Enumer

Enumer is a tool to generate Go code that adds useful methods to Go enums (constants with a specific type). It started as a fork of Rob Pike’s Stringer tool.

Generated functions and methods

When Enumer is applied to a type, it will generate:

• A method String() that returns the string representation of the enum value. This makes the enum conform the Stringer interface, so whenever you print an enum value, you'll get the string name instead of a number.
• A function <Type>String(s string) to get the enum value from its string representation. This is useful when you need to read enum values from command line arguments, from a configuration file, or from a REST API request... In short, from those places where using the real enum value (an integer) would be almost meaningless or hard to trace or use by a human.
• When the flag json is provided, two additional methods will be generated, MarshalJSON() and UnmarshalJSON(). These make the enum conform to the json.Marshaler and json.Unmarshaler interfaces. Very useful to use it in JSON APIs.
• When the flag yaml is provided, two additional methods will be generated, MarshalYAML() and UnmarshalYAML(). These make the enum conform to the gopkg.in/yaml.v2.Marshaler and gopkg.in/yaml.v2.Unmarshaler interfaces.
• When the flag sql is provided, the methods for implementing the Scanner and Valuer interfaces will be also generated. Useful when storing the enum in a database.

For example, if we have an enum type called Pill,

type Pill int

const (
	Placebo Pill = iota
	Aspirin
	Ibuprofen
	Paracetamol
	Acetaminophen = Paracetamol
)

executing enumer -type=Pill -json will generate a new file with four methods:

func (i Pill) String() string {
    //...
}

func PillString(s string) (Pill, error) {
    //...
}

func (i Pill) MarshalJSON() ([]byte, error) {
	//...
}

func (i *Pill) UnmarshalJSON(data []byte) error {
	//...
}

From now on, we can:

// Convert any Pill value to string
var aspirinString string = Aspirin.String()
// (or use it in any place where a Stringer is accepted)
fmt.Println("I need ", Paracetamol) // Will print "I need Paracetamol"

// Convert a string with the enum name to the corresponding enum value
pill, err := PillString("Ibuprofen")
if err != nil {
    fmt.Println("Unrecognized pill: ", err)
    return
}
// Now pill == Ibuprofen

// Marshal/unmarshal to/from json strings, either directly or automatically when
// the enum is a field of a struct
pillJSON := Aspirin.MarshalJSON()
// Now pillJSON == `"Aspirin"`

The generated code is exactly the same as the Stringer tool plus the mentioned additions, so you can use Enumer where you are already using Stringer without any code change.

Transforming the string representation of the enum value

By default, Enumer uses the same name of the enum value for generating the string representation (usually CamelCase in Go).

type MyType int

 ...

name := MyTypeValue.String() // name => "MyTypeValue"

Sometimes you need to use some other string representation format than CamelCase (i.e. in JSON).

To transform it from CamelCase to snake_case or kebab-case, you can use the transform flag.

For example, the command enumer -type=MyType -json -transform=snake would generate the following string representation:

name := MyTypeValue.String() // name => "my_type_value"

Note: The transformation only works form CamelCase to snake_case or kebab-case, not the other way around.

How to use

The usage of Enumer is the same as Stringer, so you can refer to the Stringer docs for more information.

There are three flags added: json, yaml and sql. If the json flag is set to true (i.e. enumer -type=Pill -json), the JSON related methods will be generated. Similarly if the yaml flag is set to true, the YAML related methods will be generated. And if the sql flag is set to true, the Scanner and Valuer interface will be implemented to seamlessly use the enum in a database model.

For enum string representation transformation the transform and trimprefix flags were added (i.e. enumer -type=MyType -json -transform=snake). Possible transform values are snake and kebab for transformation to snake_case and kebab-case accordingly. The default value for transform flag is noop which means no transformation will be performed.

If a prefix is provided via the trimprefix flag, it will be trimmed from the start of each name (before it is transformed). If a name doesn't have the prefix it will be passed unchanged.

Inspiring projects

• Stringer
• jsonenums