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amp/docs/iterators/README.md

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---
layout: docs
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title: Iterators
permalink: /iterators/
---
Iterators are the next level after promises. While promises resolve once and with one value, iterators allow a set of items to be consumed.
## Iterator Consumption
Every iterator in Amp follows the `Amp\Iterator` interface.
```php
namespace Amp;
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interface Iterator
{
public function advance(): Promise;
public function getCurrent();
}
```
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`advance()` returns a `Promise` and its resolution value tells whether there's an element to consume or not. If it resolves to `true`, `getCurrent()` can be used to consume the element at the current position, otherwise the iterator ended and there are no more values to consume. In case an exception happens, `advance()` returns a failed promise and `getCurrent()` throws the failure reason when called.
### Simple Consumption Example
```php
$iterator = foobar();
while (yield $iterator->advance()) {
$element = $iterator->getCurrent();
// do something with $element
}
```
## Iterator Creation
### Emitter
What `Deferred` is for promises, is `Emitter` for iterators. A library that returns an `Iterator` for asynchronous consumption of an iterable result creates an `Amp\Emitter` and returns the `Iterator` using `iterate()`. This ensures a consumer can only consume the iterator, but not emit values or complete the iterator.
#### `emit()`
`emit()` emits a new value to the `Iterator`, which can be consumed by a consumer. The emitted value is passed as first argument to `emit()`. `emit()` returns a `Promise` that can be waited on before emitting new values. This allow emitting values just as fast as the consumer can consume them.
#### `complete()`
`complete()` marks the `Emitter` / linked `Iterator` as complete. No further emits are allowed after completing an `Emitter` / `Iterator`.
### Producer
`Producer` is a simplified form of `Emitter` that can be used when a single coroutine can emit all values.
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`Producer` accepts a `callable` as first constructor parameter that gets run as a coroutine and passed an `$emit` callable that can be used to emit values just like the `emit()` method does in `Emitter`.
#### Example
```php
$iterator = new Producer(function (callable $emit) {
yield $emit(1);
yield $emit(new Delayed(500, 2));
yield $emit(3);
yield $emit(4);
});
```
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### `fromIterable`
Iterators can also be created from ordinary PHP arrays or `Traversable` instances, which is mainly useful in tests, but might also be used for the same reasons as `Success` and `Failure`.
```php
function fromIterable($iterable, int $delay = 0) { ... }
```
`$delay` allows adding a delay between each emission.