jsonenums is a tool to automate the creation of methods that satisfy the json.Marshaler and json.Unmarshaler interfaces. Given the name of a (signed or unsigned) integer type T that has constants defined, jsonenums will create a new self-contained Go source file implementing

func (t T) MarshalJSON() ([]byte, error)
func (t *T) UnmarshalJSON([]byte) error

The file is created in the same package and directory as the package that defines T. It has helpful defaults designed for use with go generate.

jsonenums is a simple implementation of a concept and the code might not be the most performant or beautiful to read.

For example, given this snippet,

package painkiller

type Pill int

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

running this command

jsonenums -type=Pill

in the same directory will create the file pill_jsonenums.go, in package painkiller, containing a definition of

func (r Pill) MarshalJSON() ([]byte, error)
func (r *Pill) UnmarshalJSON([]byte) error

MarshalJSON will translate the value of a Pill constant to the []byte representation of the respective constant name, so that the call json.Marshal(painkiller.Aspirin) will return the bytes []byte("\"Aspirin\"").

UnmarshalJSON performs the opposite operation; given the []byte representation of a Pill constant it will change the receiver to equal the corresponding constant. So given []byte("\"Aspirin\"") the receiver will change to Aspirin and the returned error will be nil.

Typically this process would be run using go generate, like this:

//go:generate jsonenums -type=Pill

If multiple constants have the same value, the lexically first matching name will be used (in the example, Acetaminophen will print as "Paracetamol").

With no arguments, it processes the package in the current directory. Otherwise, the arguments must name a single directory holding a Go package or a set of Go source files that represent a single Go package.

The -type flag accepts a comma-separated list of types so a single run can generate methods for multiple types. The default output file is t_jsonenums.go, where t is the lower-cased name of the first type listed. The suffix can be overridden with the -suffix flag and a prefix may be added with the -prefix flag.

This is not an official Google product (experimental or otherwise), it is just code that happens to be owned by Google.