can I expand $($(rest),)*into an array of labels? | Rust Programming Language Community | Page 1

slim lintel Sep 9, 2022, 7:35 AM

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Using macro expansion, I can expand the captured elements into an array:

let array = &[ $($rest),* ]

However, can I expand the captured elements' labels into an array? I know stringify!($foo) allowes me to get the label of $foo, what about an array of all of the labels?

Something like:

let array = &[ $( $stringify!(rest) ),* ]

minor bobcat Sep 9, 2022, 7:39 AM

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slim lintel Using macro expansion, I can expand the captured elements into an array: ```rus...

You misplaced the inner $. It should be stringify!($rest).
Otherwise that should work

slim lintel Sep 9, 2022, 7:40 AM

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oh I get

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error: variable 'rest' is still repeating at this depth

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so maybe that's because of how I am capturing it

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macro_rules! print_log {
    ($label:expr $(, $($rest:expr),* )? ) => {
        ...
    }
}

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maybe the $(, is getting in the way?

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I need to include a separator because Rust won't allow two expr to follow each other

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but I need to capture the first separately

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full macro:

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#[macro_export]
macro_rules! print_log {
    ($label:expr $(, $($rest:expr),* )? ) => {
        ::log::info!(
            "({}) {}",
            $label,
            logify(&[ stringify!($rest)* ], &[ ($($rest),*) ])
        );
    };
}```

minor bobcat Sep 9, 2022, 7:43 AM

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slim lintel ```rust #[macro_export] macro_rules! print_log { ($label:expr $(, $($rest:ex...

You forgot to repeat the first rest when you stringify

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logify(&[$(stringify!($rest)),*], ...)

slim lintel Sep 9, 2022, 7:46 AM

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nope still getting error: variable 'rest' is still repeating at this depth

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with this

#[macro_export]
macro_rules! print_log {
    ($label:expr $(, $($rest:expr),* )? ) => {
        ::log::info!(
            "({}) {}",
            $label,
            logify(&[ $(stringify!($rest)),* ], &[ $( $rest ),* ])
        );
    };
}

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I can post the full example but it depends on the logtest create to test the log output, don't know if it will be able to eval

minor bobcat Sep 9, 2022, 7:48 AM

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Wait, i just noticed that you have a nested repetition

slim lintel Sep 9, 2022, 7:48 AM

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you mean I am expanding twice?

minor bobcat Sep 9, 2022, 7:48 AM

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Or rather it's nested in a $( ... )?

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So that has to be part of your expansion

slim lintel Sep 9, 2022, 7:49 AM

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I can't see it

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the two arrays are consecutive, not nested

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should expand to logify(&[...], &[...])

minor bobcat Sep 9, 2022, 7:50 AM

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You have

$(
   $(
      $rest: expr
    ),*
)?

slim lintel Sep 9, 2022, 7:50 AM

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oh in the capture expression yeah

minor bobcat Sep 9, 2022, 7:50 AM

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So you expansion should reflect that

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But I don't think you need that additional wrapping

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The 0 case is already handled by *

slim lintel Sep 9, 2022, 7:51 AM

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right I am not sure that is the best way to include the comma at the start, e.g. why can't I remove it at the end and get rid of that nesting level
tried it but didn't work

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ok let me see

minor bobcat Sep 9, 2022, 7:52 AM

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You can allow an optional comma with $(,)?

slim lintel Sep 9, 2022, 7:52 AM

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that is not accepted

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because it's two expr

minor bobcat Sep 9, 2022, 7:53 AM

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Right, so it would probably be mandatory, hmm

slim lintel Sep 9, 2022, 7:54 AM

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I actually need a string literal in the fist and variables on the rest, so maybe I can write it like that in the capture and it will stop complainng, but I don't thing we have that level of sensibility over expressions

minor bobcat Sep 9, 2022, 7:54 AM

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I'm not the best at macros, consider reposting in #macros if no one else chips in here

slim lintel Sep 9, 2022, 7:54 AM

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I don't get why $(,($rest:expr))* doesn't work

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print_log!("hello", i); returns "error: no rules expected the token i"

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wait that would still be nested tho

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this

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($label:expr ,($rest:expr)* )

minor bobcat Sep 9, 2022, 7:57 AM

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Maybe your initial matcher was correct

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Try expanding it correctly, with the added $(...)? Around

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It would just make an empty array for the second parameter of logify which sounds like what you'd want maybe

slim lintel Sep 9, 2022, 7:58 AM

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($label:expr, ($rest:expr),* ) this should work?

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implying I will always have some rest parameters

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but it still errors out the same way as above

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oh Im missing a &

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hey that works!

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it's fine too I can get the $label only behaviour in a separate

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matcher clause

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thanks @minor bobcat

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last thing I'm struggling with is avoid polluting the public namespace with the logify() function that is only to be used inside this macro

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I can make it public and then reference it via $crate::utils::lang::logify inside the macro, but this will make it available for autocompletion everytime I type log which is annoying

minor bobcat Sep 9, 2022, 8:08 AM

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mhh, that's a good question. I think there may be visibility tricks for this but I'm not sure

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I'd suggest #macros again ;)

slim lintel Sep 9, 2022, 8:08 AM

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maybe prepending it with __ is good enough

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https://stackoverflow.com/questions/35536030/is-there-any-way-to-use-private-functions-in-public-macros-in-rust

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https://stackoverflow.com/questions/46654790/how-can-i-use-a-non-public-function-from-within-a-macro

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#can I expand $($(rest),)*into an array of labels?