Finite state machine

From Rosetta Code
Finite state machine is a draft programming task. It is not yet considered ready to be promoted as a complete task, for reasons that should be found in its talk page.

A Finite state machine (FSM) is computational abstraction which maps a finite number of states to other states within the same set, via transitions. An FSM can only be in one state at any given moment. Transitions can either be explicit or implicit; explicit transitions are triggered by an input signal and implicit transitions by the internal state of the system (that is, the current state). Implicit transitions thus represent "automatic" or sequenced states that are generally processed between explicit transitions (although they can also be used to provide an optional path when no valid transition exists for a given input signal).

Example

Consider the model of a simple vending machine. The machine is initially in the "ready" state, which maps to exactly two states in the following way:

ready -> deposit -> waiting
ready -> quit -> exit

The variables in bold-face represent transitions. Any input signal not corresponding to one of those transitions can either trigger an error or be ignored. Otherwise, the current state is updated and the process is repeated. If, for example, a deposit input signal is encountered, the FSM will move to the "waiting" state, which defines these transitions:

waiting -> select -> dispense
waiting -> refund -> refunding

The "dispense" state defines only one transition:

dispense -> remove -> ready

Note, however, that in this example the "refunding" state doesn't actually require input in order to move to the "ready" state, so an implicit transition is defined as such:

refunding -> ready


Task

Implement a finite state machine which handles both explicit and implicit transitions. Then demonstrate an example which models some real-world process.


See also



BASIC[edit]

Sinclair ZX81 BASIC[edit]

Works with 1k of RAM.

There doesn't seem much point, in BASIC, implementing a 'general' FSM that would accept a list of states and transition rules as parameters, because an unstructured BASIC program in essence already is that list.

Within each state, if the transition is implicit we just GOTO the next state. If it is explicit, we loop until the user presses a key corresponding to a valid transition. Invalid inputs are ignored.

The line 100 GOTO 110 is superfluous, because it would go there anyway; but it is worth including it in case we wanted to modify the program later and transition somewhere else out of the dispense state.

Note that the program uses no variables and makes no use of the return stack: all the state is expressed in the (so to speak) state.

 10 PRINT "PRESS D(EPOSIT) OR Q(UIT)"
20 IF INKEY$="D" THEN GOTO 50
30 IF INKEY$="Q" THEN STOP
40 GOTO 20
50 PRINT "PRESS S(ELECT) OR R(EFUND)"
60 IF INKEY$="S" THEN GOTO 90
70 IF INKEY$="R" THEN GOTO 140
80 GOTO 60
90 PRINT "DISPENSED"
100 GOTO 110
110 PRINT "PRESS R(EMOVE)"
120 IF INKEY$="R" THEN GOTO 10
130 GOTO 120
140 PRINT "REFUNDED"
150 GOTO 10
Output:

It will be seen that the user has pressed, in order, D, R, D, S, R, and Q.

PRESS D(EPOSIT) OR Q(UIT)
PRESS S(ELECT) OR R(EFUND)
REFUNDED
PRESS D(EPOSIT) OR Q(UIT)
PRESS S(ELECT) OR R(EFUND)
DISPENSED
PRESS R(EMOVE)
PRESS D(EPOSIT) OR Q(UIT)

C[edit]

This is an unapologetic implementation of goto. There have been a lot of curse words and obituaries written about it and the inventors of Java were glad to exclude it from the language, but to be fair, goto is one of the many things C inherited from languages such as Assembly or BASIC that make it truly awesome, especially when it comes to such requirements. After all, can there be a clearer and simpler implementation of a Finite State Machine (not counting BASIC ) ?

 
/*Abhishek Ghosh, 25th October 2017*/
 
#include<stdio.h>
 
int main()
{
char str[10];
 
ready: do{
printf("\nMachine is READY. (D)eposit or (Q)uit :");
scanf("%s",str);
}while(!(str[0]!='D'||str[0]!='d'||str[0]!='q'||str[0]!='Q'));
 
if(str[0]=='q'||str[0]=='Q')
goto quit;
goto waiting;
 
waiting: do{
printf("(S)elect product or choose to (R)efund :");
scanf("%s",str);
}while(!(str[0]!='s'||str[0]!='S'||str[0]!='r'||str[0]!='R'));
 
if(str[0]=='s'||str[0]=='S'){
printf("Dispensing product...");
goto dispense;
}
 
else{
printf("Please collect refund.");
goto ready;
}
 
dispense: do{
printf("\nPlease (C)ollect product. :");
scanf("%s",str);
}while(!(str[0]!='c'||str[0]!='C'));
 
goto ready;
 
quit: printf("Thank you, shutting down now.");
 
return 0;
}
 

Machine simulation :

