---
name: native-monad-result
description: "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

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

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

```typescript
// 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.