# Pattern Matching

The pattern matching module provides exhaustive branching helpers for unions and tagged unions.

## Exports

* `exhaustive`: exhaustive matching over literal unions and tagged unions.
* `exhaustive.tag`: exhaustive matching using an explicit tag key.
* `corrupt`: helper for unreachable/default branches in `switch` statements.

## exhaustive

Use `exhaustive` when all variants must be handled.

```typescript
import { exhaustive } from 'funkcia/pattern-matching';

type State = 'IDLE' | 'LOADING' | 'SUCCESS' | 'ERROR';

function message(state: State) {
  return exhaustive(state, {
    IDLE: () => 'Idle',
    LOADING: () => 'Loading',
    SUCCESS: () => 'Success',
    ERROR: () => 'Error',
  });
}
```

### Tagged unions with `_tag`

When the union has a `_tag` field, `exhaustive` matches by `_tag` automatically.

```typescript
import { TaggedError } from 'funkcia/exceptions';
import { exhaustive } from 'funkcia/pattern-matching';

class UserNotFoundError extends TaggedError('UserNotFoundError') {
  readonly userId: string;

  constructor(userId: string) {
    super(`User ${userId} was not found`);
    this.userId = userId;
  }
}

class InvalidCouponError extends TaggedError('InvalidCouponError') {
  readonly code: string;

  constructor(code: string) {
    super(`Coupon ${code} is invalid`);
    this.code = code;
  }
}

type CheckoutError = UserNotFoundError | InvalidCouponError;

function toStatus(error: CheckoutError) {
  return exhaustive(error, {
    UserNotFoundError: () => 404,
    InvalidCouponError: () => 422,
  });
}
```

### Fallback case

You can provide `_` as a runtime fallback.

```typescript
import { exhaustive } from 'funkcia/pattern-matching';

type Mode = 'ON' | 'OFF';

function toBool(mode: Mode) {
  return exhaustive(mode, {
    ON: () => true,
    OFF: () => false,
    _: () => false,
  });
}
```

## exhaustive.tag

Use `exhaustive.tag` when the discriminant key is not `_tag`.

```typescript
import { exhaustive } from 'funkcia/pattern-matching';

type Shape =
  | { kind: 'circle'; radius: number }
  | { kind: 'square'; size: number };

function area(shape: Shape) {
  return exhaustive.tag(shape, 'kind', {
    circle: (value) => Math.PI * value.radius ** 2,
    square: (value) => value.size ** 2,
  });
}
```

## corrupt

Use `corrupt` in `switch` defaults to guard impossible branches.

```typescript
import { corrupt } from 'funkcia/pattern-matching';

type Role = 'ADMIN' | 'VIEWER';

function canWrite(role: Role) {
  switch (role) {
    case 'ADMIN':
      return true;
    case 'VIEWER':
      return false;
    default:
      return corrupt(role);
  }
}
```


---

# Agent Instructions: Querying This Documentation

If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter:

```
GET https://funkcia.lukemorales.io/modules/pattern-matching.md?ask=<question>
```

The question should be specific, self-contained, and written in natural language.
The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.