func ListenTCP(network string, laddr *TCPAddr) (*TCPListener, error) {
// 使用switch,参数属于可选值时则继续往下执行,否则返回错误
switch network {
case "tcp", "tcp4", "tcp6":
default:
return nil, &OpError{Op: "listen", Net: network, Source: nil, Addr: laddr.opAddr(), Err: UnknownNetworkError(network)}
}
...
return ln, nil
}