Macro with a # pattern

1

Hello, I don't manage to setup a macro mattern for understand CSS selectors : div#foo (where # is hardcoded and div and foo can change).

#[macro_export]
macro_rules! css_selector {
    ($tag_name: tt) => {
        // TODO
    };
    ([$tag_name:tt]#[$id:tt]) => {
        //TODO
    };
}

fn main() {
    // OK
    css_selector!(div);
    // KO
    css_selector!(div#foo);
}

Please advise.
Here is the playground

Moreover, I woud be interested in having a pattern for div.class (where . is hardcoded and div and class can change).

2 
#[macro_export]
macro_rules! css_selector {
    ($tag_name: tt) => {
        println!("plain");
    };
    
    ($tag_name:tt # $id:tt) => {
        println!("#");
    };
    
    ($tag_name:tt . $id:tt) => {
        println!(".");
    };
}

fn main() {
    css_selector!(div);
    css_selector!(div#foo);
    css_selector!(div.foo);
}

Not sure why you added the brackets ([ ]) in your macro. IT works just fine without.

1 Like
3

Your second rule expects the identifiers to be surrounded by square brackets, but there's no square brackets when you call it. Removing them makes the code compile. Also note that r#ident (the first part must be r) is parsed as an identifier, so it's a single token, and that in the 2021 edition this will be the case for any sequence prefix#identifier

2 Likes
4

Thank you both ! This is clearer now how macros are working.

I though :bulb: we should add this forum as a cargo component. I always get clear and fast (less than an hour) answer from the community !

2 Likes
5

An other request. What is the macro pattern that will match div[class$="bar"].

I tries this ($tag_name:tt [$attribute_name:tt $= $attribute_value:literal ]) but this does not work as $ can not be matched.

See Macros By Example - The Rust Reference

The character $ cannot be matched or transcribed literally

6

You'll need to use :tt for it (thus making sure it is the last rule among "ambiguous" ones), and emit the validation against $ later on:

macro_rules! assert_is_dollar {
//    works because there is nothing after the dollar.
//    v
    ( $ ) => ();
}

macro_rules! css_selector {
    ( $tag_name:ident ) => (
        println!("plain");
    );

    ( $tag_name:ident # $id:ident ) => (
        println!("#");
    );

    ( $tag_name:ident . $id:ident ) => (
        println!(".");
    );

    (
        $tag_name:ident
        [ $attribute_name:ident $dol:tt = $attribute_value:literal ]
    ) => (
        assert_is_dollar!($dol);
        println!("$=");
    );
}
7

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