How to convert interface{} to string?

Question

I'm using docopt to parse command-line arguments. This works, and it results in a map, such as

map[<host>:www.google.de <port>:80 --help:false --version:false] 

Now I would like to concatenate the host and the port value to a string with a colon in-between the two values. Basically, something such as:

host := arguments["<host>"] + ":" + arguments["<port>"] 

Unfortunately, this doesn't work, as I get the error message:

invalid operation: arguments[""] + ":" (mismatched types interface {} and string)

So obviously I need to convert the value that I get from the map (which is just interface{}, so it can be anything) to a string. Now my question is, how do I do that?

Answer 1

You need to add type assertion .(string). It is necessary because the map is of type map[string]interface{}:

host := arguments["<host>"].(string) + ":" + arguments["<port>"].(string) 

Latest version of Docopt returns Opts object that has methods for conversion:

host, err := arguments.String("<host>") port, err := arguments.String("<port>") host_port := host + ":" + port 

Answer 2

You don't need to use a type assertion, instead just use the %v format specifier with Sprintf:

hostAndPort := fmt.Sprintf("%v:%v", arguments["<host>"], arguments["<port>"])