What's the resume function? about koka

[Pauan]

@zaoqi-unsafe The resume function is like a callback in JavaScript.

Let's say you are using some Node.js APIs in JavaScript:

function foo(cb) {
  fs.readFile("foo", function (err, data) {
    fs.writeFile("bar", data, cb);
  });
}

When the readFile function is finished, it will call the callback with the file contents.

Now consider this Koka code:

fun foo(): asyncx () {
  val data = read-file("foo")
  write-file("bar", data)
}

The Koka compiler will translate the above code into this JavaScript:

function read_file(path, resume) {
  // Do some stuff then call resume
}

function write_file(path, data, resume) {
  // Do some stuff then call resume
}

function foo(resume) {
  read_file("foo", function (data) {
    write_file("bar", data, resume);
  });
}

In other words, Koka automatically added in the callbacks for you.

When you write an effect handler, resume is the callback.

So if read-file never calls resume, then the read-file function will never return.

And if read-file does call resume, then the first argument to resume is the return value for read-file

[Pauan]

@zaoqi-unsafe Handlers can have extra state (in this case i). When you call resume, the first argument is the new state, and the second argument is the return value.

So in this case i is the current state, and resume(i,i) keeps the current state, and resume(j,()) sets the state to j

[zaoqi-unsafe]

@Pauan

val state = handler(i) {
  return x -> x
  get()    -> resume(i,i)
  set(j)   -> resume(j,())
}

[b-studios]

Thanks for answering @Pauan -- I'll go ahead and close the issue.