///|
/// Make a field optional: absent keys pass; explicit null is validated normally.
/// Rules chained after `.optional()` are pushed through to the inner schema
/// via `append_rule`, so `string().optional().min(3)` works correctly.
pub fn Schema::optional(self : Schema) -> Schema {
  {
    id: new_schema_id(),
    schema_type: OptionalType(self),
    rules: [],
    description: self.description,
    required_error: self.required_error,
    invalid_type_error: self.invalid_type_error,
    name: self.name,
    brand: self.brand,
  }
}

///|
pub fn Schema::parse_optional(
  self : Schema,
  inner : Schema,
  json : Json,
  path_stack : Array[String],
) -> RawSchemaResult {
  parse_inner(
    { ..self, id: new_schema_id(), schema_type: OptionalType(inner), },
    json,
    path_stack,
  )
}