How to make Go print enum fields as string?

Question

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} 

Answer 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

Answer 2

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