ts-multimethod

A powerful TypeScript library for creating multimethods with compile-time exhaustive pattern matching and type narrowing. Inspired by Clojure's multimethods, this library brings runtime polymorphism with full TypeScript type safety to your projects.

Features

• 🔒 Type-Safe Pattern Matching: Compile-time verification that all cases are handled
• 🎯 Exhaustive Checking: TypeScript ensures you handle all possible cases in a union type
• 📝 Type Narrowing: Automatic type inference in case handlers
• 🚀 Zero Dependencies: Pure TypeScript, no external dependencies
• ⚡ Runtime Efficiency: Simple dictionary lookup for case dispatch
• 📦 Up to 10 Cases: Supports up to 10 pattern matches with full type safety

Installation

npm install ts-multimethod

Or with yarn:

yarn add ts-multimethod

Quick Start

import { multimethod } from "ts-multimethod";

const fn = multimethod(
  (n: number) => n, // Predicate function
  () => "notZeroOrOne", // Default handler
  [0, () => "zero"], // Case: 0
  [1, () => "one"], // Case: 1
);

fn(0); // 'zero'
fn(1); // 'one'
fn(2); // 'notZeroOrOne'

API

multimethod(predicateFn, defaultFn, ...cases)

Creates a multimethod function with the following parameters:

Parameter	Type	Description
predicateFn	(arg: A) => P	Extracts the discriminant value from the input
defaultFn	(arg: P) => R	Fallback handler when no case matches
cases	readonly [K, (arg: K) => R][]	Up to 10 tuples of [caseValue, handler]

Returns: (arg: A) => R - A function that dispatches based on the predicate result.

Usage Examples

Basic Number Matching

const httpStatus = multimethod(
  (code: number) => code,
  () => "Unknown Status",
  [200, () => "OK"],
  [404, () => "Not Found"],
  [500, () => "Internal Server Error"],
);

httpStatus(200); // 'OK'
httpStatus(404); // 'Not Found'
httpStatus(418); // 'Unknown Status'

String Matching

const actionHandler = multimethod(
  (action: string) => action,
  () => "Unknown action",
  ["create", () => "Creating..."],
  ["read", () => "Reading..."],
  ["update", () => "Updating..."],
  ["delete", () => "Deleting..."],
);

actionHandler("create"); // 'Creating...'
actionHandler("delete"); // 'Deleting...'
actionHandler("other"); // 'Unknown action'

Boolean Matching

const boolHandler = multimethod(
  (value: boolean) => value,
  () => "default",
  [true, () => "is true"],
  [false, () => "is false"],
);

boolHandler(true); // 'is true'
boolHandler(false); // 'is false'

Symbol Matching

const symA = Symbol("a");
const symB = Symbol("b");

const symbolHandler = multimethod(
  (s: symbol) => s,
  () => "other",
  [symA, () => "Symbol A"],
  [symB, () => "Symbol B"],
);

symbolHandler(symA); // 'Symbol A'
symbolHandler(symB); // 'Symbol B'

Async Handlers

const asyncHandler = multimethod(
  (n: number) => n,
  async () => "default",
  [0, async () => "zero"],
  [1, async () => "one"],
);

await asyncHandler(0); // 'zero'
await asyncHandler(1); // 'one'

Class Usage with this Context

class Calculator {
  zero = "zero";
  one = "one";
  default = "default";

  m = multimethod(
    (n: number) => n,
    () => this.default,
    [0, () => this.zero],
    [1, () => this.one],
  );
}

const calc = new Calculator();
calc.m(0); // 'zero'
calc.m(1); // 'one'

Advanced Features

Exhaustive Pattern Matching

When your predicate returns a union type, ts-multimethod ensures at compile-time that you handle all cases:

// ✅ Complete coverage - TypeScript knows all cases are handled
const exhaustive = multimethod(
  (n: number) => n as 0 | 1 | 2, // Union type
  () => "default",
  [0, () => "zero"],
  [1, () => "one"],
  [2, () => "two"],
);
// TypeScript knows predicate result is fully covered

