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

name, description

name	description
native-monad-query	Query<T,E> API reference. ALWAYS invoke when using OkSome, OkNone, ErrNone, Query, QueryPromise, Query.from, Query.all, Query.partition, query.match(3 args), query.filter(NO errorFactory). Do not guess Query methods — consult this skill.

Query<T, E> Usage Guide

Quick Reference

Query is a three-state monad: success-with-value (OkSome), success-without-value (OkNone), error (ErrNone).

Method	OkSome	OkNone	ErrNone
isSome()	this is OkSome<T>	this is never	this is never
isNone()	this is never	this is OkNone	this is never
isErr()	this is never	this is never	this is ErrNone<E>
map(fn)	(fn: (v: T) => U) => OkSome<U>	() => OkNone	() => ErrNone<E>
mapErr(fn)	() => OkSome<T>	() => OkNone	(fn: (e: E) => F) => ErrNone<F>
flatMap(fn)	(fn: (v: T) => R) => R	() => this	() => this
flatMapErr(fn)	() => this	() => this	(fn: (e: E) => R) => R
match(a, b, c)	3 callbacks	3 callbacks	3 callbacks
filter(pred)	NO errorFactory	() => OkNone	() => ErrNone<E>
toNullable()	T	null	null
toUndefined()	T	undefined	undefined

Critical Rules

1. match() takes exactly 3 callbacks: (onSome, onNone, onErr) — NOT 2.
2. filter() takes ONLY a predicate — NO errorFactory (unlike Result). Failed filter returns OkNone, not ErrNone.
3. flatMap constraint: R extends Query<any, any> — callback must return a Query.
4. flatMapErr constraint: R extends Query<any, any> — callback must return a Query.
5. Type guards: isSome() → .value, isErr() → .err. OkNone has neither.
6. OkSome stores .value. ErrNone stores .err. OkNone has no data properties.
7. okNone() returns a frozen singleton.

Anti-Patterns

// WRONG — match with 2 callbacks (that's Result/Option, not Query)
query.match(onSome, onErr);

// CORRECT — Query.match always takes 3
query.match(
  (value) => handleValue(value),
  () => handleEmpty(),
  (error) => handleError(error),
);

// WRONG — filter with errorFactory (that's Result, not Query)
query.filter((v) => v > 0, () => new Error("bad"));

// CORRECT — Query.filter only takes predicate, returns OkNone on failure
query.filter((v) => v > 0);

Namespace & Async Methods

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