Shahzad Bhatti Welcome to my ramblings and rants!

Though, Joe Armstrong denies that Erlang is object oriented language, but according Ralph Johnson concurrent Erlang is object oriented. Erlang is based on Actor model, where each object has a mailbox associated that receives messages. In this model, the message passing takes literal form rather than method invocation. Erlang also has strong pattern matching support, so you can achieve polymorphism by pattern matching. This kind of polymorphism is similar to duck typing rather than inheritance based polymorphism in object oriented languages. One of the hardest thing in object oriented language is to know when something can be a subclass of another class because you have to measure IS-A test or Liskov substitution principle. Most people also consider inheritance bad due to encapsulation leakage so duck typing will be easier. Since, all states are immutable in Erlang, you can’t really keep state as traditional object oriented language, but you can create a new state with every mutation and attach it to the process. For example, here is a simple example of polymorphism in Java :

  1 import junit.framework.Test;
  2 import junit.framework.TestCase;
  3 import junit.framework.TestSuite;
  4
  5 import java.util.*;
  6
  7
  8 public class PolymorTest extends TestCase {
  9     Shape[] shapes;
 10
 11     protected void setUp() {
 12         shapes = new Shape[] {
 13                 new Circle(3.5), new Rectangle(20, 50), new Square(10)
 14             };
 15     }
 16
 17     protected void tearDown() {
 18     }
 19
 20     public void testArea() {
 21         assertEquals(38.484, shapes[0].area(), 0.001);
 22         assertEquals(1000.0, shapes[1].area(), 0.001);
 23         assertEquals(100.0, shapes[2].area(), 0.001);
 24     }
 25
 26     public void testPerimeter() {
 27         assertEquals(21.991, shapes[0].perimeter(), 0.001);
 28         assertEquals(140.0, shapes[1].perimeter(), 0.001);
 29         assertEquals(40.0, shapes[2].perimeter(), 0.001);
 30     }
 31
 32     public static Test suite() {
 33         TestSuite suite = new TestSuite();
 34         suite.addTestSuite(PolymorTest.class);
 35
 36         return suite;
 37     }
 38
 39     public static void main(String[] args) {
 40         junit.textui.TestRunner.run(suite());
 41     }
 42
 43     interface Shape {
 44         public double area();
 45
 46         public double perimeter();
 47     }
 48
 49     class Rectangle implements Shape {
 50         double height;
 51         double width;
 52
 53         Rectangle(double height, double width) {
 54             this.height = height;
 55             this.width = width;
 56         }
 57
 58         public double area() {
 59             return height * width;
 60         }
 61
 62         public double perimeter() {
 63             return (2 * height) + (2 * width);
 64         }
 65
 66         public void setWidth(double width) {
 67             this.width = width;
 68         }
 69
 70         public double getWidth() {
 71             return this.width;
 72         }
 73
 74         public double getHeight() {
 75             return this.height;
 76         }
 77
 78         public void setHeight(double height) {
 79             this.height = height;
 80         }
 81     }
 82
 83     class Square extends Rectangle {
 84         Square(double side) {
 85             super(side, side);
 86         }
 87     }
 88
 89     class Circle implements Shape {
 90         double radius;
 91
 92         Circle(double radius) {
 93             this.radius = radius;
 94         }
 95
 96         public double getRadius() {
 97             return this.radius;
 98         }
 99
100         public void setRadius(double radius) {
101             this.radius = radius;
102         }
103
104         public double area() {
105             return Math.PI * radius * radius;
106         }
107
108         public double perimeter() {
109             return Math.PI * diameter();
110         }
111
112         public double diameter() {
113             return 2 * radius;
114         }
115     }
116 }
117

And here is Ruby equivalent, which uses duck typing:

