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Capturing continuations with Generators

Algebraic Effects in JavaScript part 2 - Capturing continuations with Generators

This is the second part of a series about Algebraic Effects and Handlers.

Note: initially I planned a 3-part series, but since the current post on undelimited continuations ended up taking more space than intended. We'll treat delimited continuations in a separate post

In the first post we introduced the notions of continuation and control transfer. We saw how programs written in Continuation Passing Style (CPS) are more flexible in terms of control transfer manipulation. While, in direct style, control transfer is implicitly managed by the compiler via the call stack, in CPS continuations are reified as first class arguments to CPS functions.

However, a major drawback of CPS programs is that they are harder to read and write by humans, so they are more suitable to be manipulated by other programs like compilers or interpreters. This is why programming languages that expose continuations often provide a direct style syntax/API to manipulate them.

In this part, we'll do the same in JavaScript. Although the language doesn’t provide a way to access continuations we can always [try to] emulate them using Generator functions.

This post assumes the reader is familiar with Generator functions.

Driving Generators in direct style

Say we have this simple function

function greet(name) {
  const message = `Hi ${name}`;
  return message;
}

greet("Stranger");
// => "Hi Stranger"

Running this function is as simple as const result = greet(someString). Now if we take the Generator version

function* greet(name) {
  const message = yield `Hi ${name}`;
  return message;
}

greet("Stranger");
// => greet { <suspended>, __proto__: Generator, ... }

We get only the Generator object. In order to get the result we need to step the Generator until it's done. Below is the code for a function that drives the Generator and returns its result

function runGenerator(gen, arg) {
  const { done, value } = gen.next(arg);
  if (done) {
    return value;
  }
  return runGenerator(gen, value);
}

runGenerator(greet("Stranger"));
// => "Hi Stranger"

Works greet, but just like normal functions can call other normal functions, we'd like also for our Generators to call other Generators. For example, this is the Generator version of the factorial function

function* factorial(n) {
  if (n === 0) return 1;
  const n1 = yield factorial(n - 1);
  return n * n1;
}

runGenerator(factorial(10));
// => NaN

Fortunately, Generators allow us to intercept yielded values. This gives us the ability to interpret those values as desired then resume the Generator with the result of the interpretation.

In our case, interpreting child generators amounts to recursively running them and getting their result.

function isGenerator(x) {
  return x != null && typeof x.next === "function";
}

function runGenerator(gen, arg) {
  const { done, value } = gen.next(arg);
  if (done) {
    return value;
  }
  // interpret calls to child Generators
  if (isGenerator(value)) {
    const result = runGenerator(value);
    return runGenerator(gen, result);
  }
  return runGenerator(gen, value);
}

runGenerator(factorial(10));
// => 3628800

So far, we can call a Generator like a normal function, which includes nested and recursive calls. It seems like we've been able to emulate the call stack. Note here we're just reusing the underlying JavaScript call stack.

However, as we saw in the previous post, direct style can't deal with the async problem. CPS allows us to perform asynchronous calls but that comes with a price. Our next step is to allow those calls while still preserving the direct style.

Driving Generators in CPS

Let's say we want to implement a sleep function that, when yielded in a Generator, will pause its execution for some time

function* slowDouble(x) {
  yield sleep(2000);
  return x * 2;
}

In its current form, runGenerator is unable to implement the sleep behavior because it runs recursively/synchronously until completion.

In order to allow async calls, we need to rewrite the function in CPS: remember in this style we don't return function results, instead we pass them to the provided continuation(s)

function runGenerator(gen, arg, next) {
  const { done, value } = gen.next(arg);
  if (done) {
    next(value);
  } else if (isGenerator(value)) {
    runGenerator(value, null, function(result) {
      runGenerator(gen, result, next);
    });
  } else {
    runGenerator(gen, value, next);
  }
}

But we're not there yet. So far we can only yield child generators or plain values. We need a way to represent async calls and we need to interpret the given representation.

A simple solution is to represent async calls themselves as CPS functions. Let's say we write a CPS sleep version

function sleep(millis, next) {
  setTimeout(next, millis);
}

If we curry it

function sleep(millis) {
  return next => setTimeout(next, millis);
}

The curried version is more suitable to use with runGenerator. We can simply plug in a continuation that will resume the Generator with the async result. More generally, we'll represent async calls with functions taking a single callback. We'll call those functions suspended computations.

function runGenerator(gen, arg, next) {
  const { done, value } = gen.next(arg);
  if (done) {
    next(value);
  } else if (isGenerator(value)) {
    runGenerator(value, null, function continuation(result) {
      runGenerator(gen, result, next);
    });
  } else if (typeof value === "function") {
    // here we handle suspended computations
    value(function continuation(result) {
      runGenerator(gen, result, next);
    });
  } else {
    runGenerator(gen, value, next);
  }
}

runGenerator(slowDouble(10), null, console.log);
// tic tac toc
// 20

For readers already familiar with async implementation on top of Generators, this seems just like the old plumbing trick. But observe that the callback we provided to the suspended computation represents the continuation of the whole program, so now we have the full control over what to do next. Put another way, we gain the flexibility of CPS while still writing direct style code.

