Macro variant 

macro_rules! variant {
    ($pattern:pat = $input:expr => $output:expr) => { ... };
}

Maps an enum to an option for use with Iterator::filter_map

Syntax

variant!( pattern = input => output )

Example

use kai::*;

enum Foo {
    Bar(bool),
    Baz(i32),
}

let bar = Foo::Bar(true);
let baz = Foo::Baz(5);

assert_eq!(None,    variant!(Foo::Baz(i) = bar => i));
assert_eq!(Some(5), variant!(Foo::Baz(i) = baz => i));

let foos = vec![
    Foo::Baz(5),
    Foo::Bar(false),
    Foo::Baz(2),
];

// Turns this
let ints: Vec<i32> = foos
    .iter()
    .filter_map(|foo| if let Foo::Baz(i) = foo {
        Some(i)
    } else {
        None
    })
    .cloned()
    .collect();

// Into this
let ints: Vec<i32> = foos
    .iter()
    .filter_map(|foo| variant!(Foo::Baz(i) = foo => i))
    .cloned()
    .collect();

assert_eq!(vec![5, 2], ints);