C:\rosettaCode>fsm.exe

Machine is READY. (D)eposit or (Q)uit :D
(S)elect product or choose to (R)efund :S
Dispensing product...
Please (C)ollect product. :C

Machine is READY. (D)eposit or (Q)uit :D
(S)elect product or choose to (R)efund :R
Please collect refund.
Machine is READY. (D)eposit or (Q)uit :Q
Thank you, shutting down now.

C++[edit]

 
#include <map>
 
template <typename State, typename Transition = State>
class finite_state_machine
{
protected:
State
current;
std::map<State, std::map<Transition, State>>
database;
public:
finite_state_machine()
{
set(State());
}
void
set(State const& state)
{
current = state;
}
State
get() const
{
return current;
}
void
clear()
{
database.clear();
}
void
add(State const& state, Transition const& transition, State const& next)
{
database[state][transition] = next;
}
/*
Add a state which is also it's own transition (and thus a link in a chain of sequences)
*/

void
add(State const& state_and_transition, State const& next)
{
add(state_and_transition, state_and_transition, next);
}
bool
process(Transition const& transition)
{
auto const&
transitions = database[current],
found = transitions.find(transition);
if(found == transitions.end())
return false;
auto const&
next = found->second;
set(next);
return true;
}
/*
Process so-called "automatic transitions" (ie: sequencing)
*/

bool
process()
{
return process(get());
}
/*
A set of utility functions that may be helpful for displaying valid transitions to the user, etc...
*/

template <typename PushBackContainer>
bool
get_valid_transitions(State const& state, PushBackContainer& container)
{
container.clear();
auto const&
found = database.find(state);
if(found == database.end())
return false;
auto const&
transitions = found->second;
if(transitions.size() == 0)
return false;
for(auto const& iterator : transitions)
{
auto const&
transition = iterator.first;
container.push_back(transition);
}
return true;
}
template <typename Container>
bool
get_valid_transitions(Container& container)
{
return get_valid_transitions(get(), container);
}
};
 
/*
Example usage: a simple vending machine
*/

 
#include <string>
#include <vector>
#include <iostream>
 
using namespace
std;
void
print(string const& message)
{
cout << message << endl;
}
int
main()
{
finite_state_machine<string>
machine;
machine.add("ready", "quit", "exit");
machine.add("ready", "deposit", "waiting");
machine.add("waiting", "select", "dispense");
machine.add("waiting", "refund", "refunding");
machine.add("dispense", "remove", "ready");
machine.add("refunding", "ready");
machine.set("ready");
for(;;)
{
string
state = machine.get();
if(state == "ready")
print("Please deposit coins.");
else if(state == "waiting")
print("Please select a product.");
else if(state == "dispense")
print("Dispensed...please remove product from tray.");
else if(state == "refunding")
print("Refunding money...");
else if(state == "exit")
break;
else
print("Internal error: unaccounted state '" + state + "'!");
/*
Handle "automatic" transitions
*/

if(machine.process())
continue;
vector<string>
transitions;
machine.get_valid_transitions(transitions);
string
options;
for(auto const& transition : transitions)
{
if(!options.empty())
options += ", ";
options += transition;
}
print("[" + state + "] Input the next transition (" + options + "): ");
string
transition;
cout << " > ";
cin >> transition;
if(!machine.process(transition))
print( "Error: invalid transition!");
}
}
 
Output:
Please deposit coins.
[ready] Enter the next transition (deposit, quit): 
 > deposit
Please select a product.
[waiting] Enter the next transition (refund, select): 
 > refund
Refunding money...
Please deposit coins.
[ready] Enter the next transition (deposit, quit): 
 > deposit
Please select a product.
[waiting] Enter the next transition (refund, select): 
 > select
Dispensed...please remove product from tray.
[dispense] Enter the next transition (remove): 
 > remove
Please deposit coins.
[ready] Enter the next transition (deposit, quit): 
 > quit

Java[edit]

import java.util.*;
 
public class FiniteStateMachine {
 
private enum State {
Ready(true, "Deposit", "Quit"),
Waiting(true, "Select", "Refund"),
Dispensing(true, "Remove"),
Refunding(false, "Refunding"),
Exiting(false, "Quiting");
 
