tagu

Type-safe pattern matching for TypeScript discriminated unions.

Installation

npm install @naamanu/tagu

Usage

import { match } from "@naamanu/tagu";

// Define a tagged union
type Result =
  | { _tag: "Success"; data: string }
  | { _tag: "Failure"; error: string };

const result: Result = { _tag: "Success", data: "Hello" };

// Match against it
const message = match(result)({
  Success: ({ data }) => `Got: ${data}`,
  Failure: ({ error }) => `Error: ${error}`,
});

console.log(message); // "Got: Hello"

API

match<T>(value: T)

Pattern matches on a tagged union value. Returns a function that takes a pattern object with handlers for each variant.

type Shape =
  | { _tag: "Circle"; radius: number }
  | { _tag: "Rectangle"; width: number; height: number };

const shape: Shape = { _tag: "Circle", radius: 5 };

const area = match(shape)({
  Circle: ({ radius }) => Math.PI * radius * radius,
  Rectangle: ({ width, height }) => width * height,
});

Pattern<T, R>

Type helper for defining pattern objects. Ensures exhaustive matching at compile time.

import { Pattern } from "@naamanu/tagu";

const pattern: Pattern<Shape, number> = {
  Circle: ({ radius }) => Math.PI * radius * radius,
  Rectangle: ({ width, height }) => width * height,
};

Tag

Base type constraint for tagged union members. All variants must have a _tag string property.

License

MIT