Chapter 12: Understanding Closures in Rust

Closures, sometimes called lambda expressions, are anonymous functions that can capture variables from their defining scope. This allows passing small units of behavior without the boilerplate often required in languages like C, such as using function pointers paired with manually managed context data (e.g., via void*).

Typical use cases include:

• Transforming or filtering iterators (map, filter).
• Defining callbacks for asynchronous or event-driven code.
• Supplying custom comparison predicates to sorting algorithms (sort_by_key).
• Deferring work until a value is actually needed (unwrap_or_else).
• Moving data and associated logic into another thread (thread::spawn).

This chapter explains what closures are, how they capture their environment, and how Rust’s ownership and borrowing rules apply through the Fn, FnMut, and FnOnce traits. We will compare closures to functions and explore common use cases, including performance considerations relevant to C programmers.