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Delimited Continuations

Algebraic Effects in JavaScript part 3 - Delimited Continuations

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

In the preceding parts, we introduced the notions of continuations and control transfer. We saw how to capture the current continuation inside a Generator, and illustrated how to implement (the one shot version) of the famous callcc.

In this part, we're going to see how to capture delimited continuations with Generators. While callcc allowed us to capture the rest of the whole program, we can also choose to capture only a slice of it. One of the direct consequences of this concept is that delimited continuations can now return a value, and thus they can be composed inside the flow of another function. This is an important trait that will be exploited in the next part.

Back to the Call Stack

In direct style, we saw that control transfer between functions works through the Call Stack.

  • Each function call pushes a new frame (called also an activation record) onto the stack
  • Each function return pops the corresponding frame from the stack

Let's consider the following example, which computes the product of an array of numbers

function main() {
  const result = product([2, 4, 6]);
  return result;
}

function product(xs) {
  if (xs.length === 0) return 1;
  const [y, ...ys] = xs;
  return y * product(ys);
}

To visualize the call stack at a given moment, we can set a breakpoint in the browser devtools then run the above example in the console. The program will pause and we can examine the Call Stack panel of the browser

call-stack-1

Here, the program is paused on the third line of product(). The Call Stack contains already four frames:

  • anonymous can be seen as the root frame of the browser console session
  • main corresponds to the main() call executed in the console
  • The first product frame represents product([2, 4, 6]) executed in main
  • The second product frame represents the recursive call inside the return y * product(ys) statement (ie return 2 * product([4,6]))

In other words, the Call Stack tells us what part of the work has already been done. It tells us, also, what part of the work remains to do:

  • The rest of the work to do inside the current frame (product([4,6])), namely calling product([6]), multiplying the result by y (= 4) then returning the result (24) to the parent frame
  • Plus the rest of the work to do in the parent frames:
    • the call frame of product([2,4,6]) will multiply the previous result by 2 then returns 48 to the main frame
    • The call frame of main() will simply return the result 48 to its parent frame
    • The call frame of anonymous will display the result into the console

In other words, the continuation is mainly represented with the state of the Call Stack at the considered moment of execution. Therefore, if we could implement something similar to the Call Stack on top of Generators we'll be able, in principle, to capture current continuations.

Contrast this with the CPS representation of the continuation as an ordinary function. This stateless representation may be seen as superior (to the Call Stack's statefull representation) since it brings us closer to purity. However, the Call Stack representation has some advantages as well:

  • It's easier to implement more advanced stack manipulations, like delimited continuations, using the statefull representation (possible because JavaScript is single threaded)
  • It's easier to add DX features on top of the statefull approach. For example, a babel plugin can instrument the code to add some useful information (function name, line, column) to the stack frames, and some program API can dump this information in developer mode.

Modeling the Call Stack with Generators

Below is a new implementation using the statefull approach

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

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

  if (done) {
    const _return = gen._return;
    if (isGenerator(_return)) {
      runGenerator(_return, value);
    } else if (typeof _return === "function") {
      _return(value);
    }
  } else {
    if (isGenerator(value)) {
      value._return = gen;
      runGenerator(value, null);
    } else if (typeof value === "function") {
      value(gen);
    }
  }
}

function start(gen, onDone) {
  gen._return = onDone;
  runGenerator(gen, null);
}

Instead of passing a continuation argument, we now rely on the presence of a _return field in the Generator, which represents the parent frame (it may be safer to use a Symbol here). When the Generator is done, it passes the return value to its caller. When we call a child Generator, we set its _return to the current Generator.

Note also that we're now passing the Generator itself to the yielded function. So to implement something like sleep(millis) we have to write

function sleep(ms) {
  return function(gen) {
    setTimeout(x => runGenerator(gen, null), ms);
  };
}

In the statefull implementation, we're effectively building a linked list of Generators (with a callback inserted at the root by start).

The implementation of callcc can be also automatically adapted

function callcc(genFunc) {
  return function(capturedGen) {
    // this is our escape function
    function jumpToCallccPos(value) {
      // instead if resuming the current generator
      // we directly resume the one captured by callcc
      return next => runGenerator(capturedGen, value);
    }
    const gen = genFunc(jumpToCallccPos);
    gen._return = capturedGen;
    runGenerator(gen, null);
  };
}

Ok, now that we have reified the Call stack as a concrete data structure, we're ready to tackle delimited continuations.

