Macro Fragment: keyword (kw)

language design
1

pretty self explanatory, a macro fragment that takes any keyword, this wouldn't always be useful but here's one example

Examples

macro_rules! declare_x {
    ($decl_kw:kw = $v:expr) => {
        $decl_kw x = $v;
    };
}

this is one example, here's another

// Using the `macro` keyword
macro declare_fn($($keywords:kw)*) {
    $($keywords)* fn x() {}
}

Unanswered Questions

  • would this be useful in libraries, i feel its only useful for those one time macros
macro_rules! impl_marker {
    ($($safety:kw)? $Trait:ident for $($ty:ty)*) => {$(
        // we are using a variable(?) here, so a user can EASILY misuse this
        $($safety:kw)? impl $Trait for $ty {}
    )*};
}
  • what would happen with auto, safe and default? (contextual keywords)
// #![feature(specialization)]
// same macro
macro_rules! impl_marker {
    ($($safety:kw)? $Trait:ident for $($ty:ty)*) => {$(
        // we are using a variable(?) here, so a user can EASILY misuse this
        $($safety:kw)? impl $Trait for $ty {}
    )*};
}
trait MyTrait {}
impl_marker!(default MyTrait for i32);
// now what, is it ok or no?
2

You can pass keywords as ident; does that meet your need?

I don't think that can work since fn is a keyword, so the repeat doesn't know whether to end.

2 Likes
3

It's not really something for my needs, it just- something

4

The problem with this construct is that matching on any and every keyword is much less useful than matching on a set of specific keywords

That's because on every usage there will be some keywords that don't make sense in that position, but is matched anyway by the macro

2 Likes
5

For future compatibility, it seems safer to match the potential keyword as ident, and then document what you expect people to pass there. If you only want to accept a specific subset of keywords, you might want to match those keywords specifically. (I wish we had some kind of simple alternation in macro matchers.)

10 Likes
6

well if you really need that much power over your macros, use proc-macros