# PathAst.Parameter

A named path parameter with optionality and an optional regular expression.

## Signatures

```ts
type Parameter = {
    type: "parameter";
    name: string;
    optional?: boolean;
    regex?: string;
};
```

## Why

Parameter constraints stay available to both matcher registration and schema derivation.

## Ownership and lifetime

`Parameter` describes plain readonly AST data. Routes and Matchers retain the concrete nodes they receive by reference; the type itself owns no resource.

## Property: name

Name assigned to the decoded parameter field.

## Property: name: Why

Duplicate decoded names can be rejected before matcher execution.

## Property: name: Ownership and lifetime

The node retains the immutable name; decoded values are owned by the selected route's params
RefSubject, not by this AST.

## Property: optional

Whether this segment may be absent.

## Property: optional: Why

Optionality controls both matcher syntax and optional schema fields.

## Property: optional: Ownership and lifetime

The flag is immutable metadata retained with the node and creates no fallback value itself.

## Property: regex

Optional source pattern constraining the captured segment.

## Property: regex: Why

Constrained and generic parameters can compile to distinct matcher path shapes.

## Property: regex: Ownership and lifetime

The node retains the pattern string. Matcher compilation interprets it later and can surface
invalid registration syntax as `RouteDecodeError`.

## Property: type

Discriminates a named parameter node.

## Property: type: Why

Schema derivation and matcher compilation must recognize captured segments identically.

## Property: type: Ownership and lifetime

The literal tag remains on the plain node for its lifetime and owns no resource.