As a simple illustration, here is an example that simulates debugger's break. Instead of invoking the continuation, we save it in a variable and then pause the whole program.

let resume;

const BREAK = next => {
  console.log("**PAUSED**");
  resume = next;
};

function* main() {
  yield breakTest();
  yield sleep(1000);
  console.log("end of main");
}

function* breakTest() {
  for (let i = 1; i < 5; i++) {
    yield sleep(1000);
    console.log("message", i);
    if (i % 2 === 0) yield BREAK;
  }
}

// typing this in the console
runGenerator(main(), null, console.log);
/*
  message 1
  message 2
  **** PROGRAM PAUSED ****
*/
resume();
/*
  message 3
  message 4
  **** PROGRAM PAUSED ****
*/
resume();
// end of main

Another example would be an exit(result) function that, when yielded from inside a deeply nested Generator, would skip all the parents and abort the whole computation with the given result. For example consider the following code

function* main() {
  const result = yield parent();
  return `main result: (${result})`;
}

function* parent() {
  const result = yield child();
  return `parent result: (${result})`;
}

function* child() {
  return "child result";
}

runGenerator(main(), null, console.log);
// => main result: (parent result: (child result))

Using exit we could abort directly from inside child

function main() { ... }

function parent() { ... }

function* child() {
  yield exit("child result");
  throw "This shouldn't happen";
}

runGenerator(main(), null, console.log);
// should be => child result

If you recall the interpreter example in the previous post, at some point we did the same thing by providing the top-level continuation as a second argument to all child CPS functions. We can do the same trick here with runGenerator. It would be a good exercise.

The road to undelemited continuations

Ok, I assume, with good faith, that you did the last exercise. Here is the my solution

function runGenerator(gen, arg, abort, next) {
  const { done, value } = gen.next(arg);
  if (done) {
    next(value);
  } else if (isGenerator(value)) {
    runGenerator(value, null, abort, function continuation(result) {
      runGenerator(gen, result, abort, next);
    });
  } else if (typeof value === "function") {
    value(abort, function continuation(result) {
      runGenerator(gen, result, abort, next);
    });
  } else {
    runGenerator(gen, value, abort, next);
  }
}

// helper function to thread in the top-level continuation
function start(gen, next) {
  runGenerator(gen, null, next, next);
}

start(main(), console.log);
// => child result

It works, but it's not very satisfactory. We said that the promise of CPS is to empower us, end users of the API, so we can implement various control operators. But in the above solution, the control is hard coded inside the interpreter (runGenerator). We don't want to modify the interpreter each time we want to add some control construct and more importantly we don't want to implement our solutions in low level CPS code. What w're really aiming for is to provide some more general API in order to implement exit or other control flow in user land.

Let's go step by step. First, observe that what start does, essentially, is capturing the top-level continuation. But we know we can capture a continuation by yielding a suspended computation in the Generator. So, our first step would be capturing the top-level continuation.

For that, We'll make start itself a Generator and capture its continuation.

function* start(genFunc) {
  const result = yield function(abort) {
    runGenerator(genFunc(abort), null, abort);
  };
  return result;
}

We're using runGenerator manually, which is a little awkaward, but this leaves our interpreter unmodified. Later we'll see how to abstract away this code.

Next, we observe that the captured continuation is just passed as an additional argument to the nested runGenerator calls in order to keep it visible in the current scope. We can do the same by exploiting the lexical scope of Generators and passing the captured continuation as an argument to child Generators.

Our first tentative of refactoring yields the below code

function* start(genFunc) {
  const result = yield function(abort) {
    runGenerator(genFunc(abort), null, abort);
  };
  return result;
}

function* main(abort) {
  const result = yield parent(abort);
  return `main result: (${result})`;
}

function* parent(abort) {
  const result = yield child(abort);
  return `parent result: (${result})`;
}

function* child(abort) {
  yield next => abort("child result");
  throw "This shouldn't happen";
}

runGenerator(start(main), null, console.log);
// => child result

By the way, notice how, in child, the next continuation is ignored in the body of the suspended computation, which instead invokes abort. It means the next statement throw "This shouldn't happen" won't be executed and the control will jump back directly into the start Generator.

But we're not there yet, how can we implement the generic exit(result) function?

Well, given the current code, we can't. Our exit has no way to get the abort continuation without this being visible in scope. Surely this is awkward, we don't want to end up writing yield next => abort(result) each time we want to exit.

