No way to have + or * as separator token in macros

[bouk]

I'm trying to write a macro that sums a list of expressions, but it doesn't work because it's not possible to have + or * as the seperator token. For example:

macro_rules! sum {
    (
        $($e:expr)+ => (
        $($e)++ // ???
    )
}

It'll interpret is as there being no seperator token, and it seems to just stick a + at the end. Escaping the first + doesn't work either.

I'm using rust head

@bstrie

[steveklabnik]

Triage: no change. Not sure this is going to be implemented with the current macro system.

[Gankra]

@nrc will macros 3.0 or whatever potentially enable this?

[nrc]

It's not part of my plans, but it seems like something we could address separately (once the new system is in place I mean, not in the old system).

[steveklabnik]

Triage: no change

[AlphaModder]

There's a workaround for this, just use the following utility macro:

macro_rules! strip_plus {
    {+ $($rest:tt)* } => { $($rest)* }
}

Then, in your main macro instead of writing $($e)++, write strip_plus!($(+ $e)+). Obviously a slight modification makes this work for * too.

[WaffleLapkin]

There is a similar problem with separators in patterns:

macro_rules! example {
    ( $( $i:ident )+* ) => {}
}

This is allowed, however it works as 'at least one ident and a * token at the end', and not as '0 or more ident separated by + token'.

There is also a workaround:

macro_rules! workaround {
    ( $( $first:ident $( + $rest:ident )* )? ) => {}
}

But it makes the macro a lot less readable (and forces you to repeat the pattern twice).

Are there any plans to do something with this issue? (maybe reopen rust-lang/rfcs#944 ?)