tiny-pattern-ts

Pattern matching for TypeScript/ESM environments (F#-style, not regex).

Description

tiny-pattern-ts brings F#-style pattern matching to TypeScript. Patterns are ordinary objects whose matches method is a TypeScript type guard, so narrowing composes the way any other guard does. It is deliberately not a regex engine and not a macro: there is no transpiler and no DSL to learn, and the type-level contract is the feature — see development/library.md for the design decisions and Caveats for the limits.

Requirements

• Node.js >= 26 (engines field; pinned via .node-version).
• TypeScript >= 5.0 to consume the published declarations. The emitted .d.ts use const type parameters (TS 5.0) and keep their relative .ts specifiers; both resolve on TS >= 5.0 in node10 / node16 / nodenext / bundler.
• The package is ESM-only (no CommonJS shim).

API

Yet to be implemented

Caveats

• A value and its stringification collide. Object keys stringify, so a universe that mixes a member with the string it stringifies to — 1 | "1", true | "true", null | "null" — collapses to a single handler key and both members are routed to it. Use one form or the other.
• symbol and bigint are not supported. A symbol brand is a compile-time phantom with nothing to match at runtime, and a bigint is not a valid property key; neither satisfies the matcher's universe constraint.
• NaN and -0 cannot be matched specifically. They have no literal type, so both stay part of number.

License

MIT © 2025 tmu. See LICENSE.

Contributing

Contributions are documented in CONTRIBUTING.md; the reasons behind the project's decisions, rejected alternatives, and known issues live in development/. AI coding agents start at AGENTS.md.