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

name, description

name	description
native-monad-option	Option<T> API reference. ALWAYS invoke when using Some, None, Option, OptionPromise, Option.from, Option.all, Option.partition, option.map, option.flatMap, option.match(2 args), option.filter(NO errorFactory). Do not guess Option methods — consult this skill.

Option Usage Guide

Quick Reference

Method	Signature (on Some)	Signature (on None)
isSome()	this is Some<T>	this is never
isNone()	this is never	this is None
map(fn)	(fn: (v: T) => U) => Some<U>	() => None
flatMap(fn)	(fn: (v: T) => R) => R	() => this
match(onSome, onNone)	2 callbacks	2 callbacks
filter(pred)	NO errorFactory	() => None
toNullable()	T	null
toUndefined()	T	undefined
toString()	string	string

Critical Rules

1. match() takes exactly 2 callbacks: (onSome, onNone).
2. filter() takes ONLY a predicate — NO errorFactory (unlike Result).
3. Option has NO mapErr() or flatMapErr() — those are Result/Query only.
4. flatMap constraint: R extends Option<any> — callback must return an Option.
5. Type guard: after isSome(), access .value. None has no .value.
6. none() returns a frozen singleton — do not try to construct new None().
7. Option.from() treats only null | undefined as None. 0, false, "" become Some.

Anti-Patterns

// WRONG — Option.filter does NOT take errorFactory
option.filter((v) => v > 0, () => new Error("bad"));

// CORRECT — just a predicate
option.filter((v) => v > 0);

// WRONG — mapErr does not exist on Option
option.mapErr((e) => new Error(e));

// CORRECT — Option has no error channel. Use Result if you need errors.

// WRONG — fromNullable does not exist
Option.fromNullable(value);

// CORRECT
Option.from(value);

Namespace & Async Methods

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