I built a C interpreter in C# a while ago and have now begun converting it to Javascript. Everything was going fine until I realized js has no sleep function. My interpreter uses a recursive parser and it pauses for user input while it is nested several functions deep (in C# I used waithandle in a second thread). I have looked at setInterval and setTimeout but they are asynchronous /non-blocking; of course a busywait is out of the question and I looked at a timed_queue implementation I found on SO but no luck. I have tried the parser both in the main window and in a webworker. I am using jQuery. I have limited experience with js and am looking for ideas to pursue. I know little about continuation passing style, or yield and am wondering if they might hold the key. Here is a bit cut from the code to show some of the controlscript. Any ideas please...
var STATE = {
START: "START",
RUN: "RUN", //take continuous steps at waitTime delay
STEP: "STEP", //take 1 step
PAUSE: "PAUSE",//wait for next step command
STOP: "STOP",
ERROR: "ERROR"
}
var state = state.STOP;
function parsing_process() //long process we may want to pause or wait in
{
while(token !== end_of_file)//
{
//do lots of stuff - much of it recursive
//the call to getNextToken will be encountered a lot in the recursion
getNextToken();
if (state === STATE.STOP)
break;
}
}
function getNextToken()
{
//retrieve next token from lexer array
if (token === end_of_line)
{
//tell the gui to highlight the current line
if (state === STATE.STOP)
return;
if (state === STATE.STEP)//wait for next step
{
//mimick wait for user input by using annoying alert
alert("click me to continue")
}
if (state === STATE.RUN) {
//a delay here - set by a slider in the window
//a busy wait haults processing of the window
}
}
}
I have gotten this to work in Firefox using task.js
<html>
<head>
<title>task.js examples: sleep</title>
<script type="application/javascript" src="task.js"></script>
</head>
<body>
Only works in FIREFOX
<button onclick="step()">Step</button>
<button onclick="run()">Run</button>
<button onclick="stop()">Stop</button>
<pre style="border: solid 1px black; width: 300px; height: 200px;" id="out">
</pre>
<script type="application/javascript;version=1.8">
function start() {
process();
}
function step() {
if (state === STATE.STOP)
start();
state = STATE.STEP;
}
function run() {
if (state === STATE.STOP)
start();
state = STATE.RUN;
}
function stop() {
state = STATE.STOP;
}
var STATE = {
START: "START",
RUN: "RUN", //take continuous steps at sleepTime delay
STEP: "STEP", //take 1 step
PAUSE: "PAUSE",//wait for next step command
STOP: "STOP",
ERROR: "ERROR"
}
var state = STATE.STOP;
var sleepTime = 500;
function process() {
var { spawn, choose, sleep } = task;
var out = document.getElementById("out");
var i=0;
out.innerHTML = "i="+i;
var sp = spawn(function() {
while(state !== STATE.STOP)
{
i++;
out.innerHTML = "i="+i;
if (state === STATE.RUN)
{
yield sleep(sleepTime);
}
if (state === STATE.STEP)
state = STATE.PAUSE;
while (state===STATE.PAUSE)
{
yield;
}
}
});
}
</script>
</body>
</html>
I would appreciate it if someone who knew something about promises could give me some more clues. My application is not a consumer one but it would be nice if it ran in more than Firefox
As the author of JSCPP, I faced the exactly same issue when I was implementing a debugger which pauses and continues the program interpreting on the fly. In the end I decide to use generator functions from es6 but I would like to share my thought process here.
The common way is to first compile target code into a low-level recursive-free byte-code. You label each statement and then handle all the control flow with
unconditional jumpandconditional jump. Then you run a byte-code interpreter on top of that. This is a good option if you don't mind all these compiling work to be done.The other way is a "call stack save/call stack load" work flow. When you need to pause interpreting, you recursively push all arguments and all local variables to a customized stack all the way back to bottom. When you need to continue execution, you recursively load all these arguments and local variables. Your code will be transformed from
to
This method does not alter the main logic of your interpreter but you can see for yourself how crazy the code is for a simple A+B syntax.
Finally I decide to use generators. Generators are not meant for interactively altering program execution, but rather for lazy-evaluation purpose. But with some simple hacking, we can use lazy-evaluate our statements upon receiving "continue" command.
Here,
function*indicates we want theAddExpression.visitfunction to be generator function.yield*followed byvisitcall meansvisitfunction itself is a recursive generator function.This solution seems perfect at first glance but it is suffering from a huge performance decrease by using generators (http://jsperf.com/generator-performance) and that it is from es6 and not many browsers support it.
To conclude, you have three different ways of achieving interruptible execution: