CS 2135: Programming Language Concepts
The Scheme Server

After we worked through the examples of converting programs to script position, I showed an example of web programs designed to run on the MzScheme web server. Here is the code I showed.

First, here's the original count delay routine:

 (define (count-delay)
   (let ([new-delay (prompt-read-num "Delay: ")])
     (cond [(= 0 new-delay) 0]
           [else (+ new-delay (count-delay))])))

Here's the script-position version:

(define (prompt-read-num/web prompt action)
  (begin
    (printf prompt)
    (action (read))))

(define (count-delay/web action)
  (prompt-read/web
   "Delay: "
   (lambda (box)
     (let ([new-delay box])
       (cond [(= 0 new-delay) (action 0)]
	     [else
	      (count-delay/web
	       (lambda (box)
		 (action (+ new-delay box))))])))))

Here's the same program written to run with the Scheme server. Note that the count-delay program runs on the server as it was originally written, without the need to convert to script position. How is that possible? The Scheme server has a special construct called send/suspend. That construct builds the (lambda (box) ...) expression automatically and passes it along to action as a URL.

This is an example of the kind of powerful feature a programming language can provide to support a particular programming domain. With send/suspend, you get all the power of a full language like Scheme combined with the specialized web support of custom web languages.