 1 require 'test/unit'
 2 require 'test/unit/ui/console/testrunner'
 3
 4
 5 class Rectangle
 6   attr_accessor :height, :width
 7   def initialize(height, width)
 8         @height = height
 9         @width = width
10   end
11   def area()
12       height*width
13   end
14   def perimeter()
15       2*height + 2*width
16   end
17 end
18
19
20 class Square
21   attr_accessor :side
22   def initialize(side)
23         @side = side
24     end
25     def area()
26       side*side
27     end
28     def perimeter()
29       4 * side
30     end
31 end
32
33
34 class Circle
35     attr_accessor :radius
36    def initialize(radius)
37         @radius = radius
38     end
39     def area()
40      Math::PI*radius*radius
41     end
42     def perimeter()
43     Math::PI*diameter()
44       end
45      def diameter()
46       2*radius
47     end
48 end
49
50 class PolymorTest < Test::Unit::TestCase
51     def setup
52         @shapes = [Circle.new(3.5), Rectangle.new(20, 50), Square.new(10)]
53     end
54
55     def test_area
56         assert_in_delta(38.484, @shapes[0].area(), 0.001, "didn't match area0 #{@shapes[0].area()}")
57         assert_in_delta(1000.0, @shapes[1].area(), 0.001, "didn't match area1 #{@shapes[1].area()}")
58         assert_in_delta(100.0, @shapes[2].area(), 0.001, "didn't match area2 #{@shapes[2].area()}")
59     end
60     def test_perimeter
61         assert_in_delta(21.991, @shapes[0].perimeter(), 0.001, "didn't match perim0 #{@shapes[0].perimeter()}")
62         assert_in_delta(140.0, @shapes[1].perimeter(), 0.001, "didn't match perim1 #{@shapes[1].perimeter()}")
63         assert_in_delta(40.0, @shapes[2].perimeter(), 0.001, "didn't match perim2 #{@shapes[2].perimeter()}")
64     end
65 end
66
67
68 Test::Unit::UI::Console::TestRunner.run(PolymorTest)
69

And here is Erlang equivalent (obviously it’s a lot more verbose):

