To define methods on a type you don’t “own”, you need to define an alias for the type you want to extend:
package main
import (
"fmt"
"strings"
)
type MyStr string
func (s MyStr) Uppercase() string {
return strings.ToUpper(string(s))
}
func main() {
fmt.Println(MyStr("test").Uppercase())
}
http://www.golangbootcamp.com/book/methods