playground.rs

macro_rules! pipe {
    (($isn:ident | $($rest:ident)|+): $($args:tt)*) => ($isn!(($($rest)|+): $($args)*));
    (($isn:ident): $($args:tt)*) => ($isn!((): $($args)*));
    ((): $result:expr, $($rest:tt)*) => ($result);
}

macro_rules! succ {
    ($isns:tt: [$($n:expr),*], $e:expr, $($rest:tt)*) => (pipe!($isns: [$($n,)* $e], $e, $($rest)*));
}

macro_rules! double {
    ((double $(| $isns:ident)*): [$($n:expr),*], $($rest:tt)*) => (pipe!(($($isns)|*): [$($n),* $(,$n)* $(,$n)* $(,$n)*], $($rest)*));
    ($isns:tt: [$($n:expr),*], $($rest:tt)*) => (pipe!($isns: [$($n),* $(,$n)*], $($rest)*));
}

macro_rules! zero {
    ($isns:tt: $e:expr, $($rest:tt)*) => (pipe!($isns: [], $e, $($rest)*));
}

macro_rules! one {
    ($isns:tt: $e:expr, $($rest:tt)*) => (pipe!($isns: [$e], $e, $($rest)*));
}

macro_rules! four {
    (($($isns:ident)|*): $($args:tt)*) => (one!((double | double $(| $isns)*): $($args)*));
}

macro_rules! repeat {
    [$e:expr; $n:ident] => ($n!((): $e,))
}

fn main() {
    println!("{:?}", repeat![Box::new(5); four]);
}