[plt-scheme] (no subject)

On Thu, 19 Apr 2007, Shriram Krishnamurthi wrote:

> See Robby's message.  But, in BASIC, I can also say
>
> 10  GOTO 20
> 20  PRINT "HELLO WORLD"


Hmmm... ok, would this more closely capture the feeling of what's going 
on?

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
   ;; Trying to encode the spirit of
   ;;
   ;; 10 GOTO 20
   ;; 20 PRINT "HELLO WORLD"
   ;;
   ;; using Scheme's continuations.
   (define (program-2 k-end)
     (let ([line-10 #f] [line-20 #f] [line-end #f])
       (let/cc def-k
         (begin
           (let/cc k (set! line-10 k) (def-k))
           (line-20)))
       (let/cc def-k
         (begin
           (let/cc k (set! line-20 k) (def-k))
           (printf "hello world\n")
           (line-end)))
       (let/cc def-k
         (begin
           (let/cc k (set! line-end k) (def-k))
           (k-end)))
       line-10))

   (define (run prog)
     (let/ec end ((prog end))))
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


I can start it up as:

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
> (run program-2)
hello world
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;


I get the feeling, though, that this is all terribly evil.


One of the things that doesn't make me happy about this is the way I'm 
doing the linkage between lines; I guess I could write a NEXT-LINE helper 
to simulate the program counter more elegantly rather than hardcoding it.