Language Tour


Aiken programs are made up of bundles of functions and types called modules. Each module has its own namespace and can export types and values to be used by other modules in the program.

// inside module lib/straw_hats/sunny.ak
fn count_down() {
  "3... 2... 1..."
fn blast_off() {
pub fn set_sail() {

Here we can see a module named straw_hats/sunny, the name determined by the filename lib/straw_hats/sunny.ak. Typically all the modules for one project would live within a directory with the name of the project, such as straw_hats in this example.

The pub keyword makes this type usable from other modules.

For the functions count_down and blast_off we have omitted the pub keyword, so these functions are private module functions. They can only be called by other functions within the same module.

Functions, type-aliases and constants can all be exported from a module using the pub keyword.


To use functions or types from another module we need to import them using the use keyword.

// inside module src/straw_hats/laugh_tale.ak
use straw_hats/sunny
pub fn find_the_one_piece() {

The definition use straw_hats/sunny creates a new variable with the name sunny and the value of the sunny module.

In the find_the_one_piece function we call the imported module's public set_sail function using the . operator. If we had attempted to call count_down it would result in a compile time error as this function is private to the sunny module.

Named import

It is also possible to give a module a custom name when importing it using the as keyword.

use unix/dog
use animal/dog as kitty

This may be useful to differentiate between multiple modules that would have the same default name when imported.

Unqualified import

Values and types can also be imported in an unqualified fashion.

use animal/dog.{Dog, stroke}
pub fn foo() {
  let puppy = Dog { name: "Zeus" }

This may be useful for values that are used frequently in a module, but generally qualified imports are preferred as it makes it clearer where the value is defined.

You may also combine unqualified imports with custom names as such:

use animal/dog.{Dog, stroke} as kitty

Opaque types

At times it may be useful to create a type and make the constructors and fields private so that users of this type can only use the type through publicly exported functions.

For example we can create a Counter type which holds an int which can be incremented. We don't want the user to alter the Int value other than by incrementing it, so we can make the type opaque to prevent them from being able to do this.

// The type is defined with the opaque keyword
pub opaque type Counter {
  Counter(value: Int)
pub fn new() {
pub fn increment(counter: Counter) {
  Counter(counter.value + 1)

Because the Counter type has been marked as opaque it is not possible for code in other modules to construct or pattern match on counter values or access the value field. Instead other modules have to manipulate the opaque type using the exported functions from the module, in this case new and increment.

The prelude module

There are two modules that are built into the language, the first is the aiken prelude module. By default its types and values are automatically imported into every module you write, but you can still chose to import it the regular way. This may be useful if you have created a type or value with the same name as an item from the prelude.

use aiken
/// This definition locally overrides the `Option` type
/// and the `Some` constructor.
pub type Option {
/// The original `Option` and `Some` can still be used
pub fn go() -> aiken.Option<Int> {

The content of the Prelude module is documented in aiken-lang/prelude (opens in a new tab)

The builtin module

The second module that comes with the language is for exposing useful builtin functions from Plutus core. Most underlying platform functions are available here using a "snake_case" name. Much of Aiken's syntax ends up compiling to combinations of certain bultins but many aren't "exposed" through the syntax and need to be used directly. The standard library wraps these in a more Aiken-friendly interface so you'll probably never need to use these directly unless you're making your own standard library.

use aiken/builtin
fn eq(a, b) {
    builtin.equals_integer(a, b)
// is implicitly used when doing:
a == b


You may add user-facing documentation at the head of modules with a module documentation comment //// (quadruple slash!) per line. Markdown is supported and this text block will be included with the module's entry in generated HTML documentation.