Go: How to use enum as a type?

Question

I have a list of events (enum) which defines the particular event:

package events

const (
    NEW_USER       = "NEW_USER"
    DIRECT_MESSAGE = "DIRECT_MESSAGE"
    DISCONNECT     = "DISCONNECT"
)

And there is a struct that will use this enum as one of its attribute

type ConnectionPayload struct {
    EventName    string      `json:"eventName"`
    EventPayload interface{} `json:"eventPayload"`
}

Is there a way I can use enum as a type for EventName instead of string?

This is possible in typescript not sure how to do it in golang.

I want the developers to forcefully use correct event name using the enum instead of making a mistake by using any random string for eventname.

Answer 1

There is no enum type at the moment in go, and there currently isn't a direct way to enforce the same rules as what typescript does.

---

A common practice in go is to use the suggestion posted by @ttrasn :

define a custom type, and typed constants with your "enum" values :

type EventName string

const (
    NEW_USER       EventName = "NEW_USER"
    DIRECT_MESSAGE EventName = "DIRECT_MESSAGE"
    DISCONNECT     EventName = "DISCONNECT"
)

This allows you to flag, in your go code, the places where you expect such a value :

// example function signature :
func OnEvent(e EventName, id int) error { ... }

// example struct :
type ConnectionPayload struct {
    EventName    EventName  `json:"eventName"`
    EventPayload interface{} `json:"eventPayload"`
}

and it will prevent assigning a plain string to an EventName

var str string = "foo"
var ev EventName

ev = str // won't compile
OnEvent(str, 42) // won't compile

The known limitations are :

• in go, there is always a zero value :

  var ev EventName  // ev is ""
  

• string litterals are not the same as typed variables, and can be assigned :

  var ev EventName = "SOMETHING_ELSE"
  

• casting is allowed :

  var str string = "foo"
  var ev EventName = EventName(str)
  

• there is no check on unmarshalling :

  jsn := []byte(`{"eventName":"SOMETHING_ELSE","eventPayload":"some message"}`)
  err := json.Unmarshal(jsn, &payload) // no error
  

https://go.dev/play/p/vMUTpvH8DBb

If you want some stricter checking, you would have to write a validator or a custom unmarshaler yourself.