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Toward the end of the example, you will work around this in a different way The function logon takes a parameter of the IUser type: let logon (iuser: IUser) = When you call logon with a class that implements IUser, the class is implicitly downcast to this type You can add the interface members to the definition of the class if you want the methods of the interface to be available directly on the class that implements it, instead of after forcing the users of your class to cast the object to the interface in some way To revise the example, you simply add the methods Authenticate and LogonMessage as members of the class User Now it is no longer necessary to cast the identifier user (you ll learn about adding members to methods in the Classes and Methods section later in the chapter): open StrangelightsSamples.

.build file extension when you view the file via Windows Explorer. If you used Notepad to create the build file, you should enclose the filename in double quotation marks to prevent Notepad from appending a .txt file extension to the end of the filename.

def play_bounce_sound(self): channel = self.bounce_sound.play() if channel is not None: # Get the left and right volumes left, right = stereo_pan(self.position.x, SCREEN_SIZE[0]) channel.set_volume(left, right)

Helpers // a class that represents a user // it's constructor takes two parameters, the user's // name and a hash of their password type User(name, passwordHash) = interface IUser with // Authenticate implementation member xAuthenticate(password) = let hashResult = hash (password, "sha1") passwordHash = hashResult.

Listing 6-2 shows a basic C# source file you can create and save in order to test the sample build file and see how NAnt works. Listing 6-2. A File for Testing NAnt public class MyGreatProject { static void Main() { System.Console.Writeline("This is my great project."); } } Make sure you save the source file in Listing 6-2 in the same directory as the NAnt build file you just created. Save this file with a filename of MyGreatProject.cs. Then open a command-line window and navigate to the directory that has both the default.build NAnt build file and the MyGreatProject.cs C# file. Execute the NAnt build file using the default target by typing nant on the command line and pressing the Enter key. You should see output similar to what is shown in Figure 6-3.

def render(self, surface): # Draw the sprite center at self.position w, h = self.image.get_size() x, y = self.position x -= w/2 y -= h/2 surface.blit(self.image, (x, y))

// LogonMessage implementation member x.LogonMessage() = Printf.sprintf "Hello, %s" name // Expose Authenticate implementation member x.Authenticate(password) = x.Authenticate(password) // Expose LogonMessage implementation member x.LogonMessage() = x.LogonMessage()

I have already covered inheritance in a limited way in the Object Expressions and Implementing Interfaces sections. Inheritance allows you to extend a class that is already defined, as well as to add new functionality or possibly to modify or replace the original functionality. Like most modern, objectoriented languages, F# allows single inheritance (from one base class), as well as the implementation of multiple interfaces (see the previous sections, Defining interfaces and Implementing Interfaces ). This section will cover the basics of inheriting from a base class and adding new functionality. The next section, Methods and Inheritance, will show you how to implement methods to make full use of inheritance. You specify inheritance with the inherit keyword, which must come directly after the equals sign, which follows a class s constructor. After the keyword inheritance, you provide the name of the class you want to inherit from, followed by the arguments you intend to pass to its constructor. Let s kick things off by looking at a simple example of inheritance between two F# types. The following example shows an F# class, Sub, that derives from a base class, Base. The class Base has one method, GetState; and the class Sub also has one method, called GetOtherState. The example shows that how the Sub-derived class can use both methods because GetState is inherited from the base class: type Base() = member x.GetState() = 0 type Sub() = inherit Base() member x.GetOtherState() = 0 let myObject = new Sub() printfn "myObject.state = %i, myObject.otherState = %i" (myObject.GetState()) (myObject.GetOtherState()) When you compile and execute this example, you get the following results: myObject.state = 0, myObject.otherState = 0

def run(): # Initialize 44KHz 16-bit stereo sound pygame.mixer.pre_init(44100, -16, 2, 1024*4) pygame.init() screen = pygame.display.set_mode(SCREEN_SIZE, 0) pygame.mouse.set_visible(False) clock = pygame.time.Clock() ball_image = pygame.image.load("ball.png").convert_alpha() mouse_image = pygame.image.load("mousecursor.png").convert_alpha() # Load the sound file bounce_sound = pygame.mixer.Sound("bounce.wav") balls = [] while True: for event in pygame.event.get(): if event.type == QUIT: return if event.type == MOUSEBUTTONDOWN: # Create a new ball at the mouse position random_speed = ( randint(-400, 400), randint(-300, 0) ) new_ball = Ball( event.pos, random_speed, ball_image, bounce_sound ) balls.append(new_ball) time_passed_seconds = clock.tick() / 1000. screen.fill((255, 255, 255)) dead_balls = []

Try playing around with the various other targets defined in the build file. For example, you can have NAnt run the clean target by entering nant clean on the command line.