@praha/tagged

Lightweight TypeScript tagged union (discriminated union) utilities.

👏 Getting Started

Installation

npm install @praha/tagged

Usage

Basic Usage

Define a tagged type using the Tagged factory:

import { Tagged } from '@praha/tagged';

const User = Tagged({
  tag: 'User',
  fields: Tagged.fields<{ name: string; age: number }>(),
});

const user = User({ name: 'Alice', age: 30 });
// => { $tag: 'User', name: 'Alice', age: 30 }

Tag-Only (No Fields)

const Loading = Tagged({ tag: 'Loading' });

const loading = Loading();
// => { $tag: 'Loading' }

Discriminated Union

Combine multiple tagged types into a union for exhaustive type narrowing:

import { Tagged } from '@praha/tagged';

const Success = Tagged({
  tag: 'Success',
  fields: Tagged.fields<{ data: string }>(),
});

const Failure = Tagged({
  tag: 'Failure',
  fields: Tagged.fields<{ message: string }>(),
});

const Loading = Tagged({
  tag: 'Loading',
});

type Result = (
  | typeof Success.$infer
  | typeof Failure.$infer
  | typeof Loading.$infer
);

const handle = (result: Result) => {
  switch (result.$tag) {
    case 'Success':
      console.log(result.data);
      break;
    case 'Failure':
      console.error(result.message);
      break;
    case 'Loading':
      console.log('Loading...');
      break;
  }
};

Inferring Types

Use $infer to extract the TypeScript type from a factory:

type User = typeof User.$infer;
// => { $tag: 'User'; name: string; age: number }

🤝 Contributing

Contributions, issues and feature requests are welcome.

Feel free to check issues page if you want to contribute.

📝 License

Copyright © PrAha, Inc.

This project is MIT licensed.