State(boolean exp, String... in) {
inputs = Arrays.asList(in);
explicit = exp;
}
 
State nextState(String input, State current) {
if (inputs.contains(input)) {
return map.getOrDefault(input, current);
}
return current;
}
 
final List<String> inputs;
final static Map<String, State> map = new HashMap<>();
final boolean explicit;
 
static {
map.put("Deposit", State.Waiting);
map.put("Quit", State.Exiting);
map.put("Select", State.Dispensing);
map.put("Refund", State.Refunding);
map.put("Remove", State.Ready);
map.put("Refunding", State.Ready);
}
}
 
public static void main(String[] args) {
Scanner sc = new Scanner(System.in);
State state = State.Ready;
 
while (state != State.Exiting) {
System.out.println(state.inputs);
if (state.explicit){
System.out.print("> ");
state = state.nextState(sc.nextLine().trim(), state);
} else {
state = state.nextState(state.inputs.get(0), state);
}
}
}
}
[Deposit, Quit]
> Deposit
[Select, Refund]
> Refund
[Refunding]
[Deposit, Quit]
> Deposit
[Select, Refund]
> Quit
[Select, Refund]
> Select
[Remove]
> Remove
[Deposit, Quit]
> Quit

Kotlin[edit]

// version 1.1.51
 
enum class State { READY, WAITING, EXIT, DISPENSE, REFUNDING }
 
fun fsm() {
println("Please enter your option when prompted")
println("(any characters after the first will be ignored)")
var state = State.READY
var trans: String
 
while (true) {
when (state) {
State.READY -> {
do {
print("\n(D)ispense or (Q)uit : ")
trans = readLine()!!.toLowerCase().take(1)
}
while (trans != "d" && trans != "q")
state = if (trans == "d") State.WAITING else State.EXIT
}
 
State.WAITING -> {
println("OK, put your money in the slot")
do {
print("(S)elect product or choose a (R)efund : ")
trans = readLine()!!.toLowerCase().take(1)
}
while (trans != "s" && trans != "r")
state = if (trans == "s") State.DISPENSE else State.REFUNDING
}
 
State.DISPENSE -> {
do {
print("(R)emove product : ")
trans = readLine()!!.toLowerCase().take(1)
}
while (trans != "r")
state = State.READY
}
 
State.REFUNDING -> {
// no transitions defined
println("OK, refunding your money")
state = State.READY
}
 
State.EXIT -> {
println("OK, quitting")
return
}
}
}
}
 
fun main(args: Array<String>) {
fsm()
}

Sample input/output:

Please enter your option when prompted
(any characters after the first will be ignored)

(D)ispense or (Q)uit : d
OK, put your money in the slot
(S)elect product or choose a (R)efund : s
(R)emove product : r

(D)ispense or (Q)uit : d
OK, put your money in the slot
(S)elect product or choose a (R)efund : r
OK, refunding your money

(D)ispense or (Q)uit : q
OK, quitting

PASCAL (Free Pascal 3.0.0)[edit]

This version uses fairly vanilla pascal to implement the task. I have 
added some confections to vend and some simple money handeling. It uses 
the table method to implement a FSM which is an explicit table with a 
dispatch loop. 

 
{
fsm1.pas
 
Copyright 2018 Trevor Pearson <trevor @ nb-LadyNada co uk >
 
This program is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or
(at your option) any later version.
 
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
 
You should have received a copy of the GNU General Public License
along with this program; if not, write to the Free Software
Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston,
MA 02110-1301, USA.
 
Implementing a simulation of a vending machine using a finite state
machine. I have used the classic table based method and added a
* little extra stuff to give the routines something to do.
 
 
}

 
 
 
program fsm1.pas;
 
uses sysutils;
 
 
type
 
state = (Null,Ready,Waiting,Refund,Dispense,Stop);
event = (Epsilon := 1,Deposit,Select,Cancel,Remove,Quit,Error);
Item = record
Name : string[12];
Stock: shortint;
price: currency;
end;
 
 
var
amountPaid, itemPrice , changeDue: currency;
I,J : integer;
machineState: state;
newState: state;
machineEvent: event;
entry:string;
vend : array[0..4,0..4] of Item;
machine : array[1..7,1..7] of state;
 
