> For the complete documentation index, see [llms.txt](https://dibs.gitbook.io/vortex-docs/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://dibs.gitbook.io/vortex-docs/language-reference/coroutines.md).

# Coroutines

Coroutines are special functions that can pause execution to allow other code to run, and resume execution form where it left off.

Vortex functions become coroutines when the `yield` statement is found within their body.

Calling a coroutine function returns a function that is ready for execution.

```rust
const coro = () => {
    var x = 0
    while (true) {
        yield x
        x += 1
    }
}

const c = coro() // This sets up and returns a ready coroutine

c().println() // 0
c().println() // 1
c().println() // 2
```

Coroutines expose a hidden variable called `_value`. This variable starts off as the value None, however if we pass an argument into the coroutine function, it gets assigned to value.

{% code overflow="wrap" %}

```rust
const coro = () => {
    var x = 5
    while (true) {
        yield x + _value
    }
}

const c = coro() // This sets up and returns a ready coroutine

c(5).println() // 10
c(3).println() // 8
c().println() // This will not error, because the previous call has already set the _value to 3. If the first coroutine call did not provide a valid argument, an error will then be raised
```

{% endcode %}

If a coroutine finishes execution, ie. there are no more yield statements, it is considered to be done and any further calls will simply return `None`.

You can manually end a coroutine by including a return statement in the function's body.

### Checking Coroutine State

Calling the builtin function `info` on the coroutine will show you its state:

{% code overflow="wrap" %}

```rust
c.info().println()

// { name: coro_0, arity: 0, params: [], generator: true, init: true, done: false }
```

{% endcode %}

The `info` call provides information about the function, including whether or not it is a generator (coroutine), if it has been initialised or not, and if it has finished its execution.