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NOTE: Inko is still a work in progress and in the very early stages. For example, there's no website just yet, the compiler is in the super early stages, etc.

Inko is a gradually typed, interpreted, concurrent, object oriented programming language that combines the flexibility of a dynamic language with the safety of a static language. Inko at its core is a prototype-based language, drawing heavy inspiration from other languages such as Smalltalk, Self, Ruby, Erlang, and Rust.


Inko's concurrency model is heavily inspired by Erlang and relies on lightweight processes managed by the virtual machine. Each process can perform a certain amount of work before it is suspended and another process is executed. Work is evenly balanced amongst multiple OS threads, and these threads may steal jobs from each other. This ensures load is balanced evenly and you won't end up with a single thread performing all the work while the rest idles.

Object Oriented

Inko is a prototype-based object oriented programming language. Since prototype-based languages are a bit hard to work with Inko provides a simple class-like object system, allowing you to organise your code similar to other languages. This means you will very rarely have to use prototypes directly, and you don't have to invent your own way of defining classes or similar structures.

Inko relies heavily on message passing and provides no traditional if, while and similar statements. Instead you send messages to objects. For example, a simple if statement looks as follows:

x.if true: {
false: {

Here if is a message sent to x, and true: and false: are keyword arguments that each take a closure.

A while statement in turn is written as follows:

{ condition-here }.while_true {

Here while_true is a message sent to the receiving closure, and the argument is the closure to evaluate should the condition evaluate to true.

This allows objects to define whether they should evaluate to true or false, and simplifies the syntax greatly. In fact, Inko only has 14 reserved keywords!

Error Handling

For error handling Inko uses exceptions and is heavily inspired by the article "The Error Model" by Joe Duffy. The way you handle exceptions in Inko is a bit different and much more strict compared to other languages.

First of all, any method that may throw an error must specify this in the type signature using !!:

# This will produce a compile time error because the method's signature does
# not specify what can be thrown.
fn ping {

# This however is valid.
fn ping !! NetworkTimeout {

Here !! NetworkTimeout is used to indicate that the ping method may throw an error of type NetworkTimeout.

Second, a method that specifies it may throw an error must actually use the throw keyword. This means that the following code is invalid:

fn ping !! NetworkTimeout {

Third, a method can only throw a single type. This means that the following is not valid:

fn ping !! Foo, Bar, Baz {

You can however use a trait in the signature. This will allow the method to throw any type as long as those types implement the given trait. This ensures that a caller only has to deal with a single type, removing the need for giant try-catch blocks.

Calling a method that may throw an error requires you to prefix the call with the try keyword like so:

try ping

This makes it crystal clear to the reader that ping may throw an error. The default behaviour of this keyword is to re-raise the error, which in turn is bound by the rules above. This means that this code is invalid because the first method does not specify the type it may throw:

fn foo {
  try ping

fn ping !! NetworkError {

Custom behaviour in the event of an error can be specified using the else keyword:

let x = try ping else something_else

Here the ping message will be sent, and in the event of an error the something_else message will be sent. The else keyword also takes a single (optional) argument which will contain the value thrown:

let x = try ping else (error) {

Here the error argument will contain the error, and is only available to the block that follows it.

For longer snippets of code you can also use curly braces:

let x = try {
else (error) {

In all cases the return types of the try and else blocks must match.

If an error bubbles up all the way to the top of a process the process will panic, resulting in the entire program terminating.

This brings us to the final part of error handling: panics. A panic is an error that will result in the entire program terminating. Panics are used whenever an error occurs that can not be handled reasonably at run time. For example, zero division errors are panics because they are the result of incorrect program behaviour.

In Inko one should only use exceptions for errors that are expected to occur from time to time. Examples include network timeouts, file permission errors and input validation errors. Panics in turn should be used for everything else.


Unlike many other OO languages data in Inko is immutable by default, requiring you explicitly mark it as mutable. For example, the let keyword can be used to define an immutable variable while var can be used to define a mutable one:

let a = 10
var b = 10

a = 20 # => error
b = 20 # => this is OK

The same applies to method arguments, which are immutable by default but can be made mutable using the var keyword:

fn append_to(var array) {

Gradually Typed

Inko is gradually typed, with static typing being the default. This means you need to explicitly opt-in for dynamic typing, providing a safer default. Using dynamic typing is as simple as leaving out type signatures. For example, this method uses static types:

fn add(left: Integer, right: Integer) -> Integer {

This method however uses dynamic types:

fn add(left, right) {

For variables however you need to use the special Dynamic type as by default the type is inferred based on the value. This means that instead of this:

let x = something

You will have to write:

let x: Dynamic = something

Dynamic typing does not automatically allow the reassignment of variables, for this you will need to use the var keyword:

var x = 10
x = 20

Optional Values

Inko doesn't have an Option or Maybe type, instead it has "Nil" values. These are objects used to represent the lack of a value. Similar to Objective-C sending a message to Nil will simply return Nil, except for a few specific methods. This is incredibly important as it makes it trivial to deal with methods that return an optional value, and it greatly reduces the amount of conditionals necessary.

