Algebraic Effects in Practice with Flix

Summary

Algebraic effects are not just a research concept anymore. You can use them in real software, today. Here’s why you’d want to do that, in order of importance: Effects make your code testable One of the central goals of enterprise software development. Dependency injection, mocking, architecture patterns like clean, hexagonal, DDD are all meant to tackle this. Effects solve this elegantly by separating the “what” from the “how”. Effects give immediate visibility into what your own and 3rd-party code is doing Supply chain attacks are real. And they will get worse with more AI slop entering our codebases. Tools like Go’s Capslock fix this by following the whole chain of calls to stdlib functions. Effects provide this by design, as all effects are tracked by the type and effect system. Effects enable user-defined control flow abstractions Solving the “what color is your function” problem1. You can also leverage effects to implement Async/await, coroutines, backtracking search and other control flow patterns as user libraries without hard-coding these features into the language. Algebraic effects come from the pure functional world, serving a purpose similar to monads — keeping track of and having control over side effects. Like monads, they enable us to write our core logic with pure functions and push side effects like IO outwards, closer to application boundaries. Unlike monads, effects are easy to grasp for a regular developer and give immediate benefits when starting out. For me personally they’re a more natural abstraction for managing side effects — after all, effects are in the name. Starting out as an academic concept, algebraic effects were introduced to the world by research languages like Eff, Koka, Effekt, Frank, Links, and more recently Ante. People have also applied effects in practice, so far usually via a monad-based approach, by making libraries in established languages like Scala Kyo / Cats Effect / ZIO; Typescript Effect and Effector, C# language-ext,...