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These are a few little toy examples for modelling finite state machines in Scheme. There are better ways to do it. It's just really basic stuff for fun.

Finite state machines are useful for alot of things, and not just compilers. They are used heavily in video games, controls, robotics and for modelling any kind of task which involves a state and actions. There are plenty of tutorials around that discuss this much better than I can do in a paragraph.

; ; FSMP - Finite State Machine Parser in Scheme ; a Scheme interpreter for a finite state machine. ; ; A very simple language for developing finite state machines in Scheme. ; This is meant to be an example, pretty ugly, but it shows how to model these concepts ; in Scheme. ; ; test program (define prog '( FSM foo 2 (0 (1 (lambda()(display "FOO")))) (1 (0 (lambda()(display "BAR")))) ;; -1 is for end of list for now (-1))) ;; name of program we are parsing. (define prog-name "") ;; fsm-vector is a vector of all states[num] = list(next,lambda); (define fsm-vector '()) ;; number of state in vector (define num-states 0) ;; current state we are on now (define state 0) ;; add state (number next function) (define (add-state num next func) (vector-set! fsm-vector num (list next func))) ;; expand a vector with one more element at the end. copies old into it, return new vector. (define (expand-vector old) ( let* ((n (+ num-states 1))( v (make-vector n))) (let loop (( i 0 )) (if (= i (- num-states 1)) ( begin (set! num-states n) v) ( begin (vector-set! v i (vector-ref old i)) (loop (+ i 1))))))) ;; execute state n (define (exec-state n) (set! state (car n)) (eval (car(cdr n)))) ;; send message to the FSM ;; if message is recognized, it advances to the new state and evaluates it's action. (define (fsm-message n) ( let (( c (vector-ref fsm-vector state))) (if(= n (car c)) ( (exec-state c))))) ;; parse the list and setup the vector (define (parser p) (if (= (caar p) -1) #t ( let ((c (car p))) (add-state ( car c )( car (cadr c))( cadr (cadr c))) (parser (cdr p))))) ;; parse-fsm driver (define (fsm-parse program ) (let ((c (car program))) (if(null? c) (display "error")) (if(eqv? c 'FSM) ( begin (set! prog-name (cadr program)) (set! num-states (caddr program )) (set! fsm-vector (make-vector num-states)) (parser (cdddr program))) (display "Bad program")))) ;; test (fsm-parse prog) (fsm-message 1) (fsm-message 0) (fsm-message 1) (fsm-message 0)

Probabilistic state machine is similar to the FSM. But it uses a probability to choose the next state, instead of being determined. Thi s can be simulated by rolling a virtual 'die' generating a random number.

; PFSMP - Probabilistic Finite State Machine Parser ; ; Similar to FSMP, except it makes a random path from choices given to it. ; This is simulated by rolling a virtual percentile die. ; STATE RANGE(min,max) ACTION(...) (define program '( PFSM pfsm 4 ( 0 0 25 (lambda()(display "1")(newline))) ( 1 26 75 (lambda()(display "2")(newline))) ( 2 76 90 (lambda()(display "3")(newline))) ( 3 91 100 (lambda()(display "4")(newline))))) ;; roll a percentile dice (define (roll-dice) (random 100)) ;; name of program we are parsing. (define prog-name "") ;; fsm-vector is a vector of all states[num] = list(next,lambda); (define pfsm-vector '()) ;; number of state in vector (define num-states 0) ;; current state we are on now (define state 0) ;; add state (number next function) (define (add-state num min max func) (vector-set! pfsm-vector num (list min max func))) (define (pfsm-message) ( let (( d (roll-dice))(min 0)(max 0)(v (vector-ref pfsm-vector 0))) ( let loop ((i 0)) (if(= i (- num-states 1)) #t (begin (set! min (car v)) (set! max (cadr v)) (if( and (>= d min)(<= d max)) ((eval(caddr v))) (begin (set! v (vector-ref pfsm-vector (+ i 1))) (loop (+ i 1))))))))) ;; parse the list and setup the vector (define (parser p) (if(null? p) #t (begin (let ((c (car p))) ; num min max action (add-state (car c)(cadr c)(caddr c)(cadddr c)) (parser (cdr p)))))) (define (pfsm-parse program ) (let ((c (car program))) (if(null? c) (display "error")) (if(eqv? c 'PFSM) ( begin (set! prog-name (cadr program)) (set! num-states (caddr program )) (set! pfsm-vector (make-vector num-states)) (parser (cdddr program))) (display "Bad program")))) ;; test (pfsm-parse program) (let loop ((i 0)) (if (= i 10) #t (begin (pfsm-message) (loop (+ i 1)))))

I hope this is useful to anyone. Take these examples and make them better, they are very basic. I am pretty weak at scheme still, but I code in C for along while. So it probably shows ;-)

-- OpcodeFoo - 15 Nov 2005

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