Macro rpc_params 

macro_rules! rpc_params {
    ($params:expr, $id:expr => $type:ty) => { ... };
    ($params:expr, $id:expr => Option<$type:ty>) => { ... };
}

Validate and extract parameters with automatic error responses

Usage:

rpc_method!(AddMethod, "add", |params, id| {
    let numbers = rpc_params!(params, id => Vec<i32>);
    rpc_success!(numbers.iter().sum::<i32>(), id)
});