go-2-error-handling-proposal.go

// update of
// https://groups.google.com/d/msg/golang-nuts/UwH5CreZPe4/xxI8gFsVBAAJ
// in reaction to
// https://go.googlesource.com/proposal/+/master/design/go2draft-error-handling-overview.md

/*
Instead of "when", use the proposed syntax to reduce ambiguity and allow more complex use cases.
It may be nice to be able to type handlers too, like `handle err error { ... }`
*/

var ProtocolError = errors.New("protocol error")

type Remote interface {
  Read() (msg string, err error)
  Sent(msg string) (err error)
  Close()
}

func chatWithRemote(remote Remote) error {
  handle readErr {
    return fmt.Errorf("failed to read: %v", readErr)
  }
  handle writeErr {
    return fmt.Errorf("failed to send: %v", writeErr)
  }
  
  msg, check readErr := remote.Read()
  if msg != "220 test.com ESMTP Postfix" {
    return ProtocolError
  }
  
  check writeErr = remote.Sent("EHLO example.com")
  
  handle readErr {
    // failed reads will make remote unusable anymore
    remote.Sent("QUIT")
    remote.Close()
  }
  
  for {
    msg, check readErr := remote.Read()
    if msg == "250 DSN" {
      break
    }
  }
  
  check writeErr = remote.Sent("QUIT")
}

// Original example will look like that instead:

func CopyFile(src, dst string) error {
	handle err {
		return fmt.Errorf("copy %s %s: %v", src, dst, err)
	}

	r, check err := os.Open(src)
	defer r.Close()

	w, check err := os.Create(dst)
	handle err {
		w.Close()
		os.Remove(dst) // (only if a check fails)
	}

	check err = io.Copy(w, r)
	check err = w.Close()
	return nil
}