package main

import (
  "errors"
  "fmt"
)

func doubler(v interface{}) (string, error) {

  if i, ok := v.(int); ok {
    return fmt.Sprint(i * 2), nil
  }
  if s, ok := v.(string); ok {
    return s + s, nil
  }
  return "", errors.New("unsupported type passed")

}

func main() {

  res, _ := doubler(5)
  fmt.Println("5 :", res)
  res, _ = doubler("yum")
  fmt.Println("yum:", res)
  _, err := doubler(true)
  fmt.Println("true:", err)

}