Short variable declaration and “variable declared and not used” error

Question

I\'ve stumbled across a strange issue where the code below fails to compile:

func main() {
    var val reflect.Value
    var tm time.Time

    if tm, err := time         


        

Answer 1

Your if statement declares a new variable tm that exists only within the scope of the if block and is indeed never used:

if tm, err := time.Parse(time.RFC3339, "2018-09-11T17:50:54.247Z"); err != nil {
    panic(err)
}

In Go, := declares a new variable and initializes it. You probably meant:

func main() {
    var val reflect.Value
    var tm time.Time
    var err error

    // Note the change to normal assignment here instead of :=
    if tm, err = time.Parse(time.RFC3339, "2018-09-11T17:50:54.247Z"); err != nil {
        panic(err)
    }
    val = reflect.ValueOf(tm)

    fmt.Println(val, tm, reflect.TypeOf(tm))
}

The := shortcut operator is demonstrated in the Tour of Go and explained in the Go spec, the latter of which includes:

It is shorthand for a regular variable declaration with initializer expressions but no types:

"var" IdentifierList = ExpressionList .

Scoping is explained in the Go spec as well.

Answer 2

This part:

if tm, err := time.Parse(...)

creates a new variable tm that has scope only within the if statement - it is NOT the one you declared as var tm time.Time.

This new variable is not used within the if, therefore you get the error. Note you also don't get the outer-level tm assigned, so fmt.Println will print the zero time, not what time.Parse returned.

To fix this: declare err and change your if to read:

var err error
if tm, err = time.Parse(...)

NOTE this is a subtle thing in GO and a fairly common source of mistakes. The := statement can in fact be used with a mix of variables that are already declared and one or more new variables - if the already-declared ones are in the same lexical scope. Then, only the new ones are auto-declared by := and the rest are just assigned (as with =). However, if you use := in a new scope, then ALL variables are declared in that scope and mask any outer-scope variables with the same name (such as in an if; note that the if condition is not inside the braces, but is still considered as if it were within the {code} block; same happens with the for and other compound statements in GO).