# Parser.Optional

Returns `undefined` and the original input when its parser fails.

## Signatures

```ts
interface Optional<P extends Any> extends Parser<unknown> {
    readonly return: InputOf<this> extends infer Input extends string ? Apply<P, Input> extends infer R ? [
        R
    ] extends [
        never
    ] ? readonly [
        undefined,
        Input
    ] : R extends readonly [
        infer Value,
        infer Rest extends string
    ] ? readonly [
        Value,
        Rest
    ] : never : never : never;
}
```

## Why

Optional grammar does not consume input on the absent branch.

## Ownership and lifetime

TypeScript instantiates `Optional` while parsing a string-literal input. Its present or absent
result and unchanged failure remainder exist only in the resulting type.

## Property: return

Computes this type lambda's parse result for its current input.

## Property: return: Why

The local type-function protocol evaluates each parser combinator through this member without a parallel runtime object.

## Property: return: Ownership and lifetime

TypeScript evaluates `Optional.return` when the surrounding parser is applied; the associated result is erased and retains no input at runtime.