The Rust language has incrementally grown a set of patterns to support control-flow effects including error handling, iteration, and asynchronous I/O. In The registers of Rust, boats lays out four aspects of this pattern shared by Rust’s three effects. Today these effects are typically used in isolation, or at most combined in bespoke ways, but the Rust project has been working on ways to integrate them more deeply with each other, such as async gen blocks.

The theory of algebraic effects and handlers has explored this design space and offers answers to many of the questions that the Rust project has encountered during this work. This post will relate the patterns employed by Rust to the terminology and semantics of effects, to help build a shared vocabulary and understanding of the implications of combining multiple effects.