Typed Constants in Go

You can put a type in a constant declaration, such as const initial byte = 65. Now initial has type byte; Go does not treat it like an untyped number in later assignments. in The Golang Official Documentation

package main
 
import "fmt"
 
func main() {
	const initial byte = 65
	var value byte = initial
	fmt.Printf("%v %T\n", value, value)
}

If you’ll run it with go run main.go, you’ll get 65 uint8

The first declaration works because a byte constant is being used to initialize a byte variable. The value happens to be 65, but the important part is that the types match.

Try the same constant in an int64 declaration:

package main
 
import "fmt"
 
func main() {
	const initial byte = 65
	var count int64 = initial // Intentional error.
	fmt.Println(count)
}

This won’t work and you’ll get a compilation error. The compiler rejects the direct assignment because initial is a typed byte constant. If you want an int64, write int64(initial) to convert it. The fact that 65 would fit in int64 does not erase the constant’s declared type.