Working with constant in Go

In Go, only Number, String, Boolean can be constant. const keyword is used to define constant:

const (
	DefaultKey string = "test"

	CountryCodeVN = "vn"
	CountryCodeUS = "us"

	DefaultMultiplyFactor int64 = 2

	Enable  bool = true
	Disable bool = false
)

constants are final. Once initialized, their values cannot be changed:

const defaultKey string = "test"
defaultKey = "my key" // panic

Constant can be untyped. When a constant is untyped, its precision is higher than the normal number type.

const (
	bigNum = 9999999999999999999.0
)

func main() {
	v := 9999999999999999999.0
	fmt.Printf("%T, %T\n", bigNum, v)
	fmt.Printf("%t, %t", bigNum == bigNum+1, v == v+1) // false, true
}

untyped constant can be cast to any other similar types. But its precision will be reduced:

const (
	num    = 200
	bigNum = 9999999999999999999.0
)

type number int

func main() {
        // can be cast to any similar types
	var i int = num
	var j int64 = num
	var k float64 = num
	var n number = num

	fmt.Println(i, j, k, n)

	// but its precision will be reduced when assign to a specific type
	var bigFloat float64 = bigNum
	fmt.Printf("%t, %t", bigNum == bigNum+1, bigFloat == bigFloat+1) // false, true
}

iota identifier can be used in constant declarations to simplify definitions of incrementing numbers:

type ByteSize float64

const (
	_           = iota // ignore first value by assigning to blank identifier
	KB ByteSize = 1 << (10 * iota)
	MB
	GB
	TB
	PB
	EB
	ZB
	YB
)

Weekday using iota:

type WeekDay int8

const (
	Sunday WeekDay = iota + 1
	Monday
	Tuesday
	Wednesday
	Thursday
	Friday
	Staturday
)

Value of iota is its position in the const block:

const (
	US = "United State"
	VN = "Vietnam"
	Other

	Tuesday = iota
)

func main() {
	fmt.Println("Tuesday:", Tuesday) // Tuesday: 3
}

If constant is defined without value, its value is same with the nearest one:

const (
	US = "United State"
	VN = "Vietnam"
	Other
)

func main() {
	fmt.Println("VN:", VN)
	fmt.Println("Other:", Other) // Other: Vietnam
}