As an example, let's say we have an array of users and each user responds to the "name" message:

object User {
  fn init(name: String) {
    let @name = name

let array = ['Alice'),'Bob')]

In Inko the message used to access array values is [] and its signature is as follows:

fn [](index: Integer) -> T | Nil

In other words, it takes an integer as the index and either returns a value T or Nil. In most other languages using such a method would require a conditional to figure out what you're dealing with. For example, in Ruby you might do the following:

value = array[4]

if value

In Inko however you can just send the name message to Nil since it will respond to it and just return another Nil:

array[4].name # => Nil

Nil defines a few methods on its own such as to_integer and to_string to make it easier to convert Nil values to other values. Some examples:

Nil.to_string  # => ''
Nil.to_integer # => 0
Nil.to_float   # => 0.0
Nil.to_array   # => []
Nil.to_hash    # => %{}

Garbage Collection

Inko is a garbage collected language and uses Immix as the algorithm. The garbage collector is a parallel and mostly concurrent garbage collector. There is no stop-the-world phase that will pause all threads, instead the garbage collector will only suspend the process that is being collected, allowing others to continue running. Garbage collection is performed in parallel to reduce the time a process is suspended.

The garbage collector also comes with instrumentation, tracking the amount of time spent in preparing a collection, tracing through live objects, etc. This data is currently not yet exposed but will be in the future.

Code Organisation

Organising logic is done by creating objects and traits, and by having objects implement these traits where necessary. Objects can be created either on the fly, or by using the object keyword. Using the object keyword allows you to define objects that can be re-used, similar to how one might define a class in other languages.

As an example, instead of a base Object class providing a to_string method that is overwritten in child classes there is a ToString trait defined as follows:

trait ToString {
  fn to_string -> String

Each object that wishes to provide a to_string method can then simply implement the trait:

import std::string::ToString

object MyObject impl ToString {
  fn to_string -> String {


Inko uses curly braces for blocks, and only has 14 keywords. Most of the language relies heavily on message passing. Variables are defined using let and var:

let a = 10
var b = 20

Methods are defined using the fn keyword:

fn method_name {


Closures use the same syntax, except they don't include a name. If no arguments are specified you can even leave out the fn keyword:

let a = {

let b = fn(arg) {

Parenthesis are used for passing arguments but are optional:

receiver.message 10

Arguments can either be positional arguments, or keyword arguments:

receiver.message 10
receiver.message number: 10

If no parenthesis are specified then the arguments list will terminate at the end of the line.

Every argument is also a keyword argument, so you can use both whenever you like. It's preferred to use keyword arguments when the meaning of the arguments may not be clear otherwise:

# Here we can infer the meaning of the arguments quite easily.
'hello world'.replace('hello', 'HELLO')

# Here however things get a bit more tricky, especially if our list of
# arguments grows.'Alice', 24, '5th Street')

# In these cases using keyword arguments can make things more clear: 'Alice', age: 24, address: '5th Street')

Objects and traits are defined using the object and trait keywords respectively. The impl keyword can be used to implement a trait, and takes an optional list of methods to rename:

object Foo impl Bar(original_name as alias_name) {
  # The method "original_name" is now available as "alias_name"

Imports use the import keyword and :: is the namespace separator:

import std::string::ToString

The use of :: is only valid in the import statement, this means that this is not valid syntax:

This forces one to explicitly state the external modules that are necessary, and it keeps the syntax simpler.

Comments are created using the # sign and run until the end of the line:

# This is a comment!


The venerable Hello World:

import std::stdout

stdout.print('Hello, world!')

Concurrent Hello World:

import std::stdout
import std::process

process.spawn {
  stdout.print('Hello from process 1!')

process.spawn {
  stdout.print('Hello from process 2!')

Checking if a value is true or not:

import std::stdout

the_result_of_something.if true: {
  stdout.print('x is true!')
false: {
  stdout.print('x is false!')

Defining an object that can be re-used, much like a class:

object Person {
  # "init" is the constructor method, called whenever you create a new
  # instance of this object.
  fn init(name: String, age: Integer) {
    let @name = name
    let @age = age

Using dynamic typing, simply by leaving out type signatures:

fn add(left, right) {
  left + right

add(10, 20)
add(10.5, 5)
add('foo', 'bar')

Opening a file and reading data, without blocking the running thread and without the need of nested callbacks:

import std::file

let file ='')

file.read_exact(6) # => "# Inko"


  • A UNIX system, Windows is currently not tested/supported

When working on Inko itself you'll also need:

  • Rust 1.10 or newer using a nightly build (stable Rust is not supported)
  • Cargo

The following dependencies are optional but recommended:

  • Make
  • Rustup

Installation (for developers)

The easiest way to install Inko in case you want to hack on it is to first clone the repository. Once cloned you'll need to build the VM and the compiler.

Building The VM

To build the VM run the following:

cd vm

Building The Compiler

To build the compiler, run the following:

cd compiler