Koka Has a Free API: A Language With Algebraic Effects and Automatic Memory Management

Koka is a research language by Microsoft Research that pioneered algebraic effects as a first-class feature. It provides Haskell-level purity tracking with a revolutionary memory management system called Perceus.

Why Koka Matters

Algebraic effects solve the "what color is your function" problem. In Koka, effects are tracked in the type system — you know exactly what side effects a function can perform, and you can define your own effects.

What you get for free:

• Algebraic effects: compose and handle side effects cleanly
• Effect types: know what a function does from its signature
• Perceus reference counting: no GC pauses, deterministic cleanup
• Functional-but-fast: compiles to C with optimizations
• Row-polymorphic effect system
• First-class modules and interfaces

Quick Start

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

# Run
koka main.kk

# Build optimized
koka -O2 main.kk

The Basics

// Pure function — no effects
fun add(x: int, y: int): int
  x + y

// Function with console effect
fun greet(name: string): console ()
  println("Hello, " ++ name ++ "!")

// Function with multiple effects
fun ask-name(): <console, ndet> string
  println("What is your name?")
  readline()

fun main(): <console, ndet> ()
  val name = ask-name()
  greet(name)

Algebraic Effects

// Define a custom effect
effect fun ask(prompt: string): string
effect fun tell(msg: string): ()

// Use effects in functions
fun conversation(): <ask, tell> ()
  val name = ask("What is your name?")
  tell("Hello, " ++ name)
  val age = ask("How old are you?")
  tell("You are " ++ age ++ " years old")

// Handle effects differently
fun handle-console(): <console> ()
  with handler
    fun ask(prompt)
      println(prompt)
      readline()
    fun tell(msg)
      println(msg)
  conversation()

// Or handle with testing (no IO)
fun handle-test(): list<string>
  var answers := ["Alice", "30"]
  var output := []
  with handler
    fun ask(prompt)
      match answers
        Cons(a, rest) -> { answers := rest; a }
        Nil -> "default"
    fun tell(msg)
      output := output ++ [msg]
  conversation()
  output

Effect Handlers for Exceptions

effect ctl raise(msg: string): a

fun safe-div(x: int, y: int): raise int
  if y == 0 then raise("division by zero")
  else x / y

fun main(): console ()
  // Handle exception effect
  val result = with handler
    ctl raise(msg) -> println("Error: " ++ msg)
  safe-div(10, 0)

  // Or convert to Maybe
  val maybe-result = with handler
    ctl raise(msg) -> Nothing
  Just(safe-div(10, 2))

Pattern Matching

type shape
  Circle(radius: float64)
  Rectangle(width: float64, height: float64)
  Triangle(a: float64, b: float64, c: float64)

fun area(s: shape): float64
  match s
    Circle(r) -> pi * r * r
    Rectangle(w, h) -> w * h
    Triangle(a, b, c) ->
      val s = (a + b + c) / 2.0
      sqrt(s * (s - a) * (s - b) * (s - c))

fun main(): console ()
  val shapes = [Circle(5.0), Rectangle(3.0, 4.0)]
  shapes.foreach fn(s)
    println("Area: " ++ area(s).show)

Perceus Memory Management

Koka uses Perceus — a compile-time reference counting system that:

• Has no GC pauses (deterministic)
• Automatically inserts drop/reuse operations
• Achieves performance comparable to manual memory management
• Enables "functional but in-place" optimization

Useful Links

• Official Site
• GitHub
• Book
• Papers

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