Conditional Child Component Parameters

Note that providing optional argument values without Some means that None can only be specfied by not setting the parameter at all! Fortunately, it's easy to do this conditionally in the same way as for optional attributes on HTML elements:

#![allow(unused)]
fn main() {
asteracea::component! {
  Inner()(
    class?: &'bump str,
  )

  <span .class? = {class}>
}

asteracea::component! {
  Middle()(
    class?: &'bump str,
  )

  <*Inner .class? = {class}>
}

asteracea::component! {
  Outer()()

  [
    <*Middle> "\n"
    <*Middle .class = {"bourgeoisie"}>
  ]
}
}

<SPAN></SPAN>
<SPAN class=bourgeoisie></SPAN>

This also applies to any other kind of optional parameter, i.e. arguments with explicit default value.

A note for low-level component implementers

Asteracea, when using child components in templates, constructs parameter bundles using a builder pattern. It assumes that the order of distinctly named parameter assignments does not matter and, in order to reduce code size, moves some assignments of arguments that are assigned to conditionally after those of ones that are not. This is a tradeoff incurred by statically validating parameter list completeness.

Parameter value evaluation order, assignment order among any unconditionally set parameters, assignment order among conditional parameters and the order of assignments to the same name are preserved regardless.

As long as your program compiles, this optimisation is unobservable when handling child component types created using asteracea::component! { … }. You may still want to keep it in mind for components with custom argument builders.