In my previous blog post, we discussed one of my favourite code generation tools for Go. We found that they can be used to automate our trivial development tasks or even introduce features like generics and queries. Lets explore how to create our own tool.


The Go generate subcommand is a program that scans for special comments in your Go source code. The comment declares a command that should be executed. The tools is not part of Go build toolbelt. Usually it’s used by packaged developers.

This is the format of go:generate comments:

//go:generate <subcommand> <arguments>

After identifying all go:generate comments it will execute the specified commands.


We will build a command line tool that generates an implementation of io.WriterTo interface for concrete type and format.

Lets name the tool wordsmith. Note that in the sample we will use only json.

io.WriterTo provides a function that writes data to a writer. The return value n is the number of bytes written. Any error encountered during the write is also returned:

type WriterTo interface {
        WriteTo(w Writer) (n int64, err error)

Lets have a Point struct that is annotated with special go:generate comment:

type Point struct {
	X float64
	Y float64

If we implement the funcationality manually, we should end up with the following code snippet:

func (obj Point) WriteTo(writer io.Writer) (int64, error) {
	data, err := json.Marshal(&obj)
	if err != nil {
		return 0, err
	length, err := writer.Write(data)
	return int64(length), err

We can trainsform it into a text/template file:

package {{ .PackageName }}

import (

func ({{ .Object }} {{ .Type }}) WriteTo(writer io.Writer) (int64, error) {
  data, err := json.Marshal({{ .MarshalObject }})
  if err != nil {
  	return 0, err
  length, err := writer.Write(data)
  return int64(length), err

The wordsmith generation tools is a command line application that has the following arguments:

  • pointer determines whether a type is a pointer
  • type defines the name of the type that implements io.WriterTo interface
  • package defines the package that contains this type
  • format defines the serialization format that io.WriterTo is providing

The function initial reads the arguments, locates the package directory and creates the output file in the following format <type_name>_writer.go.

func main() {
	typePointer := flag.Bool("pointer", false, "Determines whether a type is a pointer or not")
	typeName := flag.String("type", "", "Type that hosts io.WriterTo interface implementation")
	packageName := flag.String("package", "", "Package name")
	format := flag.String("format", "json", "Encoding format")


	if *typeName == "" || *format != "json" {

	pkgDir, err := packageDir(*packageName)
	if err != nil {

	outputFile := formatFileName(*typeName)
	writer, err := os.Create(filepath.Join(pkgDir, outputFile))
	if err != nil {
	defer writer.Close()

	generator := &Generator{Format: JSON}

	m := metadata(*typeName, *typePointer, pkgDir)
	if err := generator.Generate(writer, m); err != nil {

	fmt.Printf("Generated %s %s\n", *format, outputFile)

The wordsmith.Generator type is responsible to execute the text template and generate the output file. It instaciates text template object from the template file and substitute it with the Metadata object properties:

type Metadata struct {
	PackageName   string
	Object        string
	MarshalObject string
	Type          string

type Generator struct {
	Format Format

func (g *Generator) Generate(writer io.Writer, metadata Metadata) error {
	tmpl, err := g.template()
	if err != nil {
		return nil

	return tmpl.Execute(writer, metadata)

func (g *Generator) template() (*template.Template, error) {
	if g.Format != JSON {
		return nil, errors.New("Unsupported format")

	resource, err := Asset("templates/writeto_json.tmpl")
	if err != nil {
		return nil, err

	tmpl := template.New("template")
	return tmpl.Parse(string(resource))

You can read more about text templates in the official golang documentation.

The wordsmith can be used from command line prompt or by annotating Point struct with the following comment:

//go:generate wordsmith -type=Point -format=json

The following command identifies the go:generate comment and executes declared wordsmith submcommand:

// it scans all
$ go generate ./...

You can use the download the full source code from github or install it immediatelly:

$ go get


I am looking forward to see more tools in the Go ecosystem. Hopefully this blog post will encourage more go developers to build such a tools that will boost our productivity.