Simple Scala-like for comprehension for rust

comp.rs

// Simple Scala-like for comprehension for rust
macro_rules! comp(
	// base case, using "map"
	(val $id:ident <- $expr:expr $(,if $cond:expr)* $(,let $assign_id:pat = $assign_val:expr)* ,yield $comp:expr) =>
	(
		($expr)$(.filtered(|&$id| $cond))*.map(|&$id| {
			$(let $assign_id = $assign_val;)* $comp
			}
		)
	);
	// recurisve case, using "flat_map"
	(val $id:ident <- $expr:expr $(,if $cond:expr)* $(,let $assign_id:pat = $assign_val:expr)*
	$(, val $id_r:ident <- $expr_r:expr $(,if $cond_r:expr)* $(,let $assign_id_r:pat = $assign_val_r:expr)*)* ,yield $comp:expr) => 
	(
		($expr)$(.filtered(|&$id| $cond))*.flat_map(|&$id| {
			$(let $assign_id = $assign_val;)*
			comp!($(val $id_r <- $expr_r $(,if $cond_r)* $(,let $assign_id_r = $assign_val_r)*),* ,yield  $comp)
		})
	)
)

fn main() {

	struct Number {
	    value : int
	}

	let result = comp!(
			val x <- [Number{value: 1}, Number{value: 2}, Number{value: 3}],
			let Number{value:y} = x,
			val z <- [6,7,8],
			if z % 2 == 0,
			yield (y,z)
		);
	println(result.to_str());
	// prints [(1, 6), (1, 8), (2, 6), (2, 8), (3, 6), (3, 8)]

	// Notes:
	// syntax: (val <var> <- generator (,if cond)* (,let <pattern> = value)*)+
	// Patterns are NOT supported in val * <- Generator
}