lib-result

name	lib-result
description	Use when writing Result-driven error handling in TypeScript — converting try/catch to typed Results, chaining fallible operations, recovering from errors without throwing, or wrapping throwing functions. Activates when the user mentions lib-result, Result/Ok/Err types, error handling patterns, or any TypeScript code that throws and would benefit from type-safe error handling, even if they don't name the library.
license	MIT
compatibility	TypeScript project with lib-result installed. Node.js or browser.
metadata	{"version":"5.0.0","source":"https://github.com/AhmedOsman101/lib-result"}

lib-result Skill Guide

A Rust-inspired Result<T, E> type for type-safe error handling.

When to use each method: See the API Quick Reference below. For full signatures and examples of any method, read references/methods.md.

Core Concepts

Result<T, E extends Error> = OkState<T, E> | ErrorState<E, T>

OkState:   { ok: T,          error: undefined }
ErrorState:{ ok: undefined,  error: E          }

E is always a real Error instance — enforced at runtime by Err().

Imports

import { Ok, Err, ErrFromText, ErrFromUnknown, ErrFromObject } from "lib-result";
import { wrap, wrapAsync, wrapThrowable, wrapAsyncThrowable } from "lib-result";
import type { Result, CustomError } from "lib-result";

Use import type for types.

---

Which Constructor to Use

Situation	Constructor
Operation succeeded	Ok(value)
Operation failed with an Error instance	Err(new Error(...))
Quick string error, no custom class needed	ErrFromText("message")
In a catch (e) block (value is unknown)	ErrFromUnknown(e)
Error needs typed extra properties (status codes, metadata)	ErrFromObject({ message, code: 500 })

Key constraint: Err() throws TypeError if given a non-Error. Use ErrFromText() or ErrFromUnknown() for strings and unknown values.

---

Which Wrap Function to Use

Situation	Function
Wrap a sync function call	wrap(() => fn())
Wrap an async function call	wrapAsync(() => fn())
Convert a sync throwing function to Result-returning	wrapThrowable(fn)
Convert an async throwing function to Result-returning	wrapAsyncThrowable(fn)

wrap()/wrapAsync() are for one-off calls. wrapThrowable()/wrapAsyncThrowable() create reusable safe functions.

---

Common Patterns

Pattern 1: Declaring a Result-returning function

class ValidationError extends Error {
  constructor(
    message: string,
    public readonly field: string,
  ) {
    super(message);
    this.name = "ValidationError";
  }
}

function validateEmail(email: string): Result<string, ValidationError> {
  if (!email.includes("@")) {
    return Err(new ValidationError("Invalid email", "email"));
  }
  return Ok(email);
}

Pattern 2: Chaining fallible ops with andThen

function processUser(id: number): Result<User, AppError> {
  return fetchUser(id)
    .andThen(user => validateUser(user))
    .andThen(user => saveUser(user));
  // short-circuits on first Err
}

Pattern 3: Converting throwing code with wrap

// Before: throws on bad input or I/O error
function readConfig(path: string) {
  return JSON.parse(fs.readFileSync(path, "utf-8"));
}

// After: returns Result
function readConfig(path: string): Result<Config, CustomError> {
  return wrap(() => JSON.parse(fs.readFileSync(path, "utf-8")));
}

Pattern 4: Error recovery with orElse

class NetworkError extends Error {}
class CacheError extends Error {}

fetchData(url)                          // Result<Data, NetworkError>
  .orElse(() => readCache(key));        // Result<Data, NetworkError | CacheError>

Pattern 5: Exhaustive handling with match

const content = readFile("/path").match({
  okFn: data => data,
  errFn: () => "default content",
});

Pattern 6: Creating an async API client

class ApiError extends Error {
  constructor(
    message: string,
    public readonly statusCode: number,
  ) {
    super(message);
  }
}

const safeGet = wrapAsyncThrowable(async (url: string) => {
  const res = await fetch(url);
  if (!res.ok) throw new ApiError("Request failed", res.status);
  return res.json() as Promise<unknown>;
});

Pattern 7: Error with cause chain

parseJson(raw)
  .andThen(data => validateData(data))
  .mapErr(e => ErrFromObject({
    message: "Validation failed",
    cause: e,
    inputType: "json",
  }));

---

Adding error properties with ErrFromObject

ErrFromObject creates typed errors with extra properties. The P type parameter describes the additional fields:

const result = ErrFromObject<{ field: string; code: number }>({
  message: "Validation failed",
  field: "email",
  code: 400,
});

if (result.isError()) {
  console.log(result.error.field);  // "email"
  console.log(result.error.code);   // 400
}

The error is still an Error instance — properties are merged via Object.assign.