{ The following routines implement the transitions }
 
procedure TOready();
 
var
i,j : integer;
 
begin
 
{ Always set the state of a state machine as the first thing you do
We also set the event to epsiion we can allways set it to error if there is a problem}

 
machineState := Ready;
machineEvent := Epsilon;
 
{ Now do whatever we need to to transition into this state and check for errors}
 
Writeln(' Trevors vending machine');
Writeln('');
WriteLn (' A B C D' );
for i:=1 to 2 do begin
write(i,' ');
for j:=1 to 4 do begin
write(vend[j,i].Name,' ':(12-length(vend[j,i].Name)));
end;
WriteLn();
Write(' ');
for j:=1 to 4 do begin
write('£',vend[j,i].price:4:2,' ');
end;
Writeln('');
end;
 
{ We should have delt with money }
if (amountPaid > 0) then machineEvent := Error;
if (changeDue > 0) then machineEvent := Error;
 
end;
 
procedure TOwaiting();
begin
machineState := Waiting;
if ((machineEvent = Select) and (amountPaid >= itemPrice)) then machineEvent := Epsilon;
if ((machineEvent = Deposit) and (amountPaid >= itemPrice)) then machineEvent := Epsilon;
 
end;
 
procedure TOrefund();
 
begin
machineState := Refund;
machineEvent := Epsilon;
 
if (amountPaid > 0) then changeDue := amountPaid;
WriteLn('REFUNDING >> £' , changeDue:2:2);
changeDue := 0;
amountPaid := 0;
end;
 
procedure TOdispense();
begin
machineState := Dispense;
 
if (amountPaid >= vend[I,J].price) then begin
machineEvent := Remove;
changeDue := amountPaid - vend[I,J].price ;
amountPaid := 0;
vend[I,J].Stock := vend[I,J].Stock - 1;
WriteLn('Vending >>',vend[I,J].Name);
end
else machineState := Waiting;
end;
 
procedure TOstop();
begin
machineState := Stop;
machineEvent := Epsilon;
{ There should not be any transaction in process }
if ((amountPaid >0) or (changeDue >0)) then machineEvent := Error;
 
end;
 
 
 
procedure Init;
var k,l: integer;
begin
 
 
{ Lets pretend we have some stuff in this machine }
 
vend[0,0].Name := 'Dummy';
vend[0,0].Stock := 0;
vend[0,0].price := 9999;
 
vend[1,1].Name := 'Snickers';
vend[1,1].Stock := 12;
vend[1,1].price := 0.50;
 
vend[2,1].Name := 'Aero';
vend[2,1].Stock := 12;
vend[2,1].price := 0.50;
 
vend[3,1].Name := 'Bounty';
vend[3,1].Stock := 10;
vend[3,1].price := 0.75;
 
vend[4,1].Name := 'Creme egg';
vend[4,1].Stock := 15;
vend[4,1].price := 0.60;
 
vend[1,2].Name := 'Coke-Cola';
vend[1,2].Stock := 6;
vend[1,2].price := 1.10;
 
vend[2,2].Name := 'Pepsi';
vend[2,2].Stock := 6;
vend[2,2].price := 1.25;
 
vend[3,2].Name := '7 up';
vend[3,2].Stock := 6;
vend[3,2].price := 1.15;
 
vend[4,2].Name := 'Dr Pepper';
vend[4,2].Stock := 6;
vend[4,2].price := 1.99;
 
{ Set up the state table }
 
for k :=1 to 7 do begin
for l :=1 to 6 do machine[k,l] := Null;
end;
 
machine[ord(Ready),ord(Deposit)] := Waiting;
machine[ord(Waiting),ord(Deposit)] := Dispense;
machine[ord(Waiting),ord(Select)] := Dispense;
machine[ord(Waiting),ord(Cancel)] := Refund;
machine[ord(Dispense),ord(Remove)] := Refund;
machine[ord(Dispense),ord(Error)] := Refund;
machine[ord(Refund),ord(epsilon)] := Ready;
machine[ord(Ready),ord(Select)] := Waiting;
machine[ord(Ready),ord(Quit)] := Stop;
 
{ There should be no money entered so no change is due
* set itemPrice to a huge dummy amount}

 
amountPaid := 0;
changeDue := 0;
itemPrice := 999;
I:= 0;
J:=0;
end;
 
