@danrabydev/match

Rust-style tagged unions and exhaustive match for TypeScript.

Zero dependencies. Constructors for every variant, compile-time exhaustiveness, and enumerable tags at runtime.

Building this repo requires Node 22.18+ so tsdown can load tsdown.config.ts with native TypeScript stripping. unrun is not a direct dependency (tsdown may still list it as an optional peer in the lockfile). The published dist/ is ES2022 and does not require that Node version.

Installation

pnpm add @danrabydev/match

Quick start

import { createMatchable, type MatchableOf } from "@danrabydev/match";

const Status = createMatchable({
  Idle: () => ({}),
  Loading: (msg: string) => ({ msg }),
  Success: (data: number) => ({ data }),
  Error: (err: Error) => ({ err }),
});

type Status = MatchableOf<typeof Status>;

const result = Status.match(Status.Loading("fetching"), {
  Idle: () => "waiting",
  Loading: ({ msg }) => `state: ${msg}`,
  Success: ({ data }) => `got ${data}`,
  Error: ({ err }) => err.message,
});
// => "state: fetching"

Omit an arm and TypeScript reports an error. Handle a value whose tag has no arm at runtime and match throws.

Reuse one matchable with different data (and optionally err) by aliasing a generic:

const ApiResult = createMatchable({
  Idle: () => ({}),
  Loading: (msg: string) => ({ msg }),
  Success: (data: unknown) => ({ data }),
  Error: (err: unknown) => ({ err }),
});

type ApiResult<TData, TErr = unknown> = MatchableOf<typeof ApiResult, TData, TErr>;

function handle<TData>(result: ApiResult<TData>): TData | undefined {
  return ApiResult.match(result, {
    Idle: () => undefined,
    Loading: () => undefined,
    Success: ({ data }) => data,
    Error: () => undefined,
  });
}

Call sites are ApiResult<User> and ApiResult<Post> — one runtime namespace. A one-argument constructor whose parameter is unknown infers data / err from the value (ApiResult.Success(post) is { tag: "Success"; data: Post }), so a function returning ApiResult<Post, ApiError> does not need a cast. Extra type arguments must extend the constructor payload (MatchableOf<typeof Status, string> is an error when data is number). A hole no variant has is never, so a surplus argument is an error.

Every feature, including a small API-client app, lives in examples/.

API

createMatchable(defs)

Takes a map of variant name → payload constructor. Returns:

• constructors for each key (Status.Loading("fetching") → { tag: "Loading", msg: "fetching" })
• match(value, arms) — exhaustive matcher bound to this union
• peek(value, arms) — optional void observers; returns the same value
• withDiagnostics(opts) — bind a diag mask (and optional reporters) at init
• merge(...results) — zip same-tag results; mixed tags become Error (TagMismatch)
• _tags — string[] of installed variant names, in definition order (undefined constructor holes are omitted)

tag is always the constructor name, even if the payload also has a tag field. It is non-writable and non-configurable so later assignment cannot reroute match. Names match, merge, peek, withDiagnostics, _tags, __proto__, prototype, and constructor are reserved (type error and runtime throw). __proto__ is rejected at runtime even when written via Object.create(null).

Constructors should return plain objects. The library copies enumerable own fields only ({ ...payload, tag }). Class instances lose methods and the prototype; that is expected. Return { value } (or another plain record), not new SomeClass().

match(value, arms)

Standalone exhaustive matcher for any { tag: string } union. Same runtime behavior as the bound match on a matchable namespace. The value’s type is inferred from the first argument, so match(result: ApiResult<User>, …) types data as User. Bound Ns.match allows extra arms for tags not present on a narrowed value; standalone match does not. A variable typed as the full union still requires every arm.

Only own, callable arms are considered — a missing arm is never taken from Object.prototype.

peek(value, arms) / Ns.peek(value, arms)

Optional observers. Write only the tags you care about; omitted tags are no-ops. Arms return void (a toast id or other incidental return is ignored). peek returns the same object (===), so reusable peekers can be imposed and match still performs one R:

const logErrors = peeker("logErrors", {
  Error: ({ err }) => console.error(err),
});

ApiResult.peek(result, logErrors);
return ApiResult.match(result, {
  Idle: () => "",
  Loading: ({ msg }) => msg,
  Success: ({ data }) => data.name,
  Error: () => "",
  Cached: ({ at }) => at.toISOString(),
});

This is not a pipe. Peek does not consume the value and does not return branch results. Bound Ns.peek allows extra arms for tags not on a narrowed value; standalone peek does not.

diagnostics / withDiagnostics / enableDiagnostics

A mask, not a required logger. { enabled, branches? } chooses whether this instance (or client) records a trail, and for which tags.

ApiResult.Success(user); // hot path
ApiResult.Error(err, diagnostics({ enabled: true, branches: ["Error"] }));

const Ns = ApiResult.withDiagnostics({
  enabled: true,
  branches: ["Error"],
});
Ns.Success(user); // silent (tag not in branches)
const failed = Ns.Error(err);
Ns.peek(failed, logErrors);
Ns.match(failed, { /* … */ });
peekTrace(failed); // trail on the tagged value

onPeek / onMatch on withDiagnostics are optional. Without them, events still record for peekTrace(value).

enableDiagnostics(["Panic", "Crit"]) is a process-wide floor: those tags always console.error, even if the instance mask is { enabled: false }. Do not put Success on the floor. disableDiagnostics() clears it (tests).

MatchableOf<T, Data?, Err?>

Extracts the tagged-union type from a createMatchable result so you can write type Status = MatchableOf<typeof Status>.

Optional type arguments specialize payload holes, in order: Data replaces data, Err replaces err. To use X<TData> at call sites, alias once:

type ApiResult<TData, TErr = unknown> = MatchableOf<typeof ApiResult, TData, TErr>;

merge(ns, ...results) / Ns.merge(...results)

Zip two or more values from the same matchable. Every tag must match. Matching Success values become Success with data: [d1, d2, …]; matching Error values zip err the same way; other payload fields (msg, at, …) become tuples. Mixed tags call Error with { reason: "tag-mismatch", tags } (TagMismatch) — not a domain error.

merge is only on namespaces whose Error constructor returns { err }. Missing Error throws. tag is locked on the result.

const page = ApiResult.merge(getUser(uid), getPost(pid));
ApiResult.match(page, {
  Success: ({ data: [user, post] }) => `${user.name}: ${post.title}`,
  Error: ({ err }) => /* TagMismatch | [ApiError, ApiError] */,
  Idle: () => "idle",
  Loading: ({ msg }) => msg.join(", "),
});

MatchArms<T, R>

The arm map type used by match: one function per T["tag"], each receiving that variant’s payload.

Why this pattern

Need	What you get
Compile-time exhaustiveness	Adding a variant without updating match is a type error
Runtime constructors	Status.Success(1) instead of hand-written { tag: "Success", data: 1 }
Enumerable keys	_tags lists variants for logs, serializers, and UI without a separate enum

This is the TypeScript analogue of a Rust enum plus match: a closed set of tagged values, constructed in one place, destructured exhaustively everywhere else.

Publishing

pnpm check typechecks, tests, builds, smoke-tests dist/, then runs publint and Are The Types Wrong. Publish with provenance:

pnpm publish:npm

(npm publish --access public --provenance. Requires a trusted publisher / OIDC environment such as GitHub Actions.)

License

MIT