Restart computation with control operators

[jpolitz]

When Pyret pauses the stack, you can restart it in more than one way.

The important one is that the client of pauseStack can choose to call resume or error callbacks; the latter simulates a throw on the original stack.

With Stopify, it's "easy" to simulate it is by looking for a special return value (e.g. turn all resumes into Either-returning evaluations). This requires rewriting more code in Pyret than is particularly nice to do, and is a fundamentally different interface.

In the limit for Stopify this could mean supporting throw, continue, break, etc as restart options.

I suspect in practice throw is the only one that matters, but it does matter.

[arjunguha]

We can simulate throwing with something like this:

function throwableControl(f) {
  const r = control(
    k =>  ({ type: 'normal', value: f(r => k({ type: 'normal', value: r})),
    r => k({ type: 'exception', value: r}))});
  const r = control(k => f(k));
  if (r.type === 'exception') {
    throw r.value;
  }
  else {
    return r.value;
  }
}

throwableControl((returnK, throwK) => ... );

I'm not sure what it means to do continue and break.

[rachitnigam]

This is addressed by 33b9f6212240d45dfe8cddd44df9619604980786.