Macro do_ 

macro_rules! do_ {
    ($F:ty; $e:expr) => { ... };
    ($F:ty; $x:ident = $e:expr; $($rest:tt)+) => { ... };
}

Monadic do-notation sugar.

Desugars sequential x = expr bindings into nested Chain::chain calls.

Example

do_! { OptionF;
    x = Some(1);
    y = Some(x + 1);
    OptionF::pure(x + y)
}