A Functional Language with Effect Types and Handlers

// A generator effect with one operation
effect yield<a>
  fun yield( x : a ) : ()

// Traverse a list and yield the elements
fun traverse( xs : list<a> ) : yield<a> () 
  match xs
    Cons(x,xx) -> { yield(x); traverse(xx) }
    Nil        -> ()

fun main() : console () 
  with fun yield(i : int)
    println("yielded " ++ i.show)   
// A generator effect with one operation
effect yieldindex/yield: V -> HX<aa: V>
  fun yieldindex/yield: forall<a> (x : a) -> (yield<a>) ()( xx: $134 : aa: V ) : (std/core/types/(): V)std/core/types/(): V

// Traverse a list and yield the elements
fun traverseindex/traverse: forall<a> (xs : list<a>) -> (yield<a>) ()( xsxs: list<$162> : liststd/core/list: V -> V<aa: V> ) : yieldindex/yield: V -> HX<aa: V> (std/core/types/(): V)std/core/types/(): V 
  match xsxs: list<$162>
    Consstd/core/Cons: forall<a> (head : a, tail : list<a>) -> list<a>(xx: $162,xxxx: list<$162>) -> { yieldindex/yield: forall<a> (x : a) -> (yield<a>) ()(xx: $162); traverseindex/traverse: forall<a> (xs : list<a>) -> (yield<a>) ()(xxxx: list<$162>) }
    Nilstd/core/Nil: forall<a> list<a>        -> (std/core/types/(): ())std/core/types/(): ()

fun mainindex/main: () -> console ()() : consolestd/core/console: X (std/core/types/(): V)std/core/types/(): V 
  withhandler: (() -> <yield<int>,console> ()) -> console () fun yieldyield: (i : int) -> console ()(ii: int : intstd/core/types/int: V)
    printlnstd/core/println: (s : string) -> console ()("yielded " ++std/core/(++).1: (x : string, y : string) -> string ii: int.showstd/core/show: (i : int) -> string)   
  [std/core/Cons: forall<a> (head : a, tail : list<a>) -> list<a>1,2,3]std/core/Nil: forall<a> list<a>.traverseindex/traverse: forall<a> (xs : list<a>) -> (yield<a>) ()

Welcome to Koka – a strongly typed functional-style language with effect types and handlers.

Install Get Started Documentation Github

Note: Koka v2 is a research language that is currently under development and not ready for production use. Nevertheless, the language is stable and the compiler implements the full specification. The main things lacking at the moment are libraries, package management, and deep IDE integration.


  • 2021-02-04 (pinned) The Context Free youtube channel posted a short and fun video about effects in Koka (and 12 (!) other languages).

  • 2021-09-01 (pinned) The ICFP'21 tutorial “Programming with Effect Handlers and FBIP in Koka” is now available on youtube.

  • 2022-02-07: Koka v2.4.0 released: improved specialization and int operations, add rbtree-fbip sample, improve grammar (pub (instead of public, remove private (as everything is private by default now)), final ctl (instead of brk), underscores in number literals, etc), rename double to float64, various bug fixes.

  • 2021-12-27: Koka v2.3.8 released: improved int performance, various bug fixes, update wasm backend, initial conan support, fix js backend.

  • 2021-11-26: Koka v2.3.6 released: maybe-like types are already value types, but now also no longer need heap allocation if not nested (and [Just(1)] uses the same heap space as [1]), improved atomic refcounting (by Anton Lorenzen), improved specialization (by Steven Fontanella), various small fixes, add std/os/readline, fix build on freeBSD

  • 2021-10-15: Koka v2.3.2 released, with initial wasm support (use --target=wasm, and install emscripten and wasmtime), improved reuse specialization (by Anton Lorenzen), and various bug fixes.

  • 2021-09-29: Koka v2.3.1 released, with improved TRMC optimizations, and improved reuse (the rbtree benchmark is as fast as C++ now), and faster effect operations. Experimental: allow elision of -> in anonymous function expressions (e.g. xs.map( fn(x) x + 1 )) and operation clauses. Command line options changed a bit with .koka as the standard output directory.

  • 2021-09-20: Koka v2.3.0 released, with new brace elision and if/match conditions without parenthesis. Updated the javascript backend using ES6 modules and BigInt. new module std/num/int64, improved effect operation performance.

  • 2021-09-05: Koka v2.2.1 released, with initial parallel tasks, the binary-trees benchmark, and brace elision.

  • 2021-08-26: Koka v2.2.0 released, improved simplification (by Rashika B), cross-module specialization (Steven Fontanella), and borrowing annotations with improved reuse analysis (Anton Lorenzen).

  • 2021-08-26: At 12:30 EST was the live Koka tutorial at ICFP'21, see it on youtube.

  • 2021-08-23: “Generalized Evidence Passing for Effect Handlers”, by Ningning Xie and Daan Leijen presented at ICFP'21. See it on youtube or read the paper.

  • 2021-08-22: “First-class Named Effect Handlers”, by Youyou Cong, Ningning Xie, and Daan Leijen presented at HOPE'21. See it on youtube or read the paper.

  • 2021-06-23: Koka v2.1.9 released, initial cross-module specialization (by Steven Fontanella).

  • 2021-06-17: Koka v2.1.8 released, initial Apple M1 support.

  • The Perceus paper won a distinguished paper award at PLDI'21!

  • 2021-06-10: Koka v2.1.6 released.

  • 2021-05-31: Koka v2.1.4 released.

  • 2021-05-01: Koka v2.1.2 released.

  • 2021-03-08: Koka v2.1.1 released.

  • 2021-02-14: Koka v2.0.16 released.

  • 2020-12-12: Koka v2.0.14 released.

  • 2020-12-02: Koka v2.0.12 released.

  • 2020-11-29: Perceus technical report publised (pdf).

Why Koka?


On macOS (x64, M1), you can install and upgrade Koka using Homebrew:

brew install koka

On Windows (x64), open a cmd prompt and use:

curl -sSL -o %tmp%\install-koka.bat https://github.com/koka-lang/koka/releases/latest/download/install.bat && %tmp%\install-koka.bat

On Linux (x64, arm64) and FreeBSD (x64) (and macOS), you can install Koka using:

curl -sSL https://github.com/koka-lang/koka/releases/latest/download/install.sh | sh

There are also installation packages for various Linux distributions: Ubuntu/Debian (x64arm64), Alpine (x64arm64), Arch (x64arm64), Red Hat (x64), and openSUSE (x64).

After installation, verify if Koka installed correctly:

$ koka
 _         _
| |       | |
| | _ ___ | | _ __ _
| |/ / _ \| |/ / _' |  welcome to the koka interactive compiler
|   ( (_) |   ( (_| |  version 2.4.0, Feb  7 2022, libc x64 (gcc)
|_|\_\___/|_|\_\__,_|  type :? for help, and :q to quit

loading: std/core
loading: std/core/types
loading: std/core/hnd

Type :q to exit the interactive environment.

For detailed installation instructions and other platforms see the releases page. It is also straightforward to build the compiler from source.

Get started with the compiler