Untyped constants in Go
Let’s start with quiz comparing two programs which differ only by single statement. Program C uses constant declaration (source code):
type N intfunc main() {
const c = 1
var n N = c
fmt.Println(n)
}
and program V uses variable declaration (source code):
type N intfunc main() {
var c = 1
var n N = c
fmt.Println(n)
}
Before scrolling down to see answers, please think for a moment about results of both C and V …
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Program C writes to stdout 1. The second one (V) might be a small surprise. It doesn’t build at all and gives compile-time error: cannot use c (type int) as type N in assignment. If you would like to understand what is exactly happening then just keep reading and in 2 minutes you’ll get it.
Constants in Go can be either typed or untyped. Untyped are created when constant declaration doesn’t specify type (source code):
const x float64 = 12
fmt.Printf("%T\n", x) // float64
const y = 12
fmt.Printf("%T\n", y) // intConstant y is untyped. When printing its type we’re getting int though…
An untyped constant has a default type which is the type to which the constant is implicitly converted in contexts where a typed value is required.
For example default type for integer constant is int or it’s float64 for floating-point number.
In program V variable c is being set to untyped constant expression 1. Default type of integer constant is int so type of c is ultimately set to int — implicit conversion happens because variables always need to have a type (source code):
var c = 1
fmt.Println(reflect.TypeOf(c).Name()) // intIn program V while executing second assignment:
var n N = cthen assignability rules from language specification apply. Since n is of type N and c is of type int then we’ve a type mismatch and assignment statement is invalid. Why the program C works then? Because in that scenario c is an untyped constant and there is a dedicated assignability rule for such case:
xis assignable toTifxis an untyped constant representable by a value of typeT.
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