  1 -module(rectangle).
  2 -compile(export_all).
  3
  4
  5 -record(rectangle, {height, width}).
  6
  7 new_rectangle(H, W) ->
  8     #rectangle{height = H, width = W}.
  9
 10 start(H, W) ->
 11     Self = new_rectangle(H, W),
 12     {Self, spawn(fun() -> loop(Self) end)}.
 13
 14 rpc(Pid, Request) ->
 15         Pid ! {self(), Request},
 16         receive
 17             {Pid, ok, Response} ->
 18                 Response
 19         end.
 20
 21
 22 area(Self) ->
 23   Self#rectangle.height * Self#rectangle.width.
 24
 25 perimeter(Self) ->
 26   2 * Self#rectangle.height + 2 * Self#rectangle.width.
 27
 28 set_width(Self, W) ->
 29     Self#rectangle{width = W}.
 30
 31 get_width(Self) ->
 32     Self#rectangle.width.
 33
 34 set_height(Self, H) ->
 35     Self#rectangle{height = H}.
 36
 37 get_height(Self) ->
 38     Self#rectangle.height.
 39
 40 loop(Self) ->
 41     receive
 42         {From, area} ->
 43             From ! {self(), ok, area(Self)},
 44             loop(Self);
 45
 46         {From, perimeter} ->
 47             From ! {self(), ok, perimeter(Self)},
 48             loop(Self);
 49
 50         {From, get_width} ->
 51             From ! {self(), ok, get_width(Self)},
 52             loop(Self);
 53
 54         {From, get_height} ->
 55             From ! {self(), ok, get_height(Self)},
 56             loop(Self);
 57
 58         {set_width, W} ->
 59             loop(set_width(Self, W));
 60
 61         {set_height, H} ->
 62             loop(set_height(Self, H))
 63     end.
 64
 65
 66
 67
 68
 69
 70
 71 -module(circle).
 72 -compile(export_all).
 73 -define(PI, 3.14159).
 74
 75 -record(circle, {radius}).
 76
 77 new_circle(R) ->
 78     #circle{radius = R}.
 79
 80 start(R) ->
 81     Self = new_circle(R),
 82     {Self, spawn(fun() -> loop(Self) end)}.
 83
 84 rpc(Pid, Request) ->
 85         Pid ! {self(), Request},
 86         receive
 87             {Pid, ok, Response} ->
 88                 Response
 89         end.
 90
 91
 92 area(Self) ->
 93   ?PI * Self#circle.radius * Self#circle.radius.
 94
 95 perimeter(Self) ->
 96   ?PI * diameter(Self).
 97
 98 set_radius(Self, R) ->
 99     Self#circle{radius = R}.
100
101 get_radius(Self) ->
102     Self#circle.radius.
103 diameter(Self) ->
104     Self#circle.radius * 2.
105
106
107 loop(Self) ->
108     receive
109         {From, area} ->
110             From ! {self(), ok, area(Self)},
111             loop(Self);
112
113         {From, perimeter} ->
114             From ! {self(), ok, perimeter(Self)},
115             loop(Self);
116
117         {From, get_radius} ->
118             From ! {self(), ok, get_radius(Self)},
119             loop(Self);
120
121         {set_radius, R} ->
122             loop(set_radius(Self, R))
123     end.
124
125
126
127
128
129 -module(square).
130 -compile(export_all).
131
132
133 -record(square, {side}).
134
135 new_square(S) ->
136     #square{side = S}.
137
138 start(S) ->
139     Self = new_square(S),
140     {Self, spawn(fun() -> loop(Self) end)}.
141
142 rpc(Pid, Request) ->
143         Pid ! {self(), Request},
144         receive
145             {Pid, ok, Response} ->
146                 Response
147         end.
148
149
150 area(Self) ->
151   Self#square.side * Self#square.side.
152
153 perimeter(Self) ->
154   4 * Self#square.side.
155
156 set_side(Self, S) ->
157     Self#square{side = S}.
158
159 get_side(Self) ->
160     Self#square.side.
161
162 loop(Self) ->
163     receive
164         {From, area} ->
165             From ! {self(), ok, area(Self)},
166             loop(Self);
167
168         {From, perimeter} ->
169             From ! {self(), ok, perimeter(Self)},
170             loop(Self);
171
172         {From, get_side} ->
173             From ! {self(), ok, get_side(Self)},
174             loop(Self);
175
176         {set_side, S} ->
177             loop(set_side(Self, S))
178     end.
179
180
181
182
183 -module(poly_test).
184 -compile(export_all).
185
186 create() ->
187    [circle:start(3.5), rectangle:start(20, 50), square:start(10)].
188
189 area(ShapeNPid) ->
190     {_Shape, Pid} = ShapeNPid,
191     Pid ! {self(), area},
192     receive
193         {Pid, ok, Response} ->
194             Response
195     end.
196
197 perimeter(ShapeNPid) ->
198     {_Shape, Pid} = ShapeNPid,
199     Pid ! {self(), perimeter},
200     receive
201         {Pid, ok, Response} ->
202             Response
203     end.
204
205 get_area(ShapesNPids) ->
206    lists:map(fun(ShapeNPid) -> area(ShapeNPid) end, ShapesNPids).
207 get_perimeter(ShapesNPids) ->
208    lists:map(fun(ShapeNPid) -> perimeter(ShapeNPid) end, ShapesNPids).
209
210 test_area() ->
211     D = create(),
212     %[CircleArea, RectangleArea, SquareArea] = [38.4845,1000,100],
213     [CircleArea, RectangleArea, SquareArea] = get_area(D).
214
215 test_perimeter() ->
216     D = create(),
217     %[CirclePerimeter, RectanglePerimeter, SquarePerimeter] = [21.9911,140,40],
218     [CirclePerimeter, RectanglePerimeter, SquarePerimeter] = get_perimeter(D).
219

Lessons Learned:

Though, Erlang supports object oriented features in its concurrent model, but it is a lot more verbose. For example, in above example Ruby was roughly 60 lines of code and Java was twice as long, whereas Erlang was about four times long.