How are functions defined in Go? What are variadic functions and named return values?

// Basic function with 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 — document what is returned
// Use sparingly; best for short functions only
func minMax(nums []int) (min, max int) {
    min, max = nums[0], nums[0]
    for _, n := range nums[1:] {
        if n < min { min = n }
        if n > max { max = n }
    }
    return  // naked return — returns named values min and max
}

// Variadic function — accepts 0 or more arguments of the given type
func sum(nums ...int) int {
    total := 0
    for _, n := range nums { total += n }
    return total
}
sum(1, 2, 3)        // 6
s := []int{4, 5, 6}
sum(s...)           // 15 — spread a slice with ...

// Function as a value (first-class)
double := func(n int) int { return n * 2 }
fmt.Println(double(7))  // 14

// Higher-order function
func apply(nums []int, f func(int) int) []int {
    result := make([]int, len(nums))
    for i, v := range nums { result[i] = f(v) }
    return result
}
fmt.Println(apply([]int{1, 2, 3}, double))  // [2 4 6]