Functions are defined using the func keyword:

// Basic function
func add(a, b int) int {
    return a + b
}

// Multiple return values
func divide(a, b float64) (float64, error) {
    if b == 0 {
        return 0, errors.New("division by zero")
    }
    return a / b, nil
}

// Named return values
func calculate(a, b int) (sum, product int) {
    sum = a + b
    product = a * b
    return // naked return
}