kyo

<img src="https://raw.githubusercontent.com/getkyo/kyo/main/kyo.png" width="200" alt="Kyo">

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Kyo is a Scala 3 toolkit for building applications. Its starting point is what it makes impossible: an error path left unhandled does not compile. An effect used without being declared in the signature is rejected at the call site. A resource cannot outlive its scope, even when something fails partway through. Concurrent work started together cannot leak its children. A durable workflow's progress survives a crash and resumes from the last completed step. One source tree compiles to JVM, JavaScript, Scala Native, and WebAssembly.

The mechanism behind those guarantees is algebraic effects with modular handlers, exposed through a compact infix type `A < S` ("A pending S"): `A` is the value the computation produces, and `S` is the open, type-level *set* of effects it needs. A function returning `A < (Sync & Abort[E] & Env[Config] & Emit[Log])` names its four effects in its return type. Handlers remove effects from the set one at a time, the call site decides where each one runs, and the compiler refuses to execute anything until the set is empty. Composition stays ordinary Scala: `map`, `flatMap`, and for-comprehensions over plain values. The theory traces back to Plotkin and Pretnar and to languages like Koka and Eff.

For a deeper context, see [Suspension: the magic behind composability](https://www.youtube.com/watch?v=y3KiuFczOFE) (Lambda Days 2025), or jump to [Modules](#modules) for the ecosystem map.

> Coming from ZIO or Cats Effect? Jump straight to [the migration table](#coming-from-zio-or-cats-effect).

## Getting Started

Add the modules you want to `build.sbt`. Most application code starts with `kyo-core` (see the [Modules](#modules) tables below for what each module provides). Add more modules as needed.

```sbt doctest:expect=skipped
libraryDependencies ++= Seq(
    "io.getkyo" %% "kyo-core" % "<version>",
    "io.getkyo" %% "kyo-http" % "<version>"    // optional: HTTP client/server
)
```

Use `%%` for JVM and Scala Native,