 
 
begin
Init;
TOready;
{ Here comes the magic bit ... We check for events and if an event
* occurs we look up on the table to see if we need to transition to
* another state. If we do we call the TO_xxxxx procedure. BUT we do
* this in the other order to check for machine generated events like
* Error and Epsilon. }

repeat
newState := machine[ord(machineState),ord(machineEvent)];
case (newState) of
Ready : TOready;
Waiting : TOwaiting;
Dispense : Todispense;
Refund: Torefund;
Stop: TOStop;
end;
 
 
{ We get some user input and assign an event to it
* If the user enters a number we convert it to currency and set a
* deposit event If we have a letter we are making a selection }

if (machineState = Ready) or (machineState = Waiting) then begin
WriteLn;
Writeln('Enter Selectian A1..D4');
Writeln('or deposit amount e.g, 0.20 -- 20p piece.');
Write('Or X to cancel, Q to stop this machine :');
ReadLn (entry);
if ((entry = 'q') or (entry = 'Q')) then machineEvent := Quit;
if ((entry = 'x') or (entry = 'X')) then machineEvent := Cancel;
if ((entry[1] in ['a'..'d']) or (entry[1] in ['A'..'D'])) then machineEvent:= Select;
if (entry[1] in ['0'..'9']) then begin
machineEvent := Deposit;
amountPaid := StrToCurr(entry);
end;
if (machineEvent = Select) then begin
I := ord(entry[1]) - 64;
if (I > 5) then I := I - 32;
J := ord(entry[2]) - ord('0');
end;
 
end;
until machineEvent = Quit;
 
end.
 
 


OUTPUT:
 *** Selection First ****

            Trevors vending machine

        A          B           C            D
1    Snickers    Aero        Bounty      Creme egg   
       £0.50      £0.50      £0.75      £0.60      
2    Coke-Cola   Pepsi       7 up        Dr Pepper   
       £1.10      £1.25      £1.15      £1.99      

Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :d1

Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :0.99
Vending  >>Creme egg
REFUNDING >> £0.39
            Trevors vending machine

        A          B           C            D
1    Snickers    Aero        Bounty      Creme egg   
       £0.50      £0.50      £0.75      £0.60      
2    Coke-Cola   Pepsi       7 up        Dr Pepper   
       £1.10      £1.25      £1.15      £1.99      

Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :q


 *** Deposit First ***
 
             Trevors vending machine

        A          B           C            D
1    Snickers    Aero        Bounty      Creme egg   
       £0.50      £0.50      £0.75      £0.60      
2    Coke-Cola   Pepsi       7 up        Dr Pepper   
       £1.10      £1.25      £1.15      £1.99      

Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :2.00

Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :b2
Vending  >>Pepsi
REFUNDING >> £0.75
            Trevors vending machine

        A          B           C            D
1    Snickers    Aero        Bounty      Creme egg   
       £0.50      £0.50      £0.75      £0.60      
2    Coke-Cola   Pepsi       7 up        Dr Pepper   
       £1.10      £1.25      £1.15      £1.99      

Enter Selectian A1..D4
or deposit amount e.g, 0.20 -- 20p piece.
Or X to cancel, Q to stop this machine :q





Racket[edit]

#lang racket
 
(define states
'((ready (deposit . waiting)
(quit . exit))
(waiting (select . dispense)
(refund . refunding))
(dispense (remove . ready))
(refunding . ready)))
 
(define (machine states prompt get-action quit)
(let recur ((state (caar states)))
(printf "CURRENT STATE: ~a~%" state)
(if (eq? state 'exit)
(quit)
(recur (match (cdr (assoc state states))
[(list (and transitions (cons actions _)) ...)
(prompt "next action (from: ~a): " actions)
(match (assoc (get-action) transitions)
[(cons action new-state)
(printf "~a -> ~a -> ~a~%" state action new-state)
new-state]
[#f (printf "invalid action for~%") state])]
[auto-state
(printf "~a -> ~a~%" state auto-state)
auto-state])))))
 
(module+ main
(let/ec quit
(with-input-from-string "deposit select remove deposit refund quit"
(λ () (machine states void read quit)))))
Output:
CURRENT STATE: ready
ready -> deposit -> waiting
CURRENT STATE: waiting
waiting -> select -> dispense
CURRENT STATE: dispense
dispense -> remove -> ready
CURRENT STATE: ready
ready -> deposit -> waiting
CURRENT STATE: waiting
waiting -> refund -> refunding
CURRENT STATE: refunding
refunding -> ready
CURRENT STATE: ready
ready -> quit -> exit
CURRENT STATE: exit

REXX[edit]

Translation of: BASIC


This version only works with:

  •   Personal REXX     --or--
  •   PC/REXX

This is essentially a one-for-one translation of the BASIC program, with the following minor differences:

  • the input allowed is either the uppercase or lowercase version of the letter(s)
  • a mixture of uppercase and lowercase text is used for the output messages
  • messages have extra blanks for readability   (and options are spelled out)
/*REXX pgm simulates a FSM (Finite State Machine), input is recognized by pressing keys.*/
10: say "Press D (deposit) or Q (quit)" /*display a prompt (message) to term. */
20: $=inkey(); upper $ /*since this a terminal, uppercase KEY.*/
if $=="D" then signal 50 /*Is response a "D" ? Process deposit.*/
if $=="Q" then exit /*Is response a "Q" ? Then exit pgm. */
signal 20 /*Response not recognized, re-issue msg*/
 
50: say "Press S (select) or R (refund)" /*display a prompt (message) to term. */
60: $=inkey(); upper $ /*since this a terminal, uppercase KEY.*/
if $=="S" then signal 90 /*Is response a "S" ? Then dispense it*/
if $=="R" then signal 140 /*Is response a "R" ? Then refund it. */
signal 60 /*Response not recognized? Re-issue msg*/
 
90: say "Dispensed" /*display what action just happened. */
signal 110 /*go and process another option. */
/* [↑] above statement isn't needed. */
110: say "Press R (remove)" /*display a prompt (message) to term. */
120: $=inkey(); upper $ /*since this a terminal, uppercase KEY.*/
if $=="R" then signal 10 /*Is response a "R" ? Then remove it. */
signal 120 /*Response not recognized, re-issue msg*/
 
140: say "Refunded" /*display what action just happened. */
signal 10 /*go & re-start process (ready state). */
output   when using (pressing) the exact same input(s) as the BASIC entry:     D   R   D   S   R   Q
press  D (deposit)   or   Q (quit)
d                                      ◄■■■■■■■■■■ user pressed this key.
Press  S (select)    or   R (refund)
r                                      ◄■■■■■■■■■■ user pressed this key.
Refunded
press  D (deposit)   or   Q (quit)
d                                      ◄■■■■■■■■■■ user pressed this key. 
Press  S (select)    or   R (refund)
s                                      ◄■■■■■■■■■■ user pressed this key. 
Dispensed
Press  R (remove)
r                                      ◄■■■■■■■■■■ user pressed this key. 
press  D (deposit)   or   Q (quit)
q                                      ◄■■■■■■■■■■ user pressed this key.

zkl[edit]

A lame FSM, we just convert text to a [hopefully valid] zkl program, compile and run it.

If we need true state to state hops, we could use tail recursion (another name for goto).

class FSM{	// our Finite State Machine
var bank=0, item=Void;
fcn deposit(coin){ bank=coin }
fcn select(item){
if(bank){ bank=0; self.item=item; }
else print("Depost coin, then select ")
}
fcn take { if(item) item=Void; else print("Select first "); }
fcn refund { coin:=bank; bank=0; return(coin) }
 
// couple of wrappers to state changes
fcn state{ println("Bank(%4d), Item(%s)".fmt(bank,item)) }
fcn act(f){ print("%-10s-->".fmt(f.name)); f(); state(); }
}
 
Vault.add(FSM); // put class FSM where I can find it
fcn run(program){  // convert text to FSM instructions and run them
program=program.replace("(",".fp("); // deposit(10)-->deposit.fp(10)
a,b,p := 0,0,Sink("class P(FSM){ state(); ");
while(Void!=(b=program.find(";",a)))
{ p.write("act(",program[a,b-a],");"); a=b + 1; }
program=p.write(program[a,*],"}").close();
// println(program); // WTH did I just do?
Compiler.Compiler.compileText(program)(); // compile and run our little FSM
}
run("select(); take(); deposit(10); select(\"snickers\"); take();");

The above is converted to:

class P(FSM){ 
state();
act(select.fp());
act( take.fp());
act( deposit.fp(10));
act( select.fp("snickers"));
act( take.fp());
}

The .fp() is function application (ie deferred execution) so I can extract the function name and print it.

Output:
Bank(   0), Item(Void)
select    -->Depost coin, then select Bank(   0), Item(Void)
take      -->Select first Bank(   0), Item(Void)
deposit   -->Bank(  10), Item(Void)
select    -->Bank(   0), Item(snickers)
take      -->Bank(   0), Item(Void)