Iterating over an Option

README.md

@@ -19,7 +19,7 @@ language.
 * TODO leak amplification ("Vec::drain sets the Vec's len to 0 prematurely so that mem::forgetting Drain "only" mem::forgets more stuff. instead of exposing uninitialized memory or having to update the len on every iteration")
 * [Finalisation in destructors](idioms/dtor-finally.md)
 * TODO interior mutability - UnsafeCell, Cell, RefCell
-* TODO treating Option like a list
+* [Iterating over an `Option`](idioms/option-iter.md)
 * TODO `Default` trait
 
 ### Design patterns

idioms/option-iter.md

@@ -0,0 +1,46 @@
+# Iterating over an `Option`
+
+## Description
+
+`Option` can be viewed as a container that contains either zero or one elements. 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 {
+    bar.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::flat_map`](https://doc.rust-lang.org/std/iter/trait.Iterator.html#method.flat_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)