Function optional 

pub fn optional<T>() -> Optional<T>
where
    T: DeserializeOwned + 'static,

Create an endpoint which parses the query string to the specified type.

This endpoint always matches and returns a None if the query string is missing.

Example

#[derive(Debug, Deserialize)]
pub struct Param {
    query: String,
    count: Option<u32>,
}

let endpoint = query::optional()
    .map(|param: Option<Param>| {
        format!("Received: {:?}", param)
    });