How to create a <tr><td> macro?

November 16, 2022

I have lots of varyingly wide sets of data, I need to insert into tables. Yew is overkill, just looking for kiss. As in

//format!("<tr><td>{e1}</td><td>{e2}</td></tr>") – no good, might be exprs
format!("<tr><td>{}</td><td>{}</td></tr>", e1, e2)

I want to simplify this for any number of columns

macro_rules! tr {
    ($($td:expr),*) => {
        format!(concat!("<tr>", $("<td>{}</td>"),* "</tr>"), $($td),*)
    }
}
tr!(e1, e2)

gives error: attempted to repeat an expression containing no syntax variables matched as repeating at this depth

So I tried an artificial code block, just to give it the variable in the 1st repeat

macro_rules! tr {
    ($($td:expr),*) => {
        format!(concat!("<tr>", $({ $td; "<td>{}</td>" }),* "</tr>"), $($td),*)
    }
}

gives error: expected token: ,

What would the right syntax be?

>Solution :

When you need to repeat some expression the same times as a metavariable but without including it in the expression, there is a trick: define an internal macro arm/another internal macro that takes the metavariable and produces the constant expression. This way, you can "use" the metavariable in the reptition without actually including it:

macro_rules! tr {
    (@trick $td:expr) => { "<td>{}</td>" };
    ($($td:expr),*) => {
        format!(concat!("<tr>", $(tr!(@trick $td),)* "</tr>"), $($td),*)
    }
}

On nightly, there is an easier way: using the ${ignore(...)} modifier that is meant exactly for that:

#![feature(macro_metavar_expr)]

macro_rules! tr {
    ($($td:expr),*) => {
        format!(concat!("<tr>", $("<td>{}</td>", ${ignore(td)})* "</tr>"), $($td),*)
    }
}