Document coroutine helpers, fixes #100

docs/coroutines/helpers.md

@@ -0,0 +1,35 @@
+# Coroutine Helpers
+
+`Amp\Coroutine` requires a already instantiated `Generator` to be passed to its constructor. Always calling a callable before passing the `Generator` to `Amp\Coroutine` is unnecessary boilerplate.
+
+## `coroutine()`
+
+Returns a new function that wraps `$callback` in a promise/coroutine-aware function that automatically runs generators as coroutines. The returned function always returns a promise when invoked. Errors have to be handled by the callback caller or they will go unnoticed.
+
+```php
+function coroutine(callable $callback): callable { ... }
+```
+
+Use this function to create a coroutine-aware callable for a promise-aware callback caller.
+
+## `asyncCoroutine()`
+
+Same as `coroutine()` but doesn't return a `Promise` when the returned callback is called. Instead, promises are passed to `Amp\Promise\rethrow()` to handle errors automatically.
+
+## `call()`
+
+```php
+function call(callable $callback, ...$args): Promise { ... }
+```
+
+Calls the given function, always returning a promise. If the function returns a `Generator`, it will be run as a coroutine. If the function throws, a failed promise will be returned.
+
+`call($callable, ...$args)` is equivalent to `coroutine($callable)(...$args)`.
+
+## `asyncCall()`
+
+```php
+function asyncCall(callable $callback, ...$args) { ... }
+```
+
+Same as `call()`, but doesn't return the `Promise`. Promises are automatically passed to `Amp\Promise\rethrow` for error handling.