Async support for hooks

[killroy42]

Hi, I propose this update to the hooks system to support async style hooks. I've made it backwards compatible to sync hooks by checking if the return value is a string. There are perhaps slightly more optimal ways of doing this, but this passes all the tests.

I used this to integrate closure compiler with the postcompile hook.

[ForbesLindesay]

I'm happy to accept the feature, but would like to use Promises instead of callbacks (although I don't mind adding support for callbacks as a fallback), and I refuse to use async ever, in any of my modules.

I would suggest we make safeHook look something like:

var Promise = require('promise');

function safeHook(hook) {
  if ('function' !== typeof hook) return function (src) { return Promise.resolve(src); };
  return function (src) {
    return new Promise(function (resolve, reject) {
      var src = hook(src, function (err, src) {
        if (err) reject(err);
        else resolve(src);
      });
      if (src) resolve(src);
    });
  }
}

This way, the result is always a promise for a string, and all hooks are always effectively asynchronous (including error handling).

Since we are now doing caching outside of the compile file, it would also be reasonable to have the entire overall compile api return a promise.

[killroy42]

Agree on the async. I prefer callbacks on broad, generic libraries as they avoid surprises and avoid further dependencies. But if we can make it dual-purpose that would be best of course. Since starting to work in a node/browser mixed environment I've gotten really paranoid about dependencies!

[ForbesLindesay]

Use browserify, then stop being paranoid. As for surprising, outside of node core, promises have won this contest. Anyway, happy to have a callback fallback.

[killroy42]

Well, for example uuid pulls in 60k of dependencies (including binary buffers, etc) and a simple version of it can be had for a couple of bytes. All of that matters in the browser, of course.

[ForbesLindesay]

Pulling in buffer is pretty expensive, pretty much anything else is usually cheap though.

[killroy42]

I'm still learning. Been doing only backend for a year, and used to do frontend in the days of tiny-as-possible. So this is a happy meeting!

[ForbesLindesay]

This has now been added, just return a promise from your hook.

[killroy42]

Thanks. Will be sure to test this out as I am about ready to work more on the deployment system.