How to convert interface{} to string?
GoGo Problem Overview
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["
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?
Go Solutions
Solution 1 - Go
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
Solution 2 - Go
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>"])
Solution 3 - Go
To expand on what Peter said: Since you are looking to go from interface{} to string, type assertion will lead to headaches since you need to account for multiple incoming types. You'll have to assert each type possible and verify it is that type before using it.
Using fmt.Sprintf (https://golang.org/pkg/fmt/#Sprintf) automatically handles the interface conversion. Since you know your desired output type is always a string, Sprintf will handle whatever type is behind the interface without a bunch of extra code on your behalf.
Solution 4 - Go
You could also use text/template:
package main
import (
"text/template"
"strings"
)
func format(s string, v interface{}) string {
t, b := new(template.Template), new(strings.Builder)
template.Must(t.Parse(s)).Execute(b, v)
return b.String()
}
func main() {
m := map[string]interface{}{"<host>": "www.google.de", "<port>": "80"}
s := format(`{{index . "<host>"}}:{{index . "<port>"}}`, m)
println(s == "www.google.de:80")
}