polygen

A Go code generator for creating polymorphic data structures with JSON serialization support.

Example

Given the following Go code:

type IsItem interface {
    isItem()
}

type TextItem struct {
    Content string `json:"content"`
}

func (TextItem) isItem() {}

type ImageItem struct {
    URL    string `json:"url"`
}

func (*ImageItem) isItem() {}

Create a .polygen.json file in your project root:

{
    "$schema": "https://raw.githubusercontent.com/ykalchevskiy/polygen/refs/tags/v0.0.6/schema.json",
    "types": [
        {
            "type": "Item",
            "interface": "IsItem",
            "package": "main",
            "subtypes": {
                "TextItem": {
                    "name": "text"
                },
                "ImageItem": {
                    "name": "image",
                    "pointer": true
                }
            }
        }
    ]
}

Then run:

polygen

The generated code allows you to marshal/unmarshal your types to/from JSON:

// unmarshaling
var item Item
json.Unmarshal([]byte(`{"type": "text", "content": "hello"}`), &item)
json.Unmarshal([]byte(`{"content": "updated"}`), &item)  // Updates just content
json.Unmarshal([]byte(`{"type": "image", "url": "pic.jpg"}`), &item)  // Changes type


// marshaling
json.Marshal(Item{IsItem: TextItem{Content: "Hello, World!"}})
// {"type": "text", "content": "Hello, World!"}
json.Marshal(Item{IsItem: &ImageItem{URL: "https://example.com/image.jpg"}})
// {"type": "image", "url": "https://example.com/image.jpg"}

Installation

go install github.com/ykalchevskiy/polygen@latest

Configuration

The JSON configuration file supports:

• Multiple type definitions in a single file
• Global and per-type discriminator field name
• Global and per-type strict mode settings
• Global and per-subtype pointer mode settings
• Custom output paths relative to config file

Schema

The configuration follows this structure:

• strictByDefault (optional): Enable strict mode by default
• pointerByDefault (optional): Mark all subtypes as pointer mode by default
• defaultDiscriminator (optional): Default JSON field name for type discrimination (default: "type")
• types (required): Array of type configurations:
  • type (required): Name of the polymorphic structure
  • interface (required): Name of the interface all subtypes implement
  • package (required): Package name for generated code
  • discriminator (optional): Override default JSON field name
  • directory (optional): Output directory path relative to config file
  • filename (optional): Output filename (defaults to _polygen.go)
  • strict (optional): Override strict mode for this type
  • defaultSubtype (optional): Default subtype to unmarshal into when the discriminator field is missing
  • subtypes (required): Map of Go type names to their configurations:
    • name (optional): JSON type name (defaults to subtype name in kebab-case)
    • pointer (optional): Use pointer for this type (defaults to pointerByDefault)