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In Inko, the collective name of methods, closures and lambdas is "blocks". These blocks define reusable chunks of code that can take input in the form of arguments, and produce output by returning or throwing data.

In the Basic types chapter we already took a look at these blocks, but we explore them in more detail in this chapter.


There are a few different types of methods in Inko: instance methods, static methods, required methods, default methods, and module methods.

A module method is simple a method defined inside a module.

Classes can have static and instance methods, while traits can have required and default methods. We cover methods for traits and classes in greater detail in the chapters Classes and Traits.

Calling methods

Calling methods is done by sending a message to the thing the method is defined on, passing zero or more arguments. Typically the message name equals the method name, though the compiler may decide to optimise method calls away. Parentheses are only required when passing arguments. Some examples:

'HELLO'.to_lowercase      # No arguments, so the parentheses can be left out
'HELLO'.starts_with?('H') # One argument, so parentheses are required
greet                     # This is the same as `self.greet`

Calling closures and lambdas

To call a closure or lambda, send the call message to it and pass whatever arguments the closure or lambda needs. For example:

{ 10 }.call

Here we define a closure without any argument and explicit throw/return types, removing the need for the do keyword. Lambdas use the fn keyword:

fn { 10 }.call

Returning from blocks

The return value of a block is the last expression evaluated:

}.call # => 20

If a method doesn't define a return type, it always returns Nil.

The return keyword is used to return from the surrounding method, and can only be used in blocks used in a method:

def example -> Integer {, 20, 30).each do (number) {
    return number

example # => 10

A return at the module-level is invalid:

def foo {}

return # This is invalid, because we are not inside a method

If you use return without a value, it will return Nil. If the surrounding method specifies a return type, the returned value must be compatible with this type. If a method doesn't define a return type, you can only return a Nil.


Methods can take zero or more arguments. Arguments can be required or optional. Each method can also have a single rest argument.

We define required arguments as follows:

def greet(name: String) {}  # A module method
do (name: String) {}        # A closure
fn (name: String) {}        # A lambda

Here name is the name of the argument, and String is its type. To define an optional argument, provide a default value for the argument:

def greet(name = 'Alice') {}
do (name = 'Alice') {}
fn (name = 'Alice') {}

When specifying a default value, you can still specify an explicit argument type:

def greet(name: String = 'Alice') {}
do (name: String = 'Alice') {}
fn (name: String = 'Alice') {}

In this case the explicit type will be used as the argument type. It's an error to assign a default value that's not compatible with the explicit type.

A rest argument is used for storing any excessive arguments passed to the method. A method can only have a single rest argument, and it must be the last argument:

def greet(name: String, *other: String) {}
do (name: String, *other: String) {}
fn (name: String, *other: String) {}

Here other is the rest argument. We can call this method like so:

greet('Alice', 'Bob', 'Eve')

When using a closure or lambda, you can leave out the argument type even for required types. When leaving these types out, the compiler will try to infer the types for you. For example:, 20, 30).each do (number) {}

Here the compiler knows that the type of number is Integer, because the values in the Array are of this type. Because of this, you can leave out the explicit argument type.


Support for inferring closure/lambda arguments types is limited, and only works when the block is directly passed as an argument. This may be improved upon in the future.

Mutable arguments

A mutable argument is an argument that can be assigned a new value, provided this value is compatible with the argument type. To make an argument mutable, add the mut keyword before the argument name:

def foo(mut number = 10) {
  number = 20

This works for all argument types.

Keyword arguments

When passing arguments along with a message, you can use either positional arguments or keyword arguments. Keyword arguments are useful when the meaning of the argument itself is not clear. Take this method for example:

def withdraw(euros: Integer) {}

Using positional arguments for this method looks like this:


When reading this line, it may not be clear what the value 10 means. Using keyword arguments, this becomes more clear:

withdraw(euros: 10)

Keyword arguments also let you pass arguments out of order. Take this method for example:

def calculate(a: Integer, b: Integer, c: Integer) -> Integer {
  a + b - c

Using keyword arguments, we can call it like so:

calculate(a: 1, b: 2, c: 3)
calculate(c: 3, a: 1, b: 2)
calculate(b: 2, c: 3, a: 1)

These three lines all translate to the same code and result. This is useful for optional arguments, as you can leave out arguments you don't want to set yourself. For example:

def bake(ingredient: String, celcius = 200, minutes = 60) {}

bake(ingredient: 'Bread', minutes: 120)

When using keyword arguments, two things should be kept in mind. First, they must come after any positional arguments. This means the following is not valid:

bake(ingredient: 'Bread', 250)

Second, you can't use keyword arguments to address rest arguments. This means the following is invalid:

def greet(*names: String) {}

greet(names: 'Alice', names: 'Bob')

In practise this means it's best to only use rest arguments when the meaning of any positional arguments is clear on their own, or when the method only defines a rest argument.

Closures and lambdas of course also support keyword arguments:

do (number: Integer) { number }.call(number: 42)

Trailing block arguments

When passing a block as the last argument, you can place it outside the parentheses. When passing only a block as the argument, you can leave out the parentheses. For example:, 20, 30).each do (number) {}

Here each only takes a single argument, so no parentheses need to be specified. If it did take an argument, we would write the following:, 20, 30).each(something) do (number) {}

Passing blocks this way is only supported if the last argument is a block, and it only works for a single block. This means the following is invalid:, 20, 30).each do (number) {}, do (foo) {}

Throw types

A method can throw a value. If so, it must specify the type of the value thrown in its signature. This is done as follows:

def example !! Integer {}

Here the example method states that it will throw a value of type Integer. For closures and lambdas, the syntax is the same:

do !! Integer {}
fn !! Integer {}

How to throw values and handle errors is covered separately.

Return types

When a method doesn't specify a return type explicitly, it will return Nil and its return type is inferred as Nil. You can specify an explicit return type as follows:

def example -> String {}

Here the method states that it returns an instance of the String type. The syntax for closures and lambdas is the same:

do -> Integer {}
fn -> Integer {}

When adding an explicit throw and return type to a block, the throw type must come first:

def example !! Integer -> String {}
do !! Integer -> String {}
fn !! Integer -> String {}

External functions

The standard library makes use of functions defined in the VM, which we call "external functions". The use of these functions allows implementing logic in the VM, without having to change the compiler whenever changing these functions.

External functions are defined using extern def. For example, here's how the external function for writing a String to STDOUT is defined:

extern def stdout_write_string(input: String) !! String -> Integer

Once defined, these functions can be called like a regular method:

try! stdout_write_string('hello')

Unlike methods, external functions can't be imported from a module, nor do they support the use of a receiver.