Vars should be splittable about airstream

[phfroidmont]

I use this pattern of splitting vars quite a bit. Since this only comes up in the context of creating child components, creating a few helpers limited to that scope made things much easier (maybe I should make a PR to Laminext @yurique ?).

extension [A](seqVar: Var[Seq[A]])
  def splitRender[Key](key: A => Key)(project: Var[A] => HtmlElement)(using CanEqual[Key, Key])
    : Mod[HtmlElement] =
    onMountInsert(ctx =>
      given Owner = ctx.owner
      children <-- seqVar.signal.split(key)((_, _, signal) =>
        val zoomedVar = Var(signal.observe.now())
        project(zoomedVar)
          .amend(signal --> zoomedVar, zoomedVar --> seqVar.updaterById(key))
      )
    )

extension [A](optionVar: Var[Option[A]])
  def splitRenderOption(project: Var[A] => HtmlElement, ifEmpty: => HtmlElement)
    : Mod[HtmlElement] =
    onMountInsert(ctx =>
      given Owner = ctx.owner
      child <-- optionVar.signal.splitOption(
        (_, signal) =>
          project(
            optionVar.zoom(_.getOrElse(signal.observe.now()))((_, value) => Some(value))
          ),
        ifEmpty = ifEmpty
      )
    )

As for zooming, I actually implemented something very similar to the LazyDerivedVar you describe (that I called ValueController). Internally it was a wrapper for Signal[State], Observer[State => State] and Lens[State, State2] from Monocle which allows support for an updater method. Zooming further is just a matter of chaining the lenses. I can dig up the actual implementation if you think that's any help.

When the zoom method was added to Laminar, I decided to use it instead. Even if the ergonomics of zoom are not ideal it felt better than maintaining my bespoke abstraction.

[filonik-cmu]

Just wanted to voice my support. I feel like this functionality would be extremely useful for working nested hierarchies of data and components. Without it, derived vars currently feel a bit like second class citizens compared to signals in Airstream/Laminar.

[rgwilton]

Just to add a +1 that I was finding it hard to get Laminar to work as I expected. I was trying to follow the similar structure of zooming into subparts of a Var and passing those onto rendering components that would zoom further. This worked, but meant that I couldn't really use split.

So, then I tried to use split instead, but found it hard to then zoom into the writer because I wanted the current context to zoom into.

[raquo]

You can now preview this feature in Airstream v17.1.0-M1. It can be used with Laminar 17.0.0. The new method names on Var are split and splitMutate – the latter for mutating mutable collections in-place in a Var.