that can in turn be used with the abstract interface:

IMPORT FGL rectangle
IMPORT FGL circle
IMPORT FGL shapes

FUNCTION main()
    DEFINE r1 rectangle.Rectangle
    DEFINE c1 circle.Circle
    DEFINE sa shapes.ShapesArray

    CALL r1.setDimensions(10, 20)
    CALL c1.setDiameter(20)

    LET sa[1] = r1
    LET sa[2] = c1

    DISPLAY sa[1].kind(), sa[1].area()
    DISPLAY sa[2].kind(), sa[2].area()
    DISPLAY "Total area:", shapes.totalArea(sa)

END FUNCTION