Variables

Run has two keywords for declaring variables: var for mutable bindings and let for immutable bindings.

Mutable variables with var

Use var to declare a variable that can be reassigned. Variables declared without an initializer are zero-initialized.

var x int        // x is 0
var y f64        // y is 0.0
var z bool       // z is false
var s string        // s is ""

You can also provide an initial value:

var x int = 42
var name string = "Run"

Immutable variables with let

Use let to declare a variable that cannot be reassigned. An initializer is required — the value is fixed once set.

let pi f64 = 3.14159
let name string = "Run"
let x = 42         // type inferred

Attempting to reassign a let variable is a compile-time error. The compiler shows where the variable was originally defined and suggests using var instead.

Short declarations

The := operator declares a mutable variable and infers its type from the right-hand side.

package main

use "fmt"

pub fun main() {
    x := 42
    name := "Run"
    pi := 3.14159

    fmt.println(x)
    fmt.println(name)
    fmt.println(pi)
}

Short declarations are the most common way to declare variables in Run. They are equivalent to var with type inference.

Zero values

Variables declared with var without an explicit initial value are given their zero value:

• 0 for numeric types
• false for bool
• "" for string
• null for nullable types