Concurrency in Scala has come a long way from the humble beginnings of `scala.concurrent.Future`. What started as a minimal abstraction over callbacks that allowed easy sequencing of operations, thanks to the monadic composition has since evolved into a rich ecosystem of effect systems, each trying to solve real-world problems around type and resource safety, composability, and performance.
After seven years in Java, I joined a Kotlin project and got curious about the native ecosystem. To learn it, I built a small Trading Accountability App — a tool that logs trades, enforces trading rules, and flags violations when discipline slips — from scratch using only Kotlin-native libraries, and this article walks you through doing the same.
The Cats library and the related Cats Effect were inspired by scalaz and scalaz-concurrent libraries (and, more broadly, by Haskell and its ecosystem). They represent the most widely adopted effect system in the Scala ecosystem. In this context, effects are values representing descriptions of computations that can be composed using typeclass-based abstractions like Functor, Monad, or Sync, as provided by Cats and Cats Effect. This approach promotes the use of pure functions and referential transparency - the principle that any expression can be replaced by its evaluated result without changing the program's behavior.
Kyo is one of the younger kids on the block - at the time of writing of this blogpost it has only reached its first release candidate for version 1.0. It is the brainchild of Flavio Brasil, a tenured Scala open source author and contributor who, in the past, created Quill, an SQL library based on Scala’s metaprogramming facilities and contributed to Twitter’s Finagle stack. Kyo attempts to take the idea of fused effects introduced in the ZIO monad and generalise it to provide a complete algebraic effects runtime and solve the “monads do not compose” problem at the same time. In practice, Kyo is also a purely functional, monadic solution but it’s built for the future and uses a lot of new features of Scala 3 to aggressively avoid allocations and inline common operations.
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