---

Gotchas

GOTCHA 1: Err() requires an Error instance

Err("string");          // TypeError — USE THIS INSTEAD:
ErrFromText("string");  // ✓
ErrFromUnknown(e);      // ✓ (catch blocks)

GOTCHA 2: Always use ErrFromUnknown in catch blocks

catch (e) {
  return ErrFromUnknown(e); // handles Error, string, object, any type
  // NOT Err(e) — throws TypeError if e is not an Error
}

GOTCHA 3: Callback throws propagate — not caught as Err

Ok(5).map(() => { throw new Error("boom"); }); // throws, does NOT return Err
// To handle, wrap in Result:
Ok(5).andThen(() => ErrFromText("boom"));

This applies to map, andThen, mapErr, orElse, unwrapOrElse, inspect, inspectErr, match.

GOTCHA 4: mapOr default is eager

mapOr(computeExpensiveDefault(), ...) // ALWAYS called
// Use mapOrElse for lazy:
mapOrElse(() => computeExpensiveDefault(), ...)

GOTCHA 5: and() ignores the value vs andThen() uses it

Ok(42).and(Ok("done"));            // Ok("done") — ignores 42
Ok(42).andThen(x => Ok(x * 2));    // Ok(84) — uses 42

GOTCHA 6: or() is eager, orElse() is lazy

Err("fail").or(Ok(42));               // Ok(42) — result already created
Err("fail").orElse(e => Ok(42));      // Ok(42) — fn called only if Err

GOTCHA 7: orElse() stays in Result, unwrapOrElse() collapses to value

result.orElse(e => Ok(0));           // Result<number, Error> ← still a Result
result.unwrapOrElse(e => e.message.length); // number ← plain value

GOTCHA 8: Naming diff from Rust

Concept	Rust	lib-result
Is success?	is_ok()	isOk()
Is error?	is_err()	isError()
Chain	and_then()	andThen()
Recover	or_else()	orElse()

GOTCHA 9: match() errFn accepts 0 or 1 args

result.match({ okFn: v => v, errFn: e => `Error: ${e.message}` });  // with error
result.match({ okFn: v => v, errFn: () => "default" });             // ignore error

Implementation uses fn.length at runtime.

GOTCHA 10: unwrap() throws the original error, expect() wraps it

result.unwrap();           // throws E directly
result.expect("context");  // throws CustomError with msg + { cause: E }

GOTCHA 11: Results are frozen (shallow)

const r = Ok(42);
r.ok = 43; // fails silently in strict mode

---

Architecture Notes

• Composition over inheritance: Methods are mixed in via Object.assign on frozen plain objects. No classes or prototypes.
• E extends Error enforced: Err() validates at runtime that the argument is an Error instance.
• Rust API alignment: Method names follow Rust conventions (andThen, orElse, mapErr, inspect, expect).
• Callback wrapping: All methods accepting callbacks wrap them in try/catch. Thrown errors are re-thrown as Error instances via toError() — they are NOT returned as Err.
• Dual ESM/CJS: Ships both module formats with type declarations.

---

API Quick Reference

For full examples, see references/methods.md.

Category	Method	Input	Output	Short-circuit?
State	isOk()	—	boolean	—
State	isError()	—	boolean	—
Extract	unwrap()	—	T / throw E	—
Extract	expect(msg)	string	T / throw	—
Extract	unwrapErr()	—	E / throw	—
Extract	expectErr(msg)	string	E / throw	—
Safe	unwrapOr(fb)	T	T	—
Safe	unwrapOrElse(fn)	E => T	T	—
Transform	map(fn)	T => U	Result<U, E>	Yes
Transform	mapErr(fn)	E => U	Result<T, U>	Yes
Transform	andThen(fn)	T => Result<U, E>	Result<U, E>	Yes
Transform	mapOr(def, fn)	U, T => U	U	—
Transform	mapOrElse(def, fn)	E => U, T => U	U	—
Combinator	and(result)	Result<U, E>	Result<U, E>	Yes
Combinator	or(result)	Result<T, F>	Result<T, F>	Yes
Combinator	orElse(fn)	E => Result<T, F>	Result<T, F>	Yes
Pattern	match({ okFn, errFn })	T => U, E => U	U	—
Side-effect	inspect(fn)	T => void	Result<T, E>	Yes
Side-effect	inspectErr(fn)	E => void	Result<T, E>	Yes
Wrap	wrap(fn)	() => T	Result<T, CustomError>	—
Wrap	wrapAsync(fn)	() => Promise<T>	Promise<Result<T, CustomError>>	—
Wrap	wrapThrowable(fn)	Fn	Fn => Result	—
Wrap	wrapAsyncThrowable(fn)	AsyncFn	AsyncFn => Promise<Result>	—