This week I learned a little bit of Go. I was fascinated by the power and simplicity goroutines and channels.

With these ideas fresh in my mind, I decided to reproduce Mark C. Chu-Carroll’s Go prime sieve in JavaScript with goroutines and channels as my toolset instead of conventional JavaScript idioms. Find my translation on GitHub.

I chose JavaScript because it shares many traits with Go, such as multiple threads of control, but doesn’t have a notion of channels.

Background: goroutines and channels

A goroutine is an independent concurrent thread of control.

A channel is a mechanism for two concurrently executing functions to communicate by passing a value. A channel has two methods, read and write, which are synchronous by default. That is, a write blocks until there is a read to consume it, and a read blocks until there is a write to consume.

JavaScript implementation

Goroutines are rather trivial to implement, a simple setTimeout will schedule an independent thread of control.

function go (fn) {
  setTimeout(fn, 0);
};

Synchronous channels will require a queue of reader and writer callbacks. I decided to call the reader callbacks before the writers, but I don’t think it matters.

var chan = function () {
  this.readers = []; // [cb, ...]
  this.writers = []; // [[value, cb], ...]
};

chan.prototype.read = function (cb) {
  if (this.writers.length) {
    // consume a writer
    var writer = this.writers.shift();
    cb(writer[0]);
    writer[1](writer[0]);
  } else {
    // queue the reader
    this.readers.push(cb);
  }
};

chan.prototype.write = function (value, cb) {
  if (this.readers.length) {
    // consumer a reader
    var reader = this.readers.shift();
    reader(value);
    cb(value);
  } else {
    // queue the writer
    this.writers.push([value, cb]);
  }
};

I also pass the writer’s value to its callback because it could be useful.

The prime sieve

Mark’s Go sieve begins with an integer generator.

func generate_integers() chan int {
    ch := make(chan int);
    go func(){
        for i := 2; ; i++ {
            ch <- i;
        }
     }();
    return ch;
}

The problem here is that the channel write (ch <- i) blocks inside of a loop. In JavaScript, we "block" by passing a callback that is called once the procedure can continue. Here, the producer function passes itself as the callback to write.

function integers () {
  var ch = new chan();
  go(function () {
    var producer = function (i) {
      ch.write(i + 1, producer);
    };
    ch.write(2, producer);
  });
  return ch;
};

This Go function excludes multiples of a given prime from the given channel.

func filter_multiples(in chan int, prime int) chan int {
   out := make(chan int);
   go func() {
      for {
         if i := <- in; i % prime != 0 {
             out <- i;
         }
      }
    }();
   return out;
}

We can rewrite this with another recursive callback, but this time we only have to block when we do a write.

function filter_multiples (ch, prime) {
  var out = new chan();
  go(function () {
      var consumer = function (i) {
        if (i % prime != 0) {
          out.write(i, function () {
            ch.read(consumer);
          });
        } else {
          ch.read(consumer);
        }
      }
      ch.read(consumer);
  });
  return out;
};

The sieve in Go will chain a series of channels to exclude multiples of all the primes we have seen.

func sieve() chan int {
   out := make(chan int);
   go func() {
      ch := generate_integers();
      for {
	     prime := <- ch;
	     out <- prime;
	     ch = filter_multiples(ch, prime);
      }
   }();
   return out;
}

We achieve the same in JavaScript with another recursive callback.

function sieve () {
   var out = new chan();
   go(function () {
      var ch = integers();
      function iteration () {
        ch.read(function (prime) {
          out.write(prime, function () {
            ch = filter_multiples(ch, prime);
            iteration();
          });
        });
      };
      iteration();
   });
   return out;
};

Mark's program simply reads from the sieve channel and prints out each prime number.

func main() {
  primes := sieve();
  for {
    fmt.Println(<-primes);
  }
}

Mine uses another recursive callback to do the same.

function main () {
  var primes = sieve();
  function iteration() {
    primes.read(function (i) {
      sys.puts(i);
      iteration();
    });
  };
  iteration();
}

main();

Demo

Go-flavored JavaScript: Prime Sieve

Discussion

Note that the call to main() returns (practically) immediately, since the goroutine in sieve(), which hasn't executed, has not yet written anything to the channel, so the read inside iteration just pushes the callback onto a read queue and returns. Once the main thread of control stops, the event loop begins running the goroutines, which drive the rest of the program. (Please correct me if I'm wrong.)

That functions return quickly is generally desirable, in JavaScript or Go, because a caller should not have to wait for a function to do some "hard work" before it returns. Instead, the hard work should occur in a separate thread of control and passed back to the caller by some layer of indirection.

JavaScript programmers typically solve this problem with callbacks which receive result of the hard work. The Go language offers synchronous channels between independent threads of control as a more sophisticated solution. These tools are easily ported to idiomatic JavaScript based on callbacks. Although unconventional, these tools are simple and may be very powerful when composed.

I hope this toy example borrowed from Go inspires JavaScript programmers to consider the unconventional.