Delimited Continuations

We'll introduce how delimited continuations work step by step through a series of examples.

We said that delimited continuations capture only a slice of the Call Stack. Our first step will be, then, some way to mark a stack frame as the limit of the continuation to be captured. This is the purpose of reset

function reset(genFunc) {
  return function(parentGen) {
    const gen = genFunc();
    gen._return = parentGen;
    // setting the limit of the continuation
    gen._reset = true;
    runGenerator(gen, null);
  };
}

reset takes a Generator function and returns a suspended computation (here a function taking the parent Generator). Like runGenerator, the suspended computation will run the provided Generator function after setting its _return field to the caller Generator. It also adds a special _reset field, which acts as a marker on the Call Stack. This field will serve us to limit the extent of the captured continuation as we'll see later.

The first thing to note is that, when invoked on an 'ordinary' Generator, reset amounts to a simple Generator call

function* main() {
  const result = yield reset(function*() {
    return "Hi";
  });
  return result;
}

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

So alone, reset is pretty useless. The interesting stuff happens when we introduce our next function shift inside a reset block.

We'll first introduce a simplified version of shift that doesn't capture the current continuation

function shift(genFunc) {
  return function(parentGen) {
    // finds the closest reset
    let resetGen = parentGen;
    while (!resetGen._reset) {
      resetGen = resetGen._return;
    }
    const gen = genFunc();
    // gen will directly return to the parent of reset
    gen._return = resetGen._return;
    runGenerator(gen, null);
  };
}

Here's an example of how it works

function* main() {
  const result = yield reset(function* resetFn() {
    const name = yield child();
    return "Hi " + name;
  });
  return result;
}

function* child() {
  const result = yield shift(function* shiftFn() {
    return "from inside shift";
  });
  return result;
}

start(main(), console.log);
// => from inside shift

In a normal sequence of calls, we'd expect the result to be 'Hi from inside shift'. However, shift isn't an ordinary function. In the above code, the Generator provided to shift will return, directly, to the parent of the closest reset block. In this case, it effectively behaves as our previous exit function. More concretely, w've transformed the following Call Stack

main() -> reset(resetFn) -> child() -> shift(shiftFn)

into this one

main -> shiftFn()

Put another way, we've discarded all the stack frames between shift and (including) reset.

What happens to the discarded frames? Well, here's the more interesting stuff, those would constitute the delimited continuation that should be provided to shift.

function shift(genFunc) {
  return function(parentGen) {
    // finds the closest reset
    let resetGen = parentGen;
    while (!resetGen._reset) {
      resetGen = resetGen._return;
    }

    function delimitedCont(value) {
      // captures the continuation from after shift up to reset
      return nextGen => {
        resetGen._return = nextGen;
        // resume from the shift's parent frame
        runGenerator(parentGen, value);
      };
    }

    const gen = genFunc(delimitedCont);
    gen._return = resetGen._return;
    runGenerator(gen, null);
  };
}

It may seem confusing how this works, so let's go step by step on a simple example

function* main() {
  const x = yield reset(function* resetFn() {
    const a = 10;
    const b = yield shift(function* shiftFn(k) {
      const c = yield k(2);
      return c + 3;
    });
    return a * b;
  });
  return x;
}

The sequence of calls until shift corresponds to

main() -> #resetFn() -> shift(shiftFn)

Where # is used to mark the reset position. We saw that the first effect of shift is to discard the frames up to the enclosing reset

main() -> shift(shiftFn) -> ...

Then the discarded frames (here #resetFn()) are provided as a continuation to shiftFn. So after the yield k(2) we obtain the following sequence

main() -> shiftFn(k) -> #resetFn()

What does #resetFn() corresponds to? it's the rest of work to do after the shift position: namely setting b with some provided value then multiplying by a (= 10). ie it's like a function: (v => a * v) -> (2 * 10) -> 20

After #resetFn() returns, shift continues by adding the obtained result 20 to 3. The final result is then 23.

Naturally, you have all the right to ask the legitimate question: why do we have to program in such a confusing style?

We have the choice between two answers:

I can repeat the arguments from the previous parts about how this can give control-flow super-powers. Which is partly true, but maybe not too concrete.

Or, you can read the next (and final) part: this time we'll be really talking about Algebraic Effects and Handlers.

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