list_builder::gen!

macro_rules! gen {
    [$e:expr => $variable:ident in $iterable:expr] => { ... };
    [$e:expr => $variable:ident in $iterable:expr, $condition:expr] => { ... };
}

Generate a new iterable witn a list comprehension. This macro tries to follow the syntax of Python's list comprehensions. This is a very flexable macro that allows the generation of any iterable that implements std::iter::FromIterator. The resulting type will be determined by the type of the variable that you are attempting to assign to. You can create a Vec:

let x: Vec<i32> = gen![i*30 => i in [1, 2, 3, 4, 5]];

You can generate a HashSet:

let x: HashSet<i32> = gen![i*30 => i in [1, 2, 3, 4, 5]];

You can even use conditionals to generate stuff:

let x: HashSet<i32> = gen![i => i in [1, 2, 3, 4, 5], x % 2 == 0];
assert_eq!(x, vec![2, 4]);

Comparisson to Python's list comprehension

Python

x = [i*4 for i in range(1, 5)]

Rust with gen! macro

let x: Vec<i32> = gen!(x*4 => x in [1, 2, 3, 4]);