# Functional Language features in Rust: Iterators and Closures

Rust’s design has taken inspiration from a lot of existing languages and techniques, and one significant influence is functional programming. Programming in a functional style often includes using functions as values in arguments or return values of other functions, assigning functions to variables for later execution, and so forth. We won’t debate here the issue of what, exactly, functional programming is or is not, but will instead show off some features of Rust that are similar to features in many languages often referred to as functional.

More specifically, we’re going to cover:

- Closures: a function-like construct you can store in a variable.
- Iterators: a way of processing a series of elements.
- How to use these features to improve on the I/O project from Chapter 12.
- The performance of these features. Spoiler alert: they’re faster than you might think!

There are other Rust features influenced by the functional style, like pattern matching and enums, that we’ve covered in other chapters as well. Mastering closures and iterators is an important part of writing idiomatic, fast Rust code, so we’re devoting an entire chapter to them here.