There is less awkward alternative, though. Instead of forwarding the captured continuation itself, then creating the suspended computation (exit) inside the exiting function, we can create exit itself inside the code that captures the top-level continuation (here in the start Generator), then pass it to child Generators.

function* start(genFunc) {
  const result = yield function(abort) {
    function exit(value) {
      return next => abort(value);
    }
    runGenerator(genFunc(exit), null, abort);
  };
  return result;
}

function* main(exit) {
  const result = yield parent(exit);
  return `main result: (${result})`;
}

function* parent(exit) {
  const result = yield child(exit);
  return `parent result: (${result})`;
}

function* child(exit) {
  yield exit("child result");
  throw "This shouldn't happen";
}

runGenerator(start(main), null, console.log);
// => child result

All we need, in order to complete the refactoring, is to abstract away the code that captures the top-level continuation inside a reusable function. But first we need to pick a suitable name for it. call_with_current_continuation looks expressive but quite verbose, so let's abbreviate it to callcc.

function callcc(genFunc) {
  return function(capturedCont) {
    // this is our previous exit
    function jumpToCallccPos(value) {
      return next => capturedCont(value);
    }
    runGenerator(genFunc(jumpToCallccPos), null, capturedCont);
  };
}

function* start() {
  const result = yield callcc(main);
  return result;
}

// rest of the code unmodified

runGenerator(start(), null, console.log);
// => child result

Note that, unlike what's found in languages like Scheme, our implementation allows only one invocation of the callcc continuation. We're here constrained by how Generators work in JavaScript. Each call to generator.next() is a one way ticket, so invoking the continuation multiple times will just keep advancing the Generator. Continuations that can be resumed only once are said to be one shot. Continuations that can be resumed many times are said to be multi shot.

In this series, we'll content ourselves with one shot continuations. If you're interested in how we could emulate multi shoot continuations Here is an example. Note this has a non negligible space/time cost.

The rest of the post illustrates the use of callcc with a couple of common examples.

Example 1: Emulating try/cacth

The previous exit example implemented a simplified version of exceptions. Next, we'll try to make a more elaborated example of structured exception handling

const handlerStack = [];

function* trycc(computation, handler) {
  return yield callcc(function*(k) {
    handlerStack.push([handler, k]);
    const result = yield computation;
    handlerStack.pop();
    return result;
  });
}

function* throwcc(exception) {
  const [handler, k] = handlerStack.pop();
  const result = yield handler(exception);
  yield k(result);
}

trycc/throwcc emulates the try/catch/throw statements. trycc starts by capturing the current continuation, saves it in a stack along with the handler, then run the computation, which may (or may not) throw. If the computation returns successfully then no exception was thrown and we can remove the handler from the stack. In the case the computation has invoked throwcc then we also pop the handler stack along with the captured continuation, run the handler then use the captured continuation to jump back to where trycc was called.

Example 2: cooperative scheduling

Another popular example is the implementation of cooperative scheduling using what we call coroutines. They are somewhat similar to Generators. Once started, a coroutine executes some code then may yield to a central scheduler. The scheduler will save the state of the coroutine then pick another coroutine to run. Below is an example

function* main() {
  yield fork(proc("1", 4));
  yield fork(proc("2", 2));
  yield dequeue();
  console.log("end main");
}

function* proc(id, n) {
  for (let i = 0; i <= n; i++) {
    yield sleep(1000);
    console.log(id, i);
    yield pause;
  }
}

Assuming we have implemented fork and pause, the result of running main() gives the following outputs

  1 0
  2 0
  1 1
  2 1
  1 2
  2 2
  1 3
  1 4
  end main

A possible implementation of coroutines is given below

const processQueue = [];

function fork(gen) {
  return next => {
    processQueue.push(
      (function*() {
        yield gen;
        yield dequeue();
      })()
    );
    next();
  };
}

const pause = callcc(function*(k) {
  processQueue.push(k());
  yield dequeue();
});

function* dequeue() {
  if (processQueue.length) {
    const next = processQueue.shift();
    yield next;
  }
}

Here's how the above code works

  • fork doesn't start the provided coroutine immediately, it just adds it to a global queue of processes
  • pause saves the state of the current coroutine by capturing its continuation, adding it to the process queue then picking the next coroutine to resume
  • dequeue is called both when a coroutine pauses and when it returns

Conclusion

Voilà! we reached the end of the second part. Just a couple more of posts to complete the understanding Algebraic Effects and Handlers.

Main takeaways of this part:

  • When driven using dierct style, Generators can emulate the call stack, but can't support async calls
  • When driven using CPS, Generators can perfom async work while still allowing the user to program in direct style
  • More important, we can capture the current contiuation of the program anytime we need it (callcc)
  • When the callcc continuation is invoked it aborts the current execution context and resumes from when callcc was invoked

Although callcc is quite powerful, it has a major limitation. The captured continuation represents the rest of the whole program. It means the yield k(someValue) can't return values since all we can do is resume until the program completes. This kind of continuations is known as undelimited continuations.

Next part, we'll see an even more powerful kind: delimited continuations, which allow us to capture only a slice of the rest of the program. A delimited continuation can return a value and thus it can be composed inside other functions.

See you next post. Thanks for being a patien reader!

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