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# andThen

Pass an `Ok` value to another function returning a `Result`. An `Err` passes through unchanged, without calling the function. Inspired by Rust's `Result::and_then`, exposed as a standalone function rather than a method.

## Usage

### `andThen(result, next)`

```typescript
import { andThen, err, ok, type Result } from 'massaman/control'

const parseCount = (raw: string): Result<number, TypeError> => {
  const count = Number(raw)
  if (Number.isNaN(count)) {
    return err(new TypeError('Expected a number'))
  }
  return ok(count)
}

const requirePositive = (count: number): Result<number, RangeError> => {
  if (count <= 0) {
    return err(new RangeError('Expected a positive count'))
  }
  return ok(count)
}

const count = andThen(parseCount('42'), requirePositive)
// Result<number, TypeError | RangeError>; Ok(42)

andThen(parseCount('invalid'), requirePositive)
// Original parse failure; requirePositive is not called.
```

#### Parameters


resultResult<T, E>required

The result to continue. Plain structurally compatible Results work too.


next(value: T) => Result<U, F>required

Called once with the success value, or not at all on failure. Must return a synchronous Result.

#### Returns

`Result<U, E | F>`. Returns the callback's Result or the original `Err` by reference. Unlike Rust's method, the input and callback may have different error types; their union is preserved.

#### Exceptions and async work

Unexpected callback exceptions propagate; `andThen` does not catch or normalize them. Use `attempt` at throwing boundaries. Async callbacks are not accepted: await `attemptAsync` first, then use `andThen` for synchronous continuation. For another async step, use an explicit `isErr` guard and await the operation.

## See also

- [`orElse`](https://massaman.dev/reference/control/orElse.md) — continue on failure instead
- [`attempt`](https://massaman.dev/reference/control/attempt.md) / [`attemptAsync`](https://massaman.dev/reference/control/attemptAsync.md) — capture unsafe work
- [Result & Errors](https://massaman.dev/concepts/result.md)