native-monad/.claude/skills/native-monad-result/SKILL.md

name, description

name	description
native-monad-result	Result<T,E> API reference. ALWAYS invoke when using Ok, Err, Result, ResultPromise, Result.from, Result.all, Result.partition, result.map, result.flatMap, result.match(2 args), result.filter(needs errorFactory). Do not guess Result methods — consult this skill.

Result<T, E> Usage Guide

Quick Reference

Method	Signature (on Ok)	Signature (on Err)
isOk()	this is Ok<T>	this is never
isErr()	this is never	this is Err<E>
map(fn)	(fn: (v: T) => U) => Ok<U>	() => Err<E>
mapErr(fn)	(fn: (e: E) => F) => Ok<T>	(fn: (e: E) => F) => Err<F>
flatMap(fn)	(fn: (v: T) => R) => R	() => this
flatMapErr(fn)	() => this	(fn: (e: E) => R) => R
match(onOk, onErr)	2 callbacks	2 callbacks
filter(pred, errorFactory)	requires errorFactory	() => Err<E>
toNullable()	T	null
toUndefined()	T	undefined
toString()	string	string

Critical Rules

1. match() takes exactly 2 callbacks: (onOk, onErr).
2. filter() REQUIRES an errorFactory parameter — it produces Err<F> when the predicate fails.
3. flatMap constraint: R extends Result<any, any> — the callback must return a Result.
4. flatMapErr constraint: R extends Result<any, any> — the callback must return a Result.
5. Type guard: after isOk(), access .value. After isErr(), access .err.
6. Ok stores .value. Err stores .err (not .error, not .value).

Anti-Patterns

// WRONG — filter without errorFactory
result.filter((v) => v > 0);

// CORRECT — filter always needs errorFactory
result.filter((v) => v > 0, (v) => new Error(`Invalid: ${v}`));

// WRONG — match with 3 callbacks (that's Query, not Result)
result.match(onOk, onNone, onErr);

// CORRECT — Result.match takes exactly 2
result.match(onOk, onErr);

Namespace & Async Methods

Read references/api-surface.md for the complete Result namespace (.from, .all, .partition, .compact, .some, .every) and ResultPromise namespace.