`@alt-stack/result`

Typed success and expected failure values for Altstack and standalone TypeScript applications.

Install

pnpm add @alt-stack/result

The implementation has no runtime dependency. Its manifest declares Zod 4 as an optional peer for ecosystem compatibility.

Quickstart

import {
  TaggedError,
  err,
  isErr,
  ok,
  type Result,
} from "@alt-stack/result";

class MissingUserError extends TaggedError<"MissingUserError"> {
  readonly _tag = "MissingUserError" as const;

  constructor(readonly userId: string) {
    super(`User ${userId} was not found`);
  }
}

function loadUser(id: string): Result<{ id: string }, MissingUserError> {
  return id === "u_123" ? ok({ id }) : err(new MissingUserError(id));
}

const result = loadUser("missing");
if (isErr(result)) {
  console.error(result.error._tag, result.error.userId);
} else {
  console.log(result.value.id);
}

A Result has an outer _tag of "Ok" or "Err". A failure carries an actual Error instance with a string _tag. A readonly literal tag is not required for ResultError conformance, but it enables exhaustive narrowing of an error union. Supplying TaggedError<"MissingUserError"> makes the base class enforce that exact literal.

Public surface

construction and guards: ok, err, isOk, isErr;
composition: map, flatMap, mapError, catchError;
extraction and matching: unwrap, defaults, match, and fold;
async boundaries: fromPromise, tryCatch, and tryCatchAsync;
collections and observation: all, firstOk, tap, and tapError;
tagged errors and inference helpers: TaggedError, ResultAggregateError, isResultError, assertResultError, and the exported helper types.

Documentation

Development

From the repository root:

pnpm --filter @alt-stack/result exec vitest run src/result.spec.ts
pnpm --filter @alt-stack/result check-types
pnpm --filter @alt-stack/result build