auroranssolis / auroranssolis.github.io Goto Github PK

Was rolling the blog post around in my brain when I decided to take a closer look at equality. Turns out, you can redefine a macro as many times as you want (which is useful for staying strictly within macro_rules! and not needing gensym or paste), and the ability to define macros within macro_rules! let's us turn an equality check where both sides are metavariables into one where only one side is a metavariable.

an example for specifically identifiers:

macro_rules! is_equal {
    ($a:ident == $b:ident ? { $($eq:tt)* } : { $($neq:tt)* }) => {
        macro_rules! __internal_test {
            ($a) => { $($eq)* };
            ($unk:ident) => { $($neq)* };
        }
        __internal_test!($b);
    }
}

is_equal!(a == a ? {} : { compile_error!("a should be equal to a"); });
is_equal!(a == b ? { compile_error!("a should not be equal to b"); } : {});

this builds without any errors or warnings. my apologies for the use of the ternary operator. playground link

I'm contemplating playing around with this a bit more. would a modifiable/queryable map structure be possible like this using macro rules?