Toggle the presence of an attribute using Option<T>

[lambda-fairy]

Alternative 1

let surprise = Some("Boo!");
html! {
    p title?=(surprise) "Hover over me"
}

Alternative 2

Like Alternative 1, but with the syntax title=?(surprise) instead.

Alternative 3

Use the same syntax as normal splices, title=(surprise), but use trait magic to omit the attribute when surprise.is_none().

Implementation: Add a method, fn is_present(&self) -> bool, to the Render trait. This method returns true for all types except Option<T>, which returns self.is_some(). Then at run time, the code will call surprise.is_present() to decide whether or not to emit the attribute.

Not sure how this composes -- what does title={ "I say: " (surprise) } mean?

[lambda-fairy]

After filing #59, I came up with another alternative: p title=(surprise)?.

[lambda-fairy]

Looking at this again, I'm not sure what use case I have for this feature. It seems too complicated at the moment. So I'm closing it for now.

If you have a use for this, feel free to make a comment!

[Korvox]

I ran into something that could use this. I have a dataset I want to prefill a form with, but the sourcing of that data isn't uniform or guaranteed so it is all optional. If I could write it like this:

html! {
   input type="text" value?=(data.value);
}

It would be much more ergonomic than how I have it written now which looks like this:

html! {
    input type="text" value=@if let Some(val) = data.value { (val) };
}

It would also be nice because right now there is no way (I think?) to have an attribute that either exists with a value or doesn't exist with no value without doing conditionals on the entire attribute.

I thought the following form might be better but it seems attributes in conditions don't work:

html! {
    input type="text" @if let Some(val) = data.value { value=(val) };
}

[lambda-fairy]

@Korvox you can make that shorter with a wrapper struct:

struct Optional<T>(Option<T>);

impl<T: Render> Render for Optional<T> {
    fn render_to(&self, buffer: &mut String) {
        if let Some(ref value) = self.0 {
            value.render_to(buffer);
        }
    }
}

Does that address your use case?

---

By the way: there's no difference between an empty value and a missing one as far as I can tell.

The only case I know of where a missing and empty attribute have different meanings is the alt attribute. An empty alt says that the image has no meaning, but a missing alt says that the image has an unknown meaning.

I haven't looked very hard though, so there might be others I've missed.