Macro req 

macro_rules! req {
    (@tail $k:ty : $v:ty) => { ... };
    (@tail $k:ty : $v:ty | $($rest:tt)+) => { ... };
    () => { ... };
    ($($pairs:tt)+) => { ... };
}

Type-level required service stack (Rust equivalent of R union requirements).

type R = effectful::req!(DbKey: DbClient | LogKey: Logger);