Add monitor function which doesn't block parent from shutting down

[jnicklas]

This comes up time and again, where we want to have a fork, but do not want the fork to block the parent from shutting down. Where this is common is in the error monitor pattern that has emerged in bigtest-server. Hence the function name.

So for example:

  let errorMonitor = fork(function*() {
    let [error]: [Error] = yield on(child, "error");
    throw error;
  });

This is tricky because the error monitor will block the fork from shutting down. This is likely not what we want. Instead we want the monitor to shut down if the parent exits. To accomplish this today, we need to wrap the entire rest of the function in a try-finally.

  let errorMonitor = fork(function*() {
    let [error]: [Error] = yield on(child, "error");
    throw error;
  });

  try {
    ...
    yield
    ...
  } finally {
    errorMonitor.halt();
  }

This is both ugly and error prone. With the monitor function we can write this more naturally:

  monitor(function*() {
    let [error]: [Error] = yield on(child, "error");
    throw error;
  });

[jnicklas]

@cowboyd I'm running into this so much now, and since we've already decided that we need this, and the implementation is easy, I thought it best to just fix it.

[frontsidejack]

A preview package of this pull request has been released to NPM with the tag monitor.
You can try it out by running the following command:

$ npm install effection@monitor

or by updating your package.json to:

{
  "effection": "monitor"
}

Once the branch associated with this tag is deleted (usually once the PR is merged or closed), it will no longer be available. However, it currently references effection@0.4.0-6a8b6aa which will be available to install forever.

Generated by 🚫 dangerJS against 6c77671

[frontsidejack]

A preview package of this pull request has been released to NPM with the tag monitor.
You can try it out by running the following command:

$ npm install effection@monitor

or by updating your package.json to:

{
  "effection": "monitor"
}

Once the branch associated with this tag is deleted (usually once the PR is merged or closed), it will no longer be available. However, it currently references effection@0.4.0-324bd7d which will be available to install forever.

Generated by 🚫 dangerJS against fdf388c

[cowboyd]

This is definite win. Like fork, I think it can be implemented as an operation using #57

[cowboyd]

superseded by #62