Multimethods

Multimethodsan introduction
Aman King
king@thoughtworks.com

What is it?

Multiple dispatch

Basically: a language feature

Before the details…

Some Java code snippets…

public class MultimethodsDemo { static class Walk { } static class Dance { } static class Attack { } static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } } public static void main(String[] args) { Robot robot = new Robot();System.out.println( robot.doThis( new Walk() ));System.out.println( robot.doThis( new Dance() ));System.out.println( robot.doThis( new Attack() )); }}

I am walking...Let's boogie woogie...Exception in thread "main" java.lang.RuntimeException: I don't believe in violence! at MultimethodsDemo$Robot.doThis(MultimethodsDemo.java:10) at MultimethodsDemo.main(MultimethodsDemo.java:18)

Polymorphism

Overloading

public class MultimethodsDemo { static class Walk { } static class Dance { } static class Attack { } static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } } public static void main(String[] args) { Robot robot = new Robot();System.out.println( robot.doThis( new Walk() ));System.out.println( robot.doThis( new Dance() ));System.out.println( robot.doThis( new Attack() )); }}

import java.util.*;public class MultimethodsDemo {static interface Command {} static class Walk implements Command { } static class Dance implements Command { } static class Attack implements Command { } static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } } public static void main(String[] args) { Robot robot = new Robot(); List<Command> commands = Arrays.asList( new Walk(), new Dance(), new Attack() );for ( Command command : commands ) {System.out.println( robot.doThis(command) ); } }}

MultimethodsDemo.java:20: cannot find symbolsymbol : method doThis(MultimethodsDemo.Command)location: class MultimethodsDemo.RobotSystem.out.println( robot.doThis(command) ); ^1 error

Solution?

Satisfy the compiler!

import java.util.*;public class MultimethodsDemo { static interface Command {} static class Walk implements Command {} static class Dance implements Command {} static class Attack implements Command {} static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); }public String doThis(Command command) { throw new RuntimeException("Unknown command: " + command.getClass()); } } public static void main(String[] args) { Robot robot = new Robot(); List<Command> commands = Arrays.asList( new Walk(), new Dance(), new Attack() ); for ( Command command : commands ) {System.out.println( robot.doThis(command) ); } }}

Made the compiler happy but…

Exception in thread "main" java.lang.RuntimeException: Unknown command: class MultimethodsDemo$Walk at MultimethodsDemo$Robot.doThis(MultimethodsDemo.java:15) at MultimethodsDemo.main(MultimethodsDemo.java:23)

Early Binding(compile-time)

Now what?

Forcing correct overloadingatcompile-time…

// … static class Robot { public String doThis(Walk walk) { return "I am walking..."; } public String doThis(Dance dance) { return "Let's boogie woogie..."; } public String doThis(Attack attack) { throw new RuntimeException("I don't believe in violence!"); } public String doThis(Command command) { if (command instanceof Walk) { return doThis( (Walk) command ); } if (command instanceof Dance) { return doThis( (Dance) command ); } if (command instanceof Attack) { return doThis( (Attack) command ); } throw new RuntimeException("Unknown command: " +command.getClass()); } } // ...

or use Visitor pattern.

So what went wrong?

Java does early binding for overloading polymorphism

But remember…

Java does late binding (runtime)for overriding polymorphism

public class MultimethodsDemo { static class Robot { public String name() { return "normal robot"; } } static class AwesomeRobot extends Robot { public String name() { return "awesome robot!!!"; } } public static void main(String[] args) {Robot robot = new Robot();Robot awesomeRobot = new AwesomeRobot();System.out.println( robot.name() ); // normal robotSystem.out.println( awesomeRobot.name() ); // awesome robot!!! }}

Now think ofrobot.doThis(command)asdoThis(robot, command)

C# 3.0 : Extension Methodsnamespace CustomExtensions{ public static class CustomExtensionsClass { public static string Times(thisobject input, int times) { return "don't care!"; } public static string Times(thisstring input, int times) { string result = "";for(inti = 0; i < times; i++) result += input; return result; } public static intTimes(thisint input, int times) { return input * times; } }}// …using CustomExtensions;varaStringObject = "aman"; aStringObject.Times(2); // "amanaman"varanIntObject = 3; anIntObject.Times(2); // 6// …

In these examples, polymorphism is based on the runtime type of a single argument: the 1st one (implicitly passed)

In other words…

Java and C# 3.0support Single Dispatch

not Double Dispatch,not Multiple Dispatch.

So what is Multiple Dispatch?

Selection aka dispatchof a method based on the runtime type of multiple arguments

Languages that support Multiple Dispatch / Multimethods:
Common Lisp
Clojure
Perl
Groovy
C# 4.0

C# 4.0 : Multimethods using ‘dynamic’using System;namespace Multimethods{ public interface ICommand { } public class Walk : ICommand { } public class Dance : ICommand { } public class Attack : ICommand { } public class Robot { public string DoThis(Walk walk) { return "I am walking..."; } public string DoThis(Dance dance) { return "Let's boogie woogie..."; } public string DoThis(Attack attack) { throw new InvalidOperationException("I don't believe in violence!"); } public string DoThis(ICommand command) { throw new InvalidOperationException("Unknown command: " + ((object) command).GetType().Name); } public string DoThisCommand(ICommand command) {dynamiccmd = command; return DoThis(cmd); } }}

// …Robot robot = new Robot();ICommandcommand = newWalk();robot.DoThisCommand( command ); // "I am walking... "robot.DoThis( command ); // Exception: "Unknown command:Walk"// …

So what is interesting about Multiple Dispatch /Multimethods?

For starters…

Thinking about…

OO as syntactic sugarfor method dispatch?

Design patterns as a way to overcome lack of language features?Visitor for Double DispatchStrategy for Closures/Lambdas

Do we really need more than Single Dispatch or at best Double Dispatch?

References
http://en.wikipedia.org/wiki/Multiple_dispatch
http://www.c2.com/cgi/wiki?MultiMethods
http://en.wikipedia.org/wiki/Extension_method
http://clojure.org/multimethods

Thank you
http://www.wikyblog.com/AmanKing

Multimethods

by Aman King on Aug 09, 2010

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Multimethods — Presentation Transcript