Improvements exclusive to Scala 3.0

[japgolly]

• impurity safe zones using context functions
• use opaque types to avoid allocations
• intersection & union types to both avoid allocations, and better represent raw facades
• use erased to remove compile-time-only constructs from JS output
• mark any nullable types for explicit nulls
• document changes

Done

• support derives for typeclasses
• Add Reusability.byRef (by default) to Reusability.derive #797 is required to realistically use derives Reusability

[japgolly]

Excerpt from TriStateCheckbox:

  final case class Props(state       : State,
                         setNextState: Callback,
                         disabled    : Boolean = false)

  private def render($: ScalaComponent.MountedPure[Props, Unit, Unit], p: Props) = {
    val setNext = $.props.flatMap(p => p.setNextState.unless_(p.disabled)) // Only access .setNextState inside the Callback for Reusability
      ...
  }

  implicit val reusabilityProps: Reusability[Props] =
    Reusability.caseClassExcept("setNextState") // .setNextState is never accessed outside of a Callback

Scala 3 could allow something like this (and with erased classes, without incurring any runtime cost!):

  final case class Props(state       : State,
                         setNextState: Xxxxxxxxxxx[Callback],
                         disabled    : Boolean = false)

  private def render($: ScalaComponent.MountedPure[Props, Unit, Unit], p: Props) = {
    val setNext = $.props.flatMap(p => p.setNextState.unless_(p.disabled))
      ...
  }

  implicit val reusabilityProps: Reusability[Props] =
    Reusability.caseClass

where

• Reusability[Xxxxxxxxxxx[A]] is Reusability.always
• given [A] Conversion[Xxxxxxxxxxx[A], A](using InRenderFn | InCallback)
• Callback methods like {apply,map,flatMap,etc} get InCallback ?=>
• Render methods get InRenderFn ?=>

[japgolly]

Oh man, this is why it'd be good to enforce at the type-level. I just made a mistake above. Accessing the Callback in the render function would void Reusability (which is why I used $.props in the first place).

Correction:

given [A] Conversion[Xxxxxxxxxxx[A], A](using InRenderFn | InCallback)
// It should actually be this to preserve Reusability
given [A] Conversion[Xxxxxxxxxxx[A], A](using InCallback)

No wait a minute.... That's easily hacked:

def render(p: Props) = {
  val hack = Callback(p.setNextState) // wrong because `p` is fixed
}

How on earth can I have this work?

def render(p: Props) = {
  val good  = $.props.flatMap(_.setNextState)
  val bad  = Callback(p.setNextState)
}

I could just solve it really simplistically for the single BackendScope case but that feels a bit limited.

def render(p: Props) = {
  val good = $.fromProps(_.setNextState) // provide evidence directly from BackendScope
  val bad  = Callback(p.setNextState)    // reject because evidence unavailable
}

trait BackendScope[P, S] {
  // obviously rename all of this
  def fromProps[A](f: XxxxxxEvidence ?=> P => Xxxxxxxxxxx[A]): A
}

?

[japgolly]

Actually if I were to do something so simple, I wouldn't even need Scala 3 features. We could have this in Scala 2 today.

final case class Xxxx[A](unsafeGet: A)

trait BackendScope[P, S] {
  def fromProps[A](f: P => Xxxxxxxxxxx[A]): CallbackTo[A] =
    props.flatMap(f(_).unsafeGet)
}

[japgolly]

• Remove runtime footprint
  • JsRepr
  • OptionLike for Option
  • Component builder
  • sourcecode (microlibs)
• Make erased versions of <:< & =:= (microlibs)
• Can comp-builder endo-fns be greