Go (golang) magic constants via "iota"

iota.go

// use "iota" for enumerated constants, like this:

const (
    a = iota // 0
    b        // 1
    c        // 2
    //...
    n        // n
)

// ALSO, "iota" for bit-shifted config vars, to be used in bitwise OR operators

const (
	OPTION_A int = 1 << iota // 1
	OPTION_B                 // 2
	OPTION_C                 // 4
	OPTION_D                 // 8
)

func demoA() {
	fmt.Println(OPTION_A)
	fmt.Println(OPTION_B)
	fmt.Println(OPTION_C)
	fmt.Println(OPTION_D)
	fmt.Println(OPTION_A | OPTION_C | OPTION_D)
}
// output:
// 1
// 2
// 4
// 8
// 14

// can also use custom types to have type-safety with our const values, like this:

type Color int

const (
	Red   Color = iota // will be Color(0)
	Green              // Automatically assigned as Color(1)
	Blue               // Automatically assigned as Color(2)
)

// we saw iota get bitshifted (above) and it can also be mathematically altered, like this

const (
	_  = 1 << (iota * 10) // ignore the first value
	KB                    // decimal:       1024 -> binary 00000000000000000000010000000000
	MB                    // decimal:    1048576 -> binary 00000000000100000000000000000000
	GB                    // decimal: 1073741824 -> binary 01000000000000000000000000000000
)