Interfaces in Chapel

In Chapel, interfaces are not a built-in language feature. However, we can achieve similar functionality using classes and inheritance. Here’s how we can implement the geometric shapes example:

use Math;

// Define an abstract class instead of an interface
abstract class Geometry {
  proc area(): real {
    halt("area() must be implemented");
  }
  proc perim(): real {
    halt("perim() must be implemented");
  }
}

// Implement the Geometry class for rectangle
class Rect: Geometry {
  var width, height: real;

  proc init(width: real, height: real) {
    this.width = width;
    this.height = height;
  }

  override proc area(): real {
    return width * height;
  }

  override proc perim(): real {
    return 2 * width + 2 * height;
  }
}

// Implement the Geometry class for circle
class Circle: Geometry {
  var radius: real;

  proc init(radius: real) {
    this.radius = radius;
  }

  override proc area(): real {
    return Math.pi * radius * radius;
  }

  override proc perim(): real {
    return 2 * Math.pi * radius;
  }
}

// A generic measure function that works on any Geometry object
proc measure(g: Geometry) {
  writeln(g);
  writeln(g.area());
  writeln(g.perim());
}

// Main function
proc main() {
  var r = new Rect(3, 4);
  var c = new Circle(5);

  measure(r);
  measure(c);

  delete r;
  delete c;
}

In this Chapel implementation:

1. We define an abstract Geometry class instead of an interface. This class declares the area() and perim() methods that must be implemented by subclasses.

2. We create Rect and Circle classes that inherit from Geometry and provide their own implementations of area() and perim().

3. The measure function takes a Geometry object as an argument, allowing it to work with any subclass of Geometry.

4. In the main function, we create instances of Rect and Circle, and pass them to the measure function.

5. Chapel uses explicit memory management, so we need to delete the objects we created with new.

To run this program, save it as geometry.chpl and use the Chapel compiler:

$ chpl geometry.chpl -o geometry
$ ./geometry

This will output something similar to:

{width = 3.0, height = 4.0}
12.0
14.0
{radius = 5.0}
78.5398163397448
31.4159265358979

While Chapel doesn’t have interfaces in the same way as some other languages, this approach using abstract classes and inheritance provides similar functionality for defining common behavior across different types.