---
name: native-monad-query
description: "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

```typescript
// 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),
);
```

```typescript
// 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.