Understanding gometalinter's warning: conn can be io.Reader/io.Writer

Question

After checking with gometalinter this part of code:

//ListenerButton is hanging listeners for contact button
func ListenerButton(number int, button *ui.Button, conn net.Conn) string {
    button.OnClicked(func(*ui.Button) {
        sliceMembers := []string{login, button.Text()}
        groupName = login + button.Text()
        _, err := conn.Write([]byte(JSONencode(login, "", "",
            0, groupName, 1,
            login, sliceMembers, " ", " ", "",
            " ", " ", " ", true, " ", "CreateGroup")))
        if err != nil {
            log.Println(err)
        }
        fmt.Println(login, groupName, number, "graphic 131")
    })
    return groupName
}

I've got this warning:

warning: conn can be io.Writer (interfacer)

What does it mean and how i have to resolve it?

Answer 1

That means that the ListenerButton function is only only using the Write method of conn. By changing the type from net.Conn to io.Writer, that allows your function to use a much larger number of io.Writer implementations. Having as small of interfaces as possible should be a goal when implementing your API.

With that change, for example, you could use an io.MultiWriter to write debug information to stderr, as well as the network connection:

func ListenerButton(number int, button *ui.Button, conn io.Writer) string {
    // ...
}

ListenerButton(number, button, io.MultiWriter(os.Stderr, networkConn))