Zig's new plan for asynchronous programs

Summary

Welcome to LWN.net The following subscription-only content has been made available to you by an LWN subscriber. Thousands of subscribers depend on LWN for the best news from the Linux and free software communities. If you enjoy this article, please consider subscribing to LWN. Thank you for visiting LWN.net! By Daroc AldenDecember 2, 2025 The designers of the Zig programming language have been working to find a suitable design for asynchronous code for some time. Zig is a carefully minimalist language, and its initial design for asynchronous I/O did not fit well with its other features. Now, the project has announced (in a Zig SHOWTIME video) a new approach to asynchronous I/O that promises to solve the function coloring problem, and allows writing code that will execute correctly using either synchronous or asynchronous I/O. In many languages (including Python, JavaScript, and Rust), asynchronous code uses special syntax. This can make it difficult to reuse code between synchronous and asynchronous parts of a program, introducing a number of headaches for library authors. Languages that don't make a syntactical distinction (such as Haskell) essentially solve the problem by making everything asynchronous, which typically requires the language's runtime to bake in ideas about how programs are allowed to execute. Neither of those options was deemed suitable for Zig. Its designers wanted to find an approach that did not add too much complexity to the language, that still permitted fine control over asynchronous operations, and that still made it relatively painless to actually write high-performance event-driven I/O. The new approach solves this by hiding asynchronous operations behind a new generic interface, Io. Any function that needs to perform an I/O operation will need to have access to an instance of the interface. Typically, that is provided by passing the instance to the function as a parameter, similar to Zig's Allocator interface for memory allocation. The s...