list_comprehensions.rs

macro_rules! make_ident_tup {
    ($i:ident) => {$i};
    ($i:ident, $($irest:ident),+) => {($i, make_ident_tup!($($irest),+))}
}

macro_rules! cross_join {
   ( $lhs:expr ) => { $lhs };
   ( $lhs:expr, $($rhss:expr),+) => {($lhs).flat_map(
       |x| (cross_join! ($($rhss),+)).map( move |y| (x, y)))}
}

macro_rules! compre {
    ( $ex:expr;$($ids:ident),+ <- [$($start_ends:expr),+]) => {
        cross_join! ($($start_ends),+).map(|make_ident_tup!($($ids),+)|{ $ex })
    };
    ( $ex:expr; $($ids:ident),+ <- [$($start_ends:expr),+]; $filter_ex:expr ) => {
        cross_join! ($($start_ends),+).filter_map(|make_ident_tup!($($ids),+)|{
            if $filter_ex { Some($ex) } else { None } })
    };
    ($key:expr => $ex:expr;$($ids:ident),+ <- [$($start_ends:expr),+]) => {
        cross_join! ($($start_ends),+).map(|make_ident_tup!($($ids),+)|{ ($key, $ex) })
    };
    ($key:expr => $ex:expr; $($ids:ident),+ <- [$($start_ends:expr),+]; $filter_ex:expr ) => {
        cross_join! ($($start_ends),+).filter_map(|make_ident_tup!($($ids),+)|{
            if $filter_ex { Some(($key, $ex)) } else { None } })
    };
}

let simple = compre![x * 2; x <- [1..10]];
let in_circle = compre![
    (x, y);
    x, y <- [-10..=10, -10..=10];
    x*x + y*y <= 64
];
let triples = compre![
    (x, y, z);
    x, y, z <- [-30..=30, -30..=30, -30..=30];
    x*x + y*y == z*z
];

let infinite_quatros = compre! {
    (x, y, z, w);
    x, y, z, w <- [0.., 0.., 0.., 0..];
    x*x + y*y + z*z == w*w
};