Language Tour
Variables & Constants

Variables & Constants

Variables / let-bindings

Aiken has let-bindings for declaring variables. A value can be given a name using the keyword let. Names can be reused by later let-bindings.

Values assigned to let-bindings are immutable, however new bindings can shadow previous bindings.

let x = 1
let y = x
let x = 2
y + x == 3


You can also introduce new bindings through the expect keyword.

expect Some(foo) = my_optional_value

We will see more about the expect keyword when talking about custom types. For now, it is sufficient to know that it exists.

Module constants

Let-bindings aren't allowed in a top-level Aiken module. Yet, Aiken provides module constants as a way to use certain fixed values in multiple places of a Aiken project.

const start_year = 2101
const end_year = 2111

Like all values in Aiken, module constants are immutable. They cannot be used as global mutable state. When a constant is referenced, its value is inlined by the compiler so they can be used in any place where you'd have written a literal to begin with (e.g. when-expression guards, if clauses ...). We'll see some example of that when dealing with control flows.

Note that you can only declare module constants for the following Aiken types: Int, ByteArray and String.

Type annotations

Variables and constants can be given type annotations. These annotations serve as documentation or can be used to provide a more specific type than the compiler would otherwise infer.

const name: ByteArray = "Aiken"
const size: Int = 100
let result: Bool = 14 > 42