patterns/src/idioms/option-iter.md

# Iterating over an `Option`

## Description

`Option` can be viewed as a container that contains either zero or one element.
In particular, it implements the `IntoIterator` trait, and as such can be used
with generic code that needs such a type.

## Examples

Since `Option` implements `IntoIterator`, it can be used as an argument to
[`.extend()`](https://doc.rust-lang.org/std/iter/trait.Extend.html#tymethod.extend):

```rust
let turing = Some("Turing");
let mut logicians = vec!["Curry", "Kleene", "Markov"];

logicians.extend(turing);

// equivalent to
if let Some(turing_inner) = turing {
    logicians.push(turing_inner);
}
```

If you need to tack an `Option` to the end of an existing iterator, you can pass
it to
[`.chain()`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.chain):

```rust
let turing = Some("Turing");
let logicians = vec!["Curry", "Kleene", "Markov"];

for logician in logicians.iter().chain(turing.iter()) {
    println!("{} is a logician", logician);
}
```

Note that if the `Option` is always `Some`, then it is more idiomatic to use
[`std::iter::once`](https://doc.rust-lang.org/std/iter/fn.once.html) on the
element instead.

Also, since `Option` implements `IntoIterator`, it's possible to iterate over it
using a `for` loop. This is equivalent to matching it with `if let Some(..)`,
and in most cases you should prefer the latter.

## See also

- [`std::iter::once`](https://doc.rust-lang.org/std/iter/fn.once.html) is an
  iterator which yields exactly one element. It's a more readable alternative to
  `Some(foo).into_iter()`.

- [`Iterator::filter_map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.filter_map)
  is a version of
  [`Iterator::map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.map),
  specialized to mapping functions which return `Option`.

- The [`ref_slice`](https://crates.io/crates/ref_slice) crate provides functions
  for converting an `Option` to a zero- or one-element slice.

- [Documentation for `Option<T>`](https://doc.rust-lang.org/std/option/enum.Option.html)