Interfaces in Modelica

In Modelica, interfaces are called “partial classes” and are used to define a set of variables and methods that can be implemented by other classes. Let’s see how we can implement the geometry example using Modelica’s partial classes.

partial class Geometry
  function area
    output Real result;
  end area;
  
  function perim
    output Real result;
  end perim;
end Geometry;

class Rectangle
  extends Geometry;
  parameter Real width;
  parameter Real height;
  
  function area
    output Real result;
  algorithm
    result := width * height;
  end area;
  
  function perim
    output Real result;
  algorithm
    result := 2*width + 2*height;
  end perim;
end Rectangle;

class Circle
  extends Geometry;
  parameter Real radius;
  
  function area
    output Real result;
  algorithm
    result := Modelica.Constants.pi * radius * radius;
  end area;
  
  function perim
    output Real result;
  algorithm
    result := 2 * Modelica.Constants.pi * radius;
  end perim;
end Circle;

function measure
  input Geometry g;
algorithm
  Modelica.Utilities.Streams.print("Area: " + String(g.area()));
  Modelica.Utilities.Streams.print("Perimeter: " + String(g.perim()));
end measure;

model Main
  Rectangle r(width=3, height=4);
  Circle c(radius=5);
equation
  measure(r);
  measure(c);
end Main;

In this Modelica code:

1. We define a partial class Geometry that declares the area and perim functions without implementing them.

2. We create Rectangle and Circle classes that extend Geometry and implement the area and perim functions.

3. The measure function takes a Geometry object as input and prints its area and perimeter.

4. In the Main model, we create instances of Rectangle and Circle and call the measure function on them.

To run this Modelica code, you would typically use a Modelica simulation environment like OpenModelica or Dymola. The output would depend on the specific environment, but it would show the area and perimeter calculations for both the rectangle and the circle.

Note that Modelica is primarily used for modeling and simulation of physical systems, so this example is somewhat artificial in the Modelica context. In practice, Modelica would be used more for describing physical components and their interactions rather than abstract geometric shapes.