Re: [Chicken-users] State machine using call/cc

[Ed Watkeys]

On Dec 8, 2004, at 9:17 AM, Joel Reymont wrote:

> Hi Ed!
>
> > Please tell me if I don't have a clue, but I think your goal should be
> > to abandon the idea of an explicit state machine and simply write your
> > code with procedure calls that wait for user input. That "simply
> > write..." part is where continuations work their magic.
>
> I have the main application loop in C++. That is where I poll for 
> network
> events. I supposed I could somehow write the server portion of my 
> project
> such that I poll for events from Scheme, maybe.

I'm sorry; I forgot about the main loop being in C++.

> I also have the client portion of the project where the main loop is 
> the
> SDL GUI loop I think that makes it harder to poll for events from 
> Scheme.
>
> I thought it would be much simpler to have Scheme be the event 
> dispatcher
> (application logic) and to leave the event gathering outside of Scheme.
>
> Am I missing something?

I think Felix's suggestion, returning the next procedure to execute or 
placing it in a hash table -- might be simpler. You could use a closure 
and lambda to save your state. This is sort of like using currying (see 
Haskell). Someone else might have discussed this but here's an example:

(define (add-two-numbers)
  (letrec ((step1
            (lambda (n1)
              (if (or (< n1 1) (> n1 11))
                  (begin
                    (display "Sorry, 1 to 11 please.\n")
                    step1)
                  (begin
                    (display "Okay, continue.\n")
                    (letrec ((step2
                              (lambda (n2)
                                (cond
                                 ((= n2 69)
                                  (display "That number disgusts me!\n")
                                  step2)
                                 ((= n1 n2)
                                  (display "Let's start over, shall we?\n")
                                  step1)
                                 (else
                                  (display "Thank you, come again.\n")
                                  (+ n1 n2))))))
                      step2))))))
    step1))

#;> (define f (add-two-numbers))
#;> f
#<procedure>
#;> (define g (f 13))
Sorry, 1 to 11 please.
#;> (define h (g 11))
Okay, continue.
#;> (define i (h 69))
That number disgusts me!
#;> (define j (i 11))
Let's start over, shall we?
#;> (define k (j 3))
Okay, continue.
#;> (define l (k 4))
Thank you, come again.
#;> l
7

The only problem I can see is the ever-increasing level of indentation, 
but there are simple ways of dealing with that too.

Regards,
Ed

>     Thanks, Joel
>
> P.S. When I replied the list was not included. Was that your intention?

I accidentally replied to you and not the list. My mistake.

Ed

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