Macro Parameter 

macro_rules! Parameter {
    (
        r#type: $type:expr
        $(,)?
    ) => { ... };
    (
        description: $description:expr,
        r#type: $type:expr
        $(,)?
    ) => { ... };
    (
        description: $description:expr,
        r#type: $type:expr,
        allowed_pattern: $allowed_pattern:expr
        $(,)?
    ) => { ... };
}