async/await

An async function always returns a promise or task. Inside it, await suspends the function until the awaited promise settles and hands control back to the caller; when the value arrives, the function resumes on the next line with ordinary variables and try/catch. The compiler does the work callbacks did by hand: in C#, as in Clojure’s go blocks, it rewrites the function into a state machine that can be paused at each await and resumed later.

C# 5.0 brought the pattern to the mainstream in August 2012, following F#’s asynchronous workflows. Python 3.5 added async def and await (PEP 492, 2015), JavaScript added async functions in ECMAScript 2017, and Rust stabilised async/.await in Rust 1.39 (November 2019). Rust’s are lazy: an async block doesn’t run at all until it’s awaited.

It’s the most readable of the explicit models, but it keeps the two colours: an async function can only be awaited from another async function, and calling it from synchronous code means blocking on it or restructuring the caller.