sketch – Generate JSON (De)serializable Object From Go Schema

sketch is tool to generate Go structs and utility methods from a schema defined in Go code.

It is aimed to provide the basic implementation for those structs that represents something that is given through a relatively free-form definition format (such as JSON and YAML), while providing basic type-safety along with the ability to work with user-defined fields in an uniform manner.

The goal of this project is to provide a foundation for projects like that need to express in Go objects that are represented as JSON.

WARNING: THIS PROJECT IS EXPERIMENTAL. It does work, but you will most likely have to implement the missing pieces if it does not do what you want it to do.

Use-Case / Assumptions

sketch should be used if you are dealing with objects that need to represent a piece of that datat that matches the following criteria:

  1. The data has a JSON/YAML/etc based schema.
  2. The data is defined with certain pre-defined fields. Fields may have been assigned a type, and you would like to handle them in a type-safe manner.
  3. The data needs to be serialized from / deserialized from their respective formats.
  4. The data may contain arbitrary user-defined fields, of which you do not know the schema before hand. These need to be serialized / deserialized the same way as the pre-defined fields.

To sum this all up: if you have a schema that defines an object, but the user is free to extend the schema, and you need to serialize/deserialize from JSON/YAML/etc, this may be the right tool for you.

How sketch works

sketch provides the schema package for you to describe an object definition.

Once you have these objects, you can use sketch command to generate the actual code to be used.

First install the sketch tool. Note that the version of your sketch tool must match the version of the library you are using.

# TODO: change @latest to a released version
go get

Then declare a module that contains your schema.

We will assume that you want to implement a module named, hosted in your ~/go directory:


We also assume that the above directory is where you want to generate the files into, but you want to “declare” the definitions under ~/go/

Given the above assumptions, create the directory ~/go/ and initialize a new go module if you have not already done so:

mkdir ~/go/
cd ~/go/

go mod init

It is important to setup a proper module, as we need to compile a small intermediate program to generate the final code, and your code must be referencable from this small program.

Note that your module does NOT need to be available at an external URL, as sketch will take care of this resolution by using the replace directive in the generate go.mod file.

Next, create the directory to store your schema.

mkdir schema

Inside this directory, you can have any number of Go files, but sketch will pick up only those structs that are declared as embedding schema.Base types:

// mymodule/schema/schema.go
package schema

import (

type Thing struct {

This will declare that you want sketch to generate code for an object Thing. The name of this package must be uppercased so that sketch can see it.

You may opt to “rename” the object. By default the name of the schema object will be used as-is, but by declaring the method Name() on the schema object, you can alter the generated object name. If below code is included, sketch will generate a struct named fooBarBaz:

func (Thing) Name() string {
  return "fooBarBaz"

You may opt to specify the package name that the object belongs to. By default last component from the output directory (in this example’s case mymodule) will be used, but you can override this by declaring a method Package() on the schema object:

func (Thing) Package() string {
  return "awesomeModule"

Finally, you ou will want to declare the list of fields in this object. This is done by declaring a method named Fields() on the schema object, which returns a list of schema.Field objects.

func (Thing) Fields() []schema.Field{
  return []schema.Field{

In the above sample, the use of schema.String() implies that a field of string type is to be declared, with an exported name of Foo. It also specifies that the value of this field will be stored in a field named foo-field when serialized to JSON.

This will in turn instruct sketch to generate code that

  1. Creates mymodule.Thing struct
  2. Creates an accessor for Foo, but will store Foo as an unexported field.
  3. Creates UnmarshalJSON/MarshalJSON methods that recognizes field Foo — that it is stored as foo-field, and that its value must be a string
  4. The JSON methods will also recognize other fields and stores them appropriately, but you will onlybe able to get to them via (*Thing).Get()


Users can specify their own templates to be processed along side with the system templates that come with this tool.

Provide your templates that define templates named as below, and they will be included automatically.

package ...
{{ ext/object/header }}
// object definition
type Object struct { ... }

// methods ...
{{ ext/object/footer }}


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