// ❌ Incomplete coverage - TypeScript shows an error
const incomplete = multimethod(
  (n: number) => n as 0 | 1 | 2,
  () => "default",
  [0, () => "zero"],
  [1, () => "one"],
  // Error: Missing case for '2'
);

Type Narrowing in Handlers

Each handler receives a properly typed argument, enabling full type safety:

const narrowed = multimethod(
  (s: string) => s as "a" | "b" | "c",
  () => "other",
  [
    "a",
    (value) => {
      // `value` is typed as 'a'
      const _check: "a" = value; // ✅ OK
      return "A";
    },
  ],
  [
    "b",
    (value) => {
      // `value` is typed as 'b'
      const _check: "b" = value; // ✅ OK
      return "B";
    },
  ],
  [
    "c",
    (value) => {
      // `value` is typed as 'c'
      const _check: "c" = value; // ✅ OK
      return "C";
    },
  ],
);

Complex Predicate Functions

The predicate function can transform input into any discriminant value:

// Extract and match specific properties
type Shape =
  | { kind: "circle"; radius: number }
  | { kind: "square"; side: number }
  | { kind: "triangle"; base: number; height: number };

const areaCalculator = multimethod(
  (shape: Shape) => shape.kind,
  () => 0,
  ["circle", (shape) => Math.PI * shape.radius * shape.radius],
  ["square", (shape) => shape.side * shape.side],
  ["triangle", (shape) => 0.5 * shape.base * shape.height],
);

areaCalculator({ kind: "circle", radius: 5 }); // 78.54...
areaCalculator({ kind: "square", side: 4 }); // 16
areaCalculator({ kind: "triangle", base: 3, height: 4 }); // 6

Numeric Range Matching

const gradeCalculator = multimethod(
  (score: number) => {
    if (score >= 90) return "A";
    if (score >= 80) return "B";
    if (score >= 70) return "C";
    if (score >= 60) return "D";
    return "F";
  },
  () => "Invalid",
  ["A", () => "Excellent"],
  ["B", () => "Good"],
  ["C", () => "Fair"],
  ["D", () => "Poor"],
  ["F", () => "Fail"],
);

gradeCalculator(95); // 'Excellent'
gradeCalculator(75); // 'Fair'

Type Safety Details

Supported Union Sizes

ts-multimethod supports exhaustive checking for unions with up to 10 members:

Overload	Max Cases	Type Safety
1 case	1	✅ Exhaustive
2 cases	2	✅ Exhaustive
...	...	...
10 cases	10	✅ Exhaustive

How Exhaustive Checking Works

The library uses TypeScript's conditional types to detect when all members of a union type are covered:

type IsLiteralType<P> = [P] extends [number]
  ? [number] extends [P]
    ? false   // P is just 'number', not a literal
    : true    // P is a literal like 0 | 1 | 2
  : /* similar checks for string, boolean, symbol */;

When you provide cases that cover all union members, TypeScript validates completeness at compile-time.

Comparison with Alternatives

Feature	ts-multimethod	switch/case	if/else chains
Type narrowing	✅	⚠️ Limited	⚠️ Manual
Exhaustive checking	✅	❌	❌
Runtime dispatch	✅	✅	✅
Composable	✅	❌	❌
Default handler	✅	⚠️ default	⚠️ else
this context	✅	❌	⚠️ Manual

Limitations

• Maximum of 10 case tuples per multimethod
• Exhaustive checking only works with literal types (number, string, boolean, symbol literals)
• First case tuple must include exhaustive check marker (handled automatically by types)

Contributing

Contributions are welcome! Please feel free to submit a Pull Request.

License

MIT License - see LICENSE for details.

Acknowledgments

• Inspired by Clojure's multimethods
• Built with TypeScript's advanced type system features
• Version 0.3.0 was vibe coded by Qwen Code