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Inko uses a familiar syntax using curly braces, and is easy to understand by both humans and computers.

If you're new to Inko you don't have to read this entire guide right away, instead we recommend using it more as a reference as you make your way through the rest of the documentation.


Each Inko source file is a module and may contain the following:

  • Imports
  • Constants
  • Classes
  • Methods
  • Trait implementations
  • Classes that are reopened
  • Comments

Unlike languages such as Ruby and Python, it's not valid to include expressions directly in a module.


Comments start with a # and continue until the end of the line:

# This is a comment that spans a single line

Multiple uninterrupted comment lines are treated as a single comment:

# This is
# a single
# comment.

# But this is a separate comment due to the empty line above.


The import statement is used to import a module or symbols from a module. The syntax is as follows:

import mod1.mod2        # This imports `mod1.mod2` and exposes it as `mod2`
import mod1.mod2.A      # This imports the symbol `A`
import mod1.mod2.(A, B) # This imports the symbol `A` and `B`
import mod1.mod2.(self) # This imports `mod2` from module `mod1`

You can also alias symbols when importing them:

import mod1.mod2.(A as B) # `A` is now exposed as `B`

You can also import module methods:

import std.process.(sleep)

This always requires the use of parentheses in the symbol list, otherwise the compiler would think you're trying to import the module std.process.sleep.

Imports may specify one or more build tags, resulting in the compiler only processing the import if all the build tags match:

import foo if linux and amd64

The syntax for the tags is import FOO if TAG1 and TAG2 and ....

External imports

The syntax import extern "NAME" is used to link against C libraries. The string "NAME" specifies the library name, not the file name or path of the library. For example, to import libm:

import extern "m"


Constants are defined using let at the module top-level. Constants are limited to integers, floats, strings, arrays of constants, and binary expressions of constants:

let A = 10
let B = 10.5
let C = 'foo'
let D = A + 5
let E = [A, 10]
let F = 10 + A

Constants are made public using let pub:

let pub A = 10


Classes are defined using the class keyword:

class Person {}

A class can define one or more fields using let:

class Person {
  let @name: String # `@name` is the field name, and `String` its type
  let @age: Int

Classes default to being private. To make a class public, use class pub like so:

class pub Person {
  let @name: String # `@name` is the field name, and `String` its type
  let @age: Int

Inko also supports algebraic data types (also known as enums), which are defined like so:

class enum Result {}

class pub enum Result {}

Enum classes allow defining of variants using the case keyword:

class enum Result {
  case Ok
  case Error

Enum classes can't define regular fields.

Generic classes are defined like so:

class enum Result[T, E] {
  case Ok(T)
  case Error(E)

Here T and E are type parameters. Type parameters can also list one or more traits that must be implemented before a type can be assigned to the parameter:

class enum Result[T, E: ToString + ToFoo + ToBar] {
  case Ok(T)
  case Error(E)

Type parameters can also specify the mut requirement, restricting the types to those that allow mutations:

class MyBox[T: mut] {}

Processes are defined as async classes like so:

class async Counter {


class pub async Counter {


Classes can define static methods, instance methods, and async methods (in case the class is an async class):

class Person {
  let @name: String

  # This is a static method, available as ``.
  fn static new(name: String) -> Person {
    # ...

  # This is an instance method, which is only available to instances of
  # `Person`.
  fn name -> String {
    # ...

class async Counter {
  fn async increment {
    # ...

C structures are defined using class extern. When used, the class can't define any methods or use generic type parameters:

class extern Timespec {
  let @tv_sec: Int64
  let @tv_nsec: Int64


Methods are defined using the fn keyword. At the top-level of a module only instance methods can be defined (i.e. static methods aren't valid directly in a module). Methods are private by default.

Methods use the following syntax:

# A private immutable instance method:
fn method_name {}

# A public immutable instance method:
fn pub method_name {}

# A private mutable instance method:
fn mut method_name {}

# A private moving method:
fn move method_name {}

# A public mutable instance method:
fn pub mut method_name {}

# A public mutable async instance method (only available in async classes):
fn pub async mut method_name {}

# Method names may end with a ?, this is used for predicate methods:
fn valid? {}

# Method names may also end with a =, this is used for setters:
fn value= {}

Methods with arguments are defined like so, optional arguments aren't supported:

fn method_name(arg1: ArgType, arg2: ArgType) {}

A return type is specified using ->:

fn method_name -> ReturnType {}

Type parameters are specified before regular arguments:

fn method_name[A, B](arg1, A, arg2: B) {}

Like classes, you can specify a list of required traits:

fn method_name[A: ToFoo + ToBar, B](arg1, A, arg2: B) {}

The method's body is contained in the curly braces:

fn method_name {
  [10, 20]

External functions

Signatures for C functions are defined using the fn extern syntax. These functions can't define any generic type parameters, can only be defined at the top-level of a module (i.e. not in a class), can't specify the mut keyword, and can't have a body. For example:

import extern "m"

fn extern ceil(value: Float64) -> Float64

Variadic functions are defined using ... as the last argument:

fn extern printf(format: Pointer[UInt8], ...) -> Int32


Traits are defined using the trait keyword, and like classes default to being private.

Traits are defined like so:

trait ToString {}

trait pub ToString {}

Traits can specify a list of other traits that must be implemented before the trait itself can be implemented:

trait Debug: ToString + ToFoo {}

Traits can define both default and required methods:

trait ToString {
  # A required instance method:
  fn to_string -> String

  # A default instance method:
  fn to_foo -> String {

Traits aren't allowed to define static methods.

Traits can also define type parameters:

trait ToArray[T] {}

And like classes and methods, these can define required traits:

trait ToArray[T: ToFoo + ToBar] {}

Type parameters can also specify the mut requirement, restricting the types to those that allow mutations:

trait ToArray[T: ToFoo + mut] {}

Implementing traits

Traits are implemented using the impl keyword:

impl ToString for String {}

The syntax is impl TraitName for ClassName { body }. Within the body only instance methods are allowed.

Reopening classes

A class can be reopened using the impl keyword like so:

impl String {}

Within the body, only methods are allowed; fields can only be defined when the class is defined for the first time.


Each method's body can contain zero or more expressions.


Identifiers are referred to by just using their name:


Identifiers can contain the following characters: a-z, A-Z, 0-9, _, $, and may end with a ?.


While identifiers can contain dollar signs (e.g. foo$bar), this is only supported to allow use of C functions using a $ in the name, such as readdir$INODE64 on macOS. Do not use dollar signs in regular Inko method or variable names.

The self keyword is used to refer to the receiver of a method. This keyword is available in all methods, including module and static methods.

Field references

Fields are referred to using @NAME where NAME is the name of the field:


Constant references

Constants are referred to as follows:

let NUMBER = 42

fn example {

It's also possible to refer to a constant in a module like so:

import foo

fn example {


Scopes are created using curly braces:


{       # <- This is the start of a new scope
}       # <- The scope ends here


Inko has two types of strings: single quoted strings and double quoted strings. Double quoted strings allow the use of escape sequences such as \n and support string interpolation:

'foo\nbar'       # => "foo\\nbar" (as in a literal \n, not a newline)
"foo\nbar"       # => "foo\nbar"
"foo{10 + 5}bar" # => "foo15bar"

Strings can span multiple lines:

"this string spans

If a string spans multiple lines and a line ends with a \, the newline and any whitespace that follows is ignored:

"foo \
bar \
baz" # => "foo bar baz"

Double quoted strings support Unicode escape sequences using the syntax \u{XXXXX}, such as this:



The syntax for integers is as follows:


Underscores in integer literals are ignored, and are useful to make large numbers more readable:



Floats are created using the usual floating point syntax:



Arrays are created using flat brackets:

[10, 20]


Booleans are created using true and false.

Inko doesn't have a dedicated negation operator such as !expression or not expression. Instead, you'd use the predicate methods Bool.true? and Bool.false?. Bool.true? returns true if its receiver is also true, while Bool.false? returns true if the receiver is false.

Here's a simple example:

if volume_is_too_loud.false? {


The nil keyword is used to create an instance of Nil.

Class literals

Class literals start with the name of the class followed by curly braces. Within the curly braces, zero or more fields are assigned a value:

class Person {
  let @name: String

Person { @name = 'Alice' }


if expressions use the if keyword:

if foo {
} else if bar {
} else {


Infinite loops are created using the loop keyword:

loop {
  # ...

Conditional loops are created using the while keyword:

while condition {
  # ...

break and next can be used to break out of a loop or jump to the next iteration respectively. Breaking a loop with a value (e.g. break 42) isn't supported.

Pattern matching

Pattern matching is performed using the match keyword:

match value {
  case PAT -> BODY

Each case starts with the case keyword, specifies one or more patterns, an optional guard, and a body to run if the pattern matches.

The following patterns are supported:

  • Integer literals: case 10 -> BODY
  • String literals: case 'foo' -> BODY
  • Constants: case FOO -> BODY
  • Bindings: case v -> BODY
  • Wildcards: case _ -> BODY
  • Variants: case Some(v) -> BODY
  • Class literals: case { @name = name } -> BODY
  • Tuples: case (a, b, c) -> BODY
  • OR patterns: case 10 or 20 -> BODY

Guards are also supported:

match foo {
  case Some(num) if num > 10 -> foo
  case _ -> bar


Closures are created using the fn keyword:

fn {}
fn (a, b) {}
fn (a: Int, b: Int) {}
fn (a, b) -> ReturnType {}

Unlike methods, the argument types are optional.

Tuples and grouping

Expressions are grouped using parentheses:

(10 + 5) / 2 # => 7

Tuples are also created using parentheses, but must contain at least a single comma:

(10,)    # => Tuple1[Int]
(10, 20) # => Tuple2[Int, Int]


References are created using ref and mut:

ref foo
mut bar

Recover expressions

Recovering is done using the recover keyword:

recover foo
recover { foo }

Returning values

The last value in a body is its return value. Explicitly returning values is done using return:

return 42

Throwing values is done using throw:

throw 42

This is syntax sugar for the following:

return Result.Error(42)

Error handling

try is available as syntax sugar for a match, and supports values of type Result and Option.

try option (where option is an Option) is syntax sugar for the following:

match option {
  case Some(val) -> val
  case None -> return Option.None

try result (where result is a Result) is syntax sugar for the following:

match result {
  case Ok(val) -> val
  case Error(error) -> return Result.Error(error)

Defining variables

Variables are defined using let:

let number = 42
let number: Int = 42
let _ = 42

By default a variable can't be assigned a new value. To allow this, use let mut:

let mut number = 42

number = 50

Pattern matching in a let isn't supported.

Assigning variables

Variables are assigned a new value using the syntax VAR = VAL. Inko also supports swapping of values using the syntax VAR := VAL:

let mut a = 42

a := 50 # => 42
a       # => 50

Method calls

Methods without arguments can be called without parentheses. If arguments are given, parentheses are required:    # calls foo() on self  # Same thing
foo(10, 20), 20)

Named arguments are also supported:

foo(name: 10, bar: 20)

When using named arguments, they must come after positional arguments:

foo(10, bar: 20)

If the last argument is a closure, it can be specified after the parentheses:

foo(10) fn { bar } # Same as `foo(10, fn { bar })`

Binary expressions

Binary operator expressions use the following syntax:

left OP right

For example:

10 + 5

Operators are left-associative. This means 5 + 10 * 2 evaluates to 30, not 25. The following binary operators are supported:

+ , - , / , * , ** , % , < , > , <= , >= , << , >> , | , & , ^ , == , !=, >>>

Inko also supports two logical operators: and and or. These operators have a higher precedence than the regular binary operators. This means 1 + 2 and 3 + 4 is parsed as (1 + 2) and (3 + 4). and and or have the same precedence as each other.


Inko doesn't have dedicated indexing syntax such as array[index] or array[index] = value, instead you'd use the methods get, get_mut, and set like so:

let numbers = [10, 20]

numbers.get(1) # => 20
numbers.set(1, 30)
numbers.get(1) # => 30

Type casts

Type casting is done using as like so:

expression as TypeName

The as keyword has the same precedence as binary operators. This means that this:

10 + 5 as ToString

Is parsed as this:

(10 + 5) as ToString

And this:

foo as Int + 5 as Foo

Is parsed as this:

(foo as Int + 5) as Foo