Utility for conditionally joining classNames together

[dashed]

I had a need for something like https://github.com/JedWatson/classnames

I came up with the following macro:

macro_rules! classnames {

    // base cases

    ($name: expr) => (
        format!("{}", $name)
    );

    ($name:expr => $should_include:expr) => (
        if $should_include {
            format!("{}", $name)
        } else {
            format!("")
        }
    );

    // expansions

    ($name:expr, $($tail:tt)+) => {
        format!("{} {}", $name, classnames!($($tail)*))

    };

    ($name:expr => $should_include:expr, $($tail:tt)+) => {
        if $should_include {
            format!("{} {}", $name, classnames!($($tail)*))
        } else {
            classnames!($($tail)*)
        };
    };

}

// usage:
html!{

    div(class = classnames!("active" => true, "button-style")) {
        : "this is a button"
    }

    div(class = classnames!("active" => false, "button-style")) {
        : "this is a button"
    }
}

rust playground with examples: https://is.gd/e6Jahb

I just learned rust macros to make this; it can probably be refactored into something more nicer.

---

I'm wondering if this utility macro would be something worth adding to horrorshow. I can try to PR.

[Stebalien]

That definitely sounds useful.

Personally, I would call it something like labels! or items! because it can be used for more than class names. I'd also use a guard syntax (labels!("a", "b" if false, "c")) as it makes it clear that the booleans are conditionals. Finally, the recursive calls to format! will have very poor performance. Instead, consider implementing the Render traits for some helper struct (you can even make this generic over items implementing Render); this will allow you to write directly to the template buffer instead of allocating intermediate strings.

At the end of the day, it should be possible to write:

    div(class = labels!("active" if false, "button-style", format_args!("nth-{}", i /* a number */)) {
        : "this is a button"
    }

and get (assuming i = 10):

<div class="button-style nth-10">this is a button</div>

[Stebalien]

If you need help with any of this, just ask. If you don't feel up for doing all of it, I can finish up anything you don't implement (i.e., partial PRs welcome).

[dashed]

The additional suggestions sounds good.

I'm not totally clear on how to allocate TemplateBuffer.

[Stebalien]

You don't. Just implement Render, RenderMut, and RenderOnce (modeled after Fn, FnMut, and FnOnce) and an &mut TemplateBuffer will get passed to you (it's an analog of Display's &mut Formatter).

[Stebalien]

Actually, you can probably just generate a closure wrapped in an FnRenderer. That is, FnRenderer::new(|tmpl| { /* append appropriate labels to template */ }).

[dashed]

Thanks. I created a PR attempt based on your suggestions.