Functions

Functions are declared with the fun keyword. Parameters use name: type syntax, and the return type follows the parameter list.

Coming from other languages?

Run uses fun, not func (Go), function (JavaScript), or def (Python). The compiler will suggest the correct keyword if you use one of these by mistake.

package main

use "fmt"

fun add(a: int, b: int) int {
    return a + b
}

pub fun main() {
    result := add(3, 4)
    fmt.println(result)
}

Visibility

Functions are private by default. Use pub to make a function accessible from other packages.

pub fun add(a: int, b: int) int {
    return a + b
}

fun helper() {
    // only visible within this package
}

No return value

Functions that do not return a value simply omit the return type.

fun greet(name: string) {
    fmt.println("Hello, " + name)
}

Failable functions

A function that can fail returns !T — an error union. A function that returns nothing but can fail uses bare !:

fun divide(a: int, b: int) !int {
    if b == 0 {
        return error.division_by_zero
    }
    return a / b
}

fun save(path: string, data: string) ! {
    file := try os.create(path)
    defer file.close()
    try file.write(data)
}

Error handling is covered in detail in the Error Handling chapter.