An explanation of Duck Typing vs Polymorphism

duck-type-vs-polymorphism.txt

// When using Polymorphism, you might define an abstract class to send a message like 
// follows, then fully implement it several times in different classes.

abstract class BaseMessage
  public void setMessageText messageText
    @messageText = messageText
  end
 
  public void send;
end
 
class EmailMessage extends BaseMessage
  public void send
    // code here to connect to mail server, send an email with @message, etc
  end
end
 
class FacebookMessage extends BaseMessage
  public void setMessageText messageText
    // We can override the setMessageText definition in BaseMessage in both 
    // Polymorphism and Duck Typing
    @messageText = messageText + "\n"
  end
  
  public void send
    // code here to connect to Facebook and send the message
  end
end

class NotAValidMessage // this class does not extend BaseMessage
  public void setMessageText messageText
    @messageText = messageText
  end
  
  public void send
    // code here to do send this message somewhere
  end
end

// If you then want to use any of these classes in a function, you do it like 
// this. Whether the first argument is valid or not depends on it extending 
// BaseMessage, thus passing NotAValidMessage would cause an error. 
public void setMessageAndSend (BaseMessage message, messageText)
  message.setMessageText(messageText)
  message.send()
end

// If using duck typing, you'd define setMessageAndSend as follows:
public void setMessageAndSend (message, messageText)
  message.setMessageText(messageText)
  message.send()
end
// Notice that we don't specify any type for the first argument, merely 
// assuming that what you pass does have functions named setMessageText 
// and send. As such all of EmailMessage, FacebookMessage and 
// NotAValidMessage can be passed to this alternative setMessageAndSend 
// implementation.

________________________________________________________

// It helps to consider why an example like this is useful.
// If you are working on a large application, you might allow users to 
// send emails and Facebook messages but also you might allow your computer 
// servers to message each other to perform different tasks. These two use 
// cases might be rather different things, both conceptually and in the code 
// base, so forcing them to extend the same class could seem inappropriate.

// If you want to let users schedule "Happy Christmas" messages for Christmas 
// Day, then you'll want to have code that can cause a message to send without 
// writing it twice for Emails and Facebook messages. Polymorphism is sufficient 
// for this use case.

// However, if you now also want to schedule messages between your computer 
// servers, you have the choice of duplicating your scheduling code used for 
// user messages, or using duck typing to just make your message scheduling work 
// for any type that can be sent.

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