Basic Backus-Naur form grammar macro

bnf.rs

macro_rules! bnf {
    
    (<$id:ident> ::= <$p1:ident> | <$p2:ident>; $($t:tt)*) => {
        fn $id(i: &'static str) -> (&str, &str) {
            let p1 = ::std::thread::spawn(move || $p1(i));
            if p1.join().is_ok() {
                $p1(i)
            } else {
                $p2(i)
            }
        }
        
        bnf!($($t)*);
    };
    
    (<$id:ident> ::= <$p1:ident> <$p2:ident>; $($t:tt)*) => {
        fn $id(i: &str) -> (&str, &str) {
            let (res1, j) = $p1(i);
            let (res2, _) = $p2(j);
            i.split_at(res1.len() + res2.len())
        }
        
        bnf!($($t)*);
    };
    
    (<$id:ident> ::= $e:expr; $($t:tt)*) => {
        fn $id(i: &str) -> (&str, &str) {
            let e = $e;
            assert!(i.starts_with(e));
            i.split_at(e.len())
        }
        
        bnf!($($t)*);
    };
    
    () => ()
}

bnf! {
    <hello> ::= "hello";
    <hi> ::= "hi";
    <hihello> ::= <hi> <hello>;
    <hi_hello> ::= <hi> | <hello>;
}