active_match.rs

#![feature(macro_rules)]

macro_rules! active_match {
    //Single zero parameter case
    ($m:expr, $i:ident() => $b:block) => {
        match $i($m) {
            Some(()) => $b,
            _ => panic!("Encountered an unmatched active case.")
        }
    };
    
    //Single one parameter case
    ($m:expr, $i:ident($p:pat) => $b:block) => {
        match $i($m) {
            Some($p) => $b,
            _ => panic!("Encountered an unmatched active case.")
        }
    };
    
    //Single multiple parameter case
    ($m:expr, $i:ident($($ps:pat),*) => $b:block) => {
        match $i($m) {
            Some(($($ps),*)) => $b,
            _ => panic!("Encountered an unmatched active case.")
        }
    };

    //Zero parameter case followed by one or more cases
    ($m:expr, $i:ident() => $b:block $(, $i_rest:ident($($ps_rest:pat),*) => $b_rest:block)+) => {
        match $i($m) {
            Some(()) => $b,
            _ => active_match! { $m, $($i_rest($($ps_rest),*) => $b_rest),+ }
        }
    };
    
    //One parameter case followed by one or more cases
    ($m:expr, $i:ident($p:pat) => $b:block $(, $i_rest:ident($($ps_rest:pat),*) => $b_rest:block)+) => {
        match $i($m) {
            Some($p) => $b,
            _ => active_match! { $m, $($i_rest($($ps_rest),*) => $b_rest),+ }
        }
    };
    
    //Multiple parameter case followed by one or more cases
    ($m:expr, $i:ident($($ps:pat),*) => $b:block $(, $i_rest:ident($($ps_rest:pat),*) => $b_rest:block)+) => {
        match $i($m) {
            Some(($($ps),*)) => $b,
            _ => active_match! { $m, $($i_rest($($ps_rest),*) => $b_rest),+ }
        }
    };
}

fn is_even(n: i32) -> Option<()> {
    if n % 2 == 0 { Some(()) } else { None }
}

fn is_odd(n: i32) -> Option<()> {
    if n % 2 == 1 { Some(()) } else { None }
}

fn double_up(n:i32) -> Option<i32> {
    Some(n*2)
}

fn double_and_triple_if_even(n: i32) -> Option<(i32, i32)> {
    if n % 2 == 0 {
        Some((n*2, n*3))
    } else {
        None
    }
}

//Fake wildcard
fn otherwise<T>(_: T) -> Option<()> {
    Some(())
}

fn main() {
    println!("Testing two cases with no parameters.");

    for i in range(1,10) {
        active_match! { i,
            is_even() => { println!("{} is an even number.", i) },
            is_odd() => { println!("{} is an odd number.", i) }
        }
    }
    
    println!("----");
    
    println!("Testing one case with no parameters and a wildcard.");

    for i in range(1,10) {
        active_match! { i,
            is_even() => { println!("{} is an even number.", i) },
            otherwise() => { println!("{} is an odd number.", i) }
        }
    }
    
    println!("----");
    
    println!("Testing one case with one parameter.");

    for i in range(1,10) {
        active_match! { i,
            double_up(n) => { println!("{} doubled is {}.", i, n) }
        }
    }
    
    println!("----");
    
    println!("Testing one case with two parameters and a wildcard.");

    for i in range(1,10) {
        active_match! { i,
            double_and_triple_if_even(n2, n3) => { println!("{} doubled is {}, and tripled is {}.", i, n2, n3) },
            otherwise() => { println!("{} is odd.", i); }
        }
    }
    
    println!("----");
    
    println!("Testing one case with two parameters, one case with a pattern and a parameter, and a wildcard.");

    for i in range(1,10) {
        active_match! { i,
            double_and_triple_if_even(4, n3) => { println!("Two doubled is 4, and tripled is {}.", n3) },
            double_and_triple_if_even(n2, n3) => { println!("{} doubled is {}, and tripled is {}.", i, n2, n3) },
            otherwise() => { println!("{} is odd.", i); }
        }
    }
    
    println!("----");
}