Golang Enum pattern that can be serialized to json

enum.go

package enum_example

import (
	"bytes"
	"encoding/json"
)

// TaskState represents the state of task, moving through Created, Running then Finished or Errorred
type TaskState int

const (
	// Created represents the task has been created but not started yet
	Created TaskState = iota
	//Running represents the task has started
	Running
	// Finished represents the task is complete
	Finished
	// Errorred represents the task has encountered a problem and is no longer running
	Errorred
)

func (s TaskState) String() string {
	return toString[s]
}

var toString = map[TaskState]string{
	Created:  "Created",
	Running:  "Running",
	Finished: "Finished",
	Errorred: "Errorred",
}

var toID = map[string]TaskState{
	"Created":  Created,
	"Running":  Running,
	"Finished": Finished,
	"Errorred": Errorred,
}

// MarshalJSON marshals the enum as a quoted json string
func (s TaskState) MarshalJSON() ([]byte, error) {
	buffer := bytes.NewBufferString(`"`)
	buffer.WriteString(toString[s])
	buffer.WriteString(`"`)
	return buffer.Bytes(), nil
}

// UnmarshalJSON unmashals a quoted json string to the enum value
func (s *TaskState) UnmarshalJSON(b []byte) error {
	var j string
	err := json.Unmarshal(b, &j)
	if err != nil {
		return err
	}
	// Note that if the string cannot be found then it will be set to the zero value, 'Created' in this case.
	*s = toID[j]
	return nil
}

WOW. Thanks for this.

Really helpful, thanks for sharing!

Cheers

For me, MarshalJSON only works when its receiver is a TaskState, rather than a *TaskState:

func (s TaskState) MarshalJSON() ([]byte, error)

For me, MarshalJSON only works when its receiver is a TaskState, rather than a *TaskState:

Good spot, thanks I'd replaced the original gist with a cleaner example and introduced that bug 😛
Gist fixed.

Very Cool, 👍

Thanks, very helpful!

cool

Very useful, thanks. It is also nice to know that editors support regex replace which allows you to generate the contents of toString and toId very easily from the list of enum values.

For example toString can be generated by copying the list of values, and doing this regex replacement on it:
find: (\w+).*
replace with: $1: "$1"

So cooooool👍

Another option is:

type TaskState string

const (

	// Created represents the task has been created but not started yet

	Created TaskState = "Created"

	//Running represents the task has started

	Running = "Running"

	// Finished represents the task is complete

	Finished = "Finished"

	// Errorred represents the task has encountered a problem and is no longer running

	Errorred ="Errored"

)

The one disadvantage is that if you do a bunch a comparisons in Go, they will be string comparisons instead of int. This could effect performance, but for most cases is probably insignificant.

Thank you!

thanks 👍

Thanks!

my two cents:

type Kind int

const (
	Sent Kind = iota
	Received
	Failed
	Report
)

func (t Kind) String() string {
	return [...]string{"Sent", "Received", "Failed", "Report"}[t]
}

func (t *Kind) FromString(kind string) Kind {
	return map[string]Kind{
		"Sent":     Sent,
		"Received": Received,
		"Failed":   Failed,
		"Report":   Report,
	}[kind]
}

func (t Kind) MarshalJSON() ([]byte, error) {
	return json.Marshal(t.String())
}

func (t *Kind) UnMarshalJSON(b []byte) error {
	var s string
	err := json.Unmarshal(b, &s)
	if err != nil {
		return err
	}
	*t = t.FromString(s)
	return nil
}

my two cents:

type Kind int

const (
	Sent Kind = iota
	Received
	Failed
	Report
)

func (t Kind) String() string {
	return [...]string{"Sent", "Received", "Failed", "Report"}[t]
}

func (t *Kind) FromString(kind string) Kind {
	return map[string]Kind{
		"Sent":     Sent,
		"Received": Received,
		"Failed":   Failed,
		"Report":   Report,
	}[kind]
}

func (t Kind) MarshalJSON() ([]byte, error) {
	return json.Marshal(t.String())
}

func (t *Kind) UnMarshalJSON(b []byte) error {
	var s string
	err := json.Unmarshal(b, &s)
	if err != nil {
		return err
	}
	*t = t.FromString(s)
	return nil
}

Nice, the same approach comes to my mind. When you put the maps in a function they are immutable. I think that is a better approach.

Thank you, for this example.

type Kind int

const (
	Unknown Kind = iota
	Sent
	Received
	Failed
	Report
)

func (t Kind) String() string {
	return [...]string{"Unknown", "Sent", "Received", "Failed", "Report"}[t]
}

func (t *Kind) FromString(kind string) Kind {
	return map[string]Kind{
		"Unknown":  Unknown,
		"Sent":     Sent,
		"Received": Received,
		"Failed":   Failed,
		"Report":   Report,
	}[kind]
}

func (t Kind) MarshalJSON() ([]byte, error) {
	return json.Marshal(t.String())
}

func (t *Kind) UnmarshalJSON(b []byte) error {
	var s string
	err := json.Unmarshal(b, &s)
	if err != nil {
		return err
	}
	*t = t.FromString(s)
	return nil
}

func (t *Kind) UnMarshalJSON(b []byte) error {

Note that this should be UnmarshalJSON, without the capital "M" in the middle... other than that, works brilliantly, thanks!

I ran into an edge case today - you need to add the following method if you plan to unmarshal json into a map that uses a custom enum as a key (e.g. map[TaskState]string):

func (s *TaskState) UnmarshalText(b []byte) error {
	return s.UnmarshalJSON(b)
}

Without it you will get an error like this when you unmarshal:

json: cannot unmarshal number Sent into Go struct field <some field with type map[TaskState]string> of type TaskState

---

FWIW I found the docs on this confusing. The paragraph below from https://pkg.go.dev/encoding/json#Unmarshal seems to indicate that implementing json.Unmarshaler is sufficient (emphasis mine):

To unmarshal a JSON object into a map, Unmarshal first establishes a map to use. If the map is nil, Unmarshal allocates a new map. Otherwise Unmarshal reuses the existing map, keeping existing entries. Unmarshal then stores key-value pairs from the JSON object into the map. The map's key type must either be any string type, an integer, implement json.Unmarshaler, or implement encoding.TextUnmarshaler.

However, this source code comment indicates otherwise: https://cs.opensource.google/go/go/+/refs/tags/go1.21.3:src/encoding/json/decode.go;drc=b9b8cecbfc72168ca03ad586cc2ed52b0e8db409;l=630