Macro monadic::wrdo

macro_rules! wrdo {
    (pure $e:expr                           ) => { ... };
    (let $v:ident = $e:expr ; $($rest:tt)*) => { ... };
    (_ <- $monad:expr ; $($rest:tt)* ) => { ... };
    ($v:ident <- pure $e:expr ; $($rest:tt)* ) => { ... };
    ($v:ident <- $monad:expr ; $($rest:tt)* ) => { ... };
    ($monad:expr                            ) => { ... };
}

macro for a Writer<A, W = String> monad that holds a pair (A, Monoid)

The logger type (a local Monoid instance) can be established by using a tell_...() generator or by constraining the type of the macro result.