How to make Go print enum fields as string?

You print an enum that implements Stringer using "%v", it will print its string value. If you declare the same enum inside a struct and print the struct using "%v", it will print enum's numeric value. Is there a way to print the string value of a enum field?

Sample (https://play.golang.org/p/AP_tzzAZMI):

package main

import (
	"fmt"
)

type MyEnum int

const (
	Foo MyEnum = 1
	Bar MyEnum = 2
)

func (e MyEnum) String() string {
	switch e {
	case Foo:
		return "Foo"
	case Bar:
		return "Bar"
	default:
		return fmt.Sprintf("%d", int(e))
	}
}

type MyStruct struct {
	field MyEnum
}

func main() {
	info := &MyStruct{
		field: MyEnum(1),
	}
	fmt.Printf("%v\n", MyEnum(1))
	fmt.Printf("%v\n", info)
	fmt.Printf("%+v\n", info)
	fmt.Printf("%#v\n", info)
}

Prints:

Foo
&{1}
&{field:1}
&main.MyStruct{field:1}

You need to make the field exported,ie you may declare the struct as

type MyStruct struct {
    Field MyEnum
}

Here is a sample program with exported and unexported fields
#Code

package main

import (
	"fmt"
)

type MyEnum int

const (
	Foo MyEnum = 1
	Bar MyEnum = 2
)

func (e MyEnum) String() string {
	switch e {
	case Foo:
		return "Foo"
	case Bar:
		return "Bar"
	default:
		return fmt.Sprintf("%d", int(e))
	}
}

type MyStruct struct {
	Field1 MyEnum
	field2 MyEnum
}

func main() {
	info := &MyStruct{
		Field1: MyEnum(1),
		field2: MyEnum(2),
	}
	fmt.Printf("%v\n", MyEnum(1))
	fmt.Printf("%v\n", info)
	fmt.Printf("%+v\n", info)
	fmt.Printf("%#v\n", info)
}

#Output

Foo
&{Foo 2}
&{Field1:Foo field2:2}
&main.MyStruct{Field1:1, field2:2}

Here is play link : https://play.golang.org/p/7knxM4KbLh

I am going to expand on the accepted answer a little with this method:

type MyEnum int

const (
	Foo MyEnum = iota
	Bar
)

func (me MyEnum) String() string {
	return [...]string{"Foo", "Bar"}[me]
}

// ...

fmt.Println(Foo, Bar)  // Prints: Foo Bar

The above code assumes that the enum values starts from 0, which works out nicely because the first element in the array in the method String can be referenced by the enum value directly.

But the first enum value in the original question has a value of 1. We can modify it the method accordingly.

const (
	Foo MyEnum = iota + 1
	Bar
)

func (me MyEnum) String() string {
	return [...]string{"", "Foo", "Bar"}[me]
}

Here's the playlink: https://play.golang.org/p/6pmyVlsAeV2

Use stringer. It is an integral part of the Golang tool chain and is made specifically for this task.

You might want to use a standard enumerator package as mentioned in godocs: https://godoc.org/github.com/alvaroloes/enumer

PS: There are some forks which have worked on the above project. But for your requirement, this should be sufficient.

All you have to do is to implement String method on the type that defines the ENUM

take a look at this example

package main

import "fmt"

// Day is the type that represents the day
// and  here will be an alias of unit8
type Day uint8

// Enum of week days
const (
	InvalidDay Day = iota // invalid day will be 0
	Monday
	Tuesday
	Wednesday
	Thursday
	Friday
	Saturday
	Sunday
)

// String method is responsible for how enum values
// will be converted to string and printed
// it will be called by default when you call fmt.Println()
// or other formatting functions
func (d Day) String() string {
	switch d {
	case Monday:
		return "Monday"
	case Tuesday:
		return "Tuesday"
	case Wednesday:
		return "Wednesday"
	case Thursday:
		return "Thursday"
	case Friday:
		return "Friday"
	case Saturday:
		return "Saturday"
	case Sunday:
		return "Sunday"
	default:
		return "Invalid Week Day"
	}
}

func main() {
	fmt.Printf("My favourite day is: %v\n", Monday) // My favourite day is: Monday
}