errors.go

package main

import "errors"
import "fmt"

func f1(arg int) (int, error) {
  if arg == 42 {
    return -1, errors.New("can't work with 42")
  }

  return arg + 3, nil
}

type argError struct {
  arg int
  prob string
}

func (e *argError) Error() string {
  return fmt.Sprintf("%d - %s", e.arg, e.prob)
}

func f2(arg int) (int, error) {
  if arg == 42 {
    return -1, &argError{arg, "can't work with it"}
  }
  return arg + 3, nil
}

func main() {
  for _, i := range []int{7, 42} {
    if r, e := f1(i); e != nil {
      fmt.Println("f1 failed:", e)
    } else {
      fmt.Println("f1 worked:", r)
    }
  }
  for _, i := range []int{7, 42} {
    if r, e := f2(i); e != nil {
      fmt.Println("f2 failed:", e)
    } else {
      fmt.Println("f2 worked:", r)
    }
  }

  _, e := f2(42)
  if ae, ok := e.(*argError); ok {
    fmt.Println(ae.arg)
    fmt.Println(ae.prob)
  }
}