Programming Assignment

• Due on 12 February by 8:00 PM, submit a file Asst2.dyl containing class, function, and method definitions necessary to implement a class <egg-timer> that can be used by the following main program: define method main () let x = read-char(); let e = make (<egg-timer>); until (x == 'q') select (x) 'c' => press (e.c-button); 's' => press (e.s-button); 'm' => press (e.m-button); otherwise => print ("I don't know which button that is."); end select; x := read-char (); // to discard carriage return x := read-char (); end until; values (); // to return no printable value end method main;

Effective Use of the Marlais interpreter

• Make sure to use dylan mode for editing dylan programs in emacs. You can add the following to the bottom of your .emacs file to make sure dylan mode is employed whenever you edit a program with filename extension .dyl: (setq auto-mode-alist (append (list (cons "\\.dyl$" 'dylan-mode)) auto-mode-alist)) (autoload 'dylan-mode "dylan-mode" "Dylan Editing Mode" t) (setq load-path (cons (expand-file-name "/cis/homes/jnw/emacs") load-path))
• Make sure you use marlais mode to get a dylan listener within an emacs session. You do this by adding the following at the bottom of your .emacs file: (autoload 'run-dylan "marlais" "Run an inferior Dylan process." t) Then, when you want to execute a marlais interpreter from an emacs buffer, you execute the emacs command M-x run-dylan.
• Find out about manipulating multiple windows in emacs from the Files menu item and using multiple buffers with the Buffers menu.
• Make sure you either link to ~jnw/bin/sun4/marlais from a directory in your execution path, or change your .cshrc file to include set path = ($path ~jnw/bin/sun4)

More on Generic Functions

• One more example of generic function use. Consider the factorial function. define generic factorial (n :: <integer>) => result :: <integer>; One can define multiple methods for factorial, because method dispatch is based on the type, not the class of the arguments in the method invocation.
• The usual case treated by the factorial function is the following: define method factorial (n :: <integer>) n * factorial (n - 1); end method factorial;
• The previous factorial function doesn't work if its argument is 0. We can create an argument whose type is the single value 0. (Such a type can be created by evaluating the expression singleton (value)). define method factorial (n == 0) 1; end method factorial;
• We still haven't handled all the cases of possible argument values. If the argument is a negative, factorial has been asked to do an impossible job. We can create a type that consists of a limited range of integer values and use that to provide a behavior for the rest of the cases: define method factorial (n :: limited(<integer>, max: -1)) error ("factorial: Bad argument", n); end method factorial;
• Why is the right method used if we evaluate factorial (1), factorial (0), or factorial (-1)?
  • Which type do you believe is more specific to the object 0, the singleton type that contains only the value 0 or the class <integer>?
  • Which type do you believe is more specific to the object -1, the range of integers containing only negative integers or the class <integer>?

Using Message Passing Style in Dylan

• The function curry is used to transform a function of n arguments into a function of n - 1 arguments by fixing the first argument to some specific value. Consider the function \+. It takes two arguments. If we curry the first argument, then we must fix it to some particular value. Find out the value of each of the following expressions:
  • 3
  • \+
  • curry
  • curry (\+, 3)
  • curry (\+, 3) (4)
• What if we define a constant to have curry (\+, 3) as its value? What will it do when we apply it?
• Now consider the following class definition: define class <c> (<object>) slot s :: <integer>, init-value: 1; end class <c>;
• We can make this class employe message-passing syntax for the behavior called message by currying a function whose first argument is of type <c>. Consider the following: define method do-message (self :: <c>, x :: <integer>) self.s + x; end method do-message; define method message (self :: <c>) curry (do-message, self); end method message;