Assignment 9

Consider the following Scheme function:

(define ladd
  (lambda ()
    (let ((more? (read)))
      (if more? 
          (+ (read) (ladd))
          0))))

which repeatedly asks for more numbers and then sums all the numbers. For example,

> (load "ladd.ss")
> (ladd)
#t
10
#t
20
#t
30
#t
40
#f
100

You are to write a web server application which implements the above function by taking its input from web forms and producing its output as HTML pages. Your application should be robust: it should work in the presence of crazy control actions by the user (cloning windows, bookmarking intermediate pages, using the back button, submitting forms more than once, etc).

You will need to download and install the tomcat server. See http://jakarta.apache.org/tomcat/ for details. The easiest way I found to work with the server is to put the Java files under webapps/examples/WEB-INF/classes.

You will probably need some guidance with this. I will try to go through the details in lectures. Meanwhile you can test my implementation by going to http://dogfish.cs.indiana.edu:8080/examples/c311.html. Please let me know if you encounter any problems!

Turn in a URL to sabry@cs.indiana.edu.

Clarification

In this particular example, the continuation can be represented as the sum of numbers seen so far. Taking advantage of this can greatly simplify the code but misses the point of the assignment, which is to explore a general technique that should work in other situations. For example, consider a slightly modified version of our assignment in which the operation to perform on the number is not hardwired to be "addition". Instead after the user enters all the numbers, the user then supplies an operation to apply which could be addition, multiplication, or even a Java method. It is clear that the code now must maintain the entire list of numbers that has been entered: doing so in the URL is not practical, and using hidden parameters is not much better. Furthermore the information saved at every point might be much more complex than simple numbers. Converting the code to CPS and using the general technique explained in the paper addresses all these issues in general. If you can still find another way to solve the problem in its full generality as explained above, I'd love to see your solution.