Golang：：=左侧没有新变量，而类似的错误未发生

Question

I have written some code to prints the content found at a URL following the guide of a book named The Go Programming Language. The compiler complained about the following code that no new variables on left side of :=.

package main

import (
    "fmt"
    "net/http"
    "os"
    "io"
)

func main() {
    for _, url := range os.Args[1:] {
        resp, err := http.Get(url)
        if err != nil {
            fmt.Fprintf(os.Stderr, "fetch: %v
", err)
            os.Exit(1)
        }
        _, err := io.Copy(os.Stdout, resp.Body)
        resp.Body.Close()
        if err != nil {
            fmt.Fprintf(os.Stderr, "fetch: reading %s: %v
", url, err)
            os.Exit(1)
        }
    }
}

And I have known that this was caused by re-declaration of certain variables. While the following one passed the compilation.

package main

import (
    "fmt"
    "io/ioutil"
    "net/http"
    "os"
)

func main() {
    for _, url := range os.Args[1:] {
        resp, err := http.Get(url)
        if err != nil {
            fmt.Fprintf(os.Stderr, "fetch: %v
", err)
            os.Exit(1)
        }
        b, err := ioutil.ReadAll(resp.Body)
        resp.Body.Close()
        if err != nil {
            fmt.Fprintf(os.Stderr, "fetch: reading %s: %v
", url, err)
            os.Exit(1)
        }
        fmt.Printf("%s", b)
    }
}

Didn't it re-declare the variable err? Then how could it pass the compilation?

Answer 1

If there are any new variables in the assignment (e.g. b) then := will create it. If all variables are not new then you get the error you're getting. _ is not a new variable.

Since you have all existing variables you can just use = for your io line:

_, err = io.Copy(os.Stdout, resp.Body)

When you used b instead of _ then := declared b but treated err like a standard assignment (i.e. just as if a = was used)