rpominov/promises.md

## promises.md

      
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              promises.md
            
          
    Promises how I would do them

1. No over polymorphic .then

Four methods instead: .map(a -> b), .mapError(a -> b), .flatMap(a -> Promise), .flatMapError(a -> Promise).
Why?

more predictable code
easier to understand what is going on
easy to create a Promise that contain another Promise (using .map)
no need for duck-typing because it guaranteed that .flatMap/.flatMapError callback returns a Promise
Single .then() method is confusing
Too much polymorphism isn't a good idea

2. Ability to unsubscribe (cancelation)

The four methods above should not activate the promise, instead we should have additional method to subscribe:
let p = Promise((resolve, reject) => {
  // we get here when the promise gets a first subscriber (i.e. activates)
  return () => {
    // we get here when/if the promise loses all subscribers
  };
});

let unsubscribe = p.map(foo).flatMap(bar).subscribe(onSuccess, onFail);

// maybe later ...
unsubscribe();
Why?
See "RxJS Observables vs Promises", and a practical example of this feature being used in ajax() function from this demo http://jsfiddle.net/fwo0toLx/6/
3. No exceptions catching

Make it up to the developer. If it needed (which is very rare), one can do it manually:
foo.flatMap(x => {
  try {
    return Promise.resolved(fnThatMightThrow(x));
  } catch (e) {
    return Promise.rejected(e);
  }
});
Why?

It makes debugging harder

Actually, not 100% sure about this one :)