为什么golang编译器认为变量已声明但未使用？

Question

I am a newbee to golang, and I write a program to test io package:

func main() {
    readers := []io.Reader{
         strings.NewReader("from string reader"),
         bytes.NewBufferString("from bytes reader"),
    }

    reader := io.MultiReader(readers...)
    data := make([]byte, 1024)

    var err error
    //var n int

    for err != io.EOF {
        n, err := reader.Read(data)
        fmt.Printf("%s
", data[:n])
    }
    os.Exit(0)
}

The compile error is "err declared and not used". But I think I have used err in for statement. Why does the compiler outputs this error?

Answer 1

The err inside the for is shadowing the err outside the for, and it's not being used (the one inside the for). This happens because you are using the short variable declaration (with the := operator) which declares a new err variable that shadows the one declared outside the for.