Go Compilation Error: Declared but not used (Although used)

I have the following Go example:

package main
import (
	"fmt"
	"log"
	"os"
	"strconv"
)
func main() {
	numberOfUsers := 10
	numberOfUsersStr, found := os.LookupEnv("NUMBER_OF_USERS")
	if found {
		numberOfUsers, err := strconv.Atoi(numberOfUsersStr)
		if err != nil {
			log.Fatalln(err)
		}
	}
	fmt.Printf("Number of users: %d", numberOfUsers)
}

When building this snipper, I get the following error:

> go build -o app .
.\app.go:14:3: numberOfUsers declared but not used

Clearly the variable is used in the last print statement, however it seems hidden from the compiler. What am I missing?

When using :=, you're declaring a new variable. That means the numberOfUsers here:

numberOfUsers, err := strconv.Atoi(numberOfUsersStr)

is actually shadowing your other numberOfUsers variable.

You can fix it by declaring err beforehand and then using just = instead of := so that you are only assigning a new value to the variable and not declaring a new variable.

var err error
numberOfUsers, err = strconv.Atoi(numberOfUsersStr)

The numberOfUsers variable in this line is unused: numberOfUsers, err := strconv.Atoi(numberOfUsersStr).

Here you are redeclaring a new variable numberOfUsers in the scope of the if statement. This variable is not used in that scope, hence the error.

You probably want something like this:

func main() {
    numberOfUsers := 10
    numberOfUsersStr, found := os.LookupEnv("NUMBER_OF_USERS")
    if found {
        var err error
        numberOfUsers, err = strconv.Atoi(numberOfUsersStr)
        if err != nil {
            log.Fatalln(err)
        }
    }
    fmt.Printf("Number of users: %d", numberOfUsers)
}

Notice the difference between := (declare an assign a new variable) and = (only assign).