Concurrency and recovery

The Hello, concurrency! tutorial provides a basic overview of running code concurrently. Let's take a look at the details of what makes concurrency safe in Inko.

This guide assumes you've read Hello, concurrency! and Memory management, as these guides explain the basics of what we'll build upon in this guide.

To recap, Inko uses lightweight processes for concurrency. These processes don't share memory, instead values are moved between processes. Processes are defined using class async:

class async Counter {
  let @number: Int

Interacting with processes is done using async methods. Such methods are defined like so:

class async Counter {
  let @number: Int

  fn async mut increment(amount: Int) {
    @number += amount

In the Hello, concurrency! tutorial we only used value types as the arguments for async methods, which are easy to move between processes as they're copied upon moving. What if we want to move more complex values around?

Inko's approach to making this safe is to restrict moving data between processes to values that are "sendable". A value is sendable if it's either a value type (String or Int for example), or a unique value, of which the type signature syntax is uni T (e.g. uni Array[User]).

Unique values

If you're familiar with Pony, Inko's unique values are the same as Pony's isolated values, just using a name we feel better captures their purpose/intent.

A unique value is unique in the sense that only a single reference to it can exist. The best way to explain this is to use a cardboard box as a metaphor: a unique value is a box with items in it. Within that box these items are allowed to refer to each other using borrows, but none of the items are allowed to refer to values outside of the box or the other way around. This makes it safe to move the data between processes, as no data race conditions can occur.

Creating unique values

Unique values are created using the recover expression, and the return value of such an expression is turned from a T into uni T, or from a uni T into a T, depending on what the original type is:

let a = recover [10, 20] # => uni Array[Int]
let b = recover a        # => Array[Int]

This is why this process is known as "recovery": when the returned value is owned we "recover" the ability to move it between processes. If the returned value is instead a unique value, we recover the ability to perform more operations on it (i.e. we lift the restrictions that come with a uni T value).

Capturing variables

When capturing variables defined outside of the recover expression, they are exposed using the following types:

Type on the outsideType on the inside
Tuni mut T
uni Tuni T
mut Tuni mut T
ref Tuni ref T

If a recover returns a captured uni T variable, the variable is moved such that the original one is no longer available.

Borrowing unique values

Unique values can be borrowed using ref and mut, resulting in values of type uni ref T and uni mut T respectively. These borrows come with signifiant restrictions:

  1. They can't be assigned to variables
  2. They're not compatible with ref T and mut T, meaning you can't pass them as arguments.
  3. They can't be used in type signatures

This effectively means they can only be used as method call receivers, provided the method is available as discussed below.

Unique values and method calls

Methods can be called on unique values provided the methods meet the following criteria:

  1. If a method takes any arguments and/or specifies a return type, these types must be sendable. If any of these types isn't sendable, the method isn't available.
  2. If a method doesn't take any arguments and is immutable, and returns an owned value, the method is available if and only if these types are sendable (including any sub values they may store).

These restrictions can make working with unique values a bit tricky at times. We aim to implement more sophisticated compiler analysis over time to make working with unique values as easy as possible.

To illustrate this, consider the following expression:

let a = recover 'testing'


The variable a contains a value of type uni String. The expression a.to_upper is valid because to_upper doesn't take any arguments, and its return type (String) is a value type, which is a sendable type.

Because a is a unique value, we can also write the following:

let a = recover 'testing'  # => uni String
let b = recover a.to_upper # => uni String

Here's a more complicated example:


class async Main {
  fn async main {
    let server = recover TcpServer
      .new(IpAddress.v4(127, 0, 0, 1), port: 40_000)

    let client = recover server.accept.unwrap

Here server is of type uni TcpServer. The expression server.accept is valid because server is unique and thus we can capture it, and because accept meets rule two: it doesn't mutate its receiver, doesn't take any arguments, and its return type is sendable.

Here's an example of something that isn't valid:

let a = recover []


This isn't valid because a is of type uni Array[ByteArray], and push takes an argument of type ByteArray which isn't sendable, thus the push method isn't available.


Aside from passing arguments along with async method calls, you can also use the Channel type to send data between processes. Channel is a fixed size, multiple-producer multiple-consumer, first-in-first-out queue. Channel is a value type, so you can share it between processes.

Channels are useful if one process schedules work to be performed by a group of other processes, and wants to wait for the results to come in. In this case the process gives the other processes a reference to a Channel, then waits for these processes to send over the channel.

Spawning processes with fields

When spawning a process, the values assigned to its fields must be sendable:

class async Example {
  let @numbers: Array[Int]

class async Main {
  fn async main {
    Example { @numbers = recover [10, 20] }

Defining async methods

When defining an async method, the following rules are enforced by the compiler:

  • The arguments must be sendable
  • Return types aren't allowed, instead you can use the Channel type to send data back (if needed)

Calling async methods

Calling async methods is done using the same syntax as for calling regular methods:

class async Counter {
  let @value: Int

  fn async mut increment {
    @value += 1

  fn async value(output: Channel[Int]) {

class async Main {
  fn async main {
    let counter = Counter { @value = 0 }
    let output = 1)

    output.receive # => 1

Dropping processes

Processes are value types, making it easy to share references to a process with other processes. Internally processes use atomic reference counting to keep track of the number of incoming references. When the count reaches zero, the process is instructed to drop itself after it finishes running any remaining messages. This means that there may be some time between when the last reference to a process is dropped, and when the process itself is dropped.