Runtime System

Runtime Configuration

The Stopify runtime system takes a dictionary of options with the following type:

interface RuntimeOpts {
  estimator?: "velocity" | "reservoir" | "exact" | "countdown",
  yieldInterval?: number    /* must be greater than zero */,
  stackSize?: number        /* must be greater than zero */
  restoreFrames?: number    /* must be greater than zero */

The first two options control how frequently Stopify yields control to the browser (yieldInterval) and the mechanism that it uses to determine elapsed time (estimator). The last two options can be used to simulate a larger stack than what JavaScript natively provides.

Time estimator (.estimator)

By default, Stopify uses the velocity estimator that samples the current time (using and tries to yield every 100 milliseconds. The velocity estimator dynamically measures the achieved yield interval and adapts how frequently it yields accordingly. This mechanism is inexact, but performs well. You can adjust the yield interval, but we do not recommend using a value lower than 100.

The reservoir estimator samples the current time using reservoir sampling (i.e., the probability of resampling the current time decreases as the program runs longer). This technique is less robust than velocity to fluctuations in program behavior, but still outperforms other methods. This usually has a lower runtime overhead than velocity, but sacrifices accuracy. We recommend velocity for a more general, nondeterministic estimator.

The countdown estimator yields after exactly n yield points have passed. With this estimator, the yieldInterval is interpreted as the value of $n$ and not a duration. We do not recommend using this estimator in practice, since a good value of $n$ will depend on platform performance and program characteristics that are very hard to predict. However, it is useful for reproducing bugs in Stopify, since the velocity estimator is nondeterministic.

Finally, the exact estimator checks the current time at every yield point, instead of sampling the time. This has a higher runtime overhead than velocity and we do not recommend it.

Unbounded stacks (.stackSize and .restoreFrames)

On certain browsers, the JavaScript stack is very shallow. This is a problem for programming languages that rely heavily on recursion (e.g., idiomatic functional code). If this is not a concern, you can ignore these options.

To support heavily recursion code, Stopify can spill stack frames on to the heap. Therefore, a program will never throw a stack overflow error (however, it may run out of memory). To do so, it tracks the depth of the JavaScript stack and spills stack frames when the stack depth exceeds stackSize. Similarly, when resuming computation, the restoreFrames parameter determines how many saved stack frames are turned into JavaScript stack frames.

To maximize performance, stackSize should be as high as possible and restoreFrames should be equal to stackSize. The largest possible value of stackSize depends on the source language and browser. In our experience, a value of 500 works well.

The AsyncRun Interface

interface NormalResult {
  type: 'normal';
  value: any;

interface ExceptionResult {
  type: 'exception';
  value: any;
  stack: string[]

type Result = NormalResult | ExceptionResult;

interface AsyncRun {
  run(onDone: (result: Result) => void,
      onYield?: () => void,
      onBreakpoint?: (line: number) => void): void;
  pause(onPaused: (line?: number) => void): void;
  resume(): void;
  setBreakpoints(line: number[]): void;
  step(onStep: (line: number) => void): void;
  pauseImmediate(callback: () => void): void;
  continueImmediate(result: Result): void;
  processEvent(body: () => any, receiver: (x: Result) => void): void;

The AsyncRun interface provides methods to run, stop, and control the execution of a stopified program. The interface provides several methods, none of which should be used directly by the stopified program. The following methods are meant to be used by the driver program that controls execution (e.g., a web-based IDE):

  • The run method starts execution and requires a callback that gets invokes when execution completes. You may provide optional callbacks that are invoked when the program yields control and when a breakpoint is reached.
  • The setBreakpoint method sets the active breakpoints.
  • The pause method pauses the program at the next yield point and requires an optional callback that is invoked when the program has paused.
  • The resume method resumes execution after a pause.
  • The step method resumes execution and pauses again at the next yield point.

The following methods are are meant to be used by non-blocking JavaScript functions to provide simulated blocking interface to the stopified program:

  • The pauseImmediate method suspends the stopified program and invokes the provided callback. A function should not execute anything after invoking pauseImmediate. Typically, a function that uses pauseImmediate will use it in a return statement.
  • The continueImmediate function resumes execution with the provided value.

Illustrative Examples has several examples that use these methods to implement simulated blocking operations.

Finally, the processEvent(f, onDone) method allows external event-handlers to call a stopified function f. Since f may pause execution and thus not return immediately, Stopify passes its result to the onDone callback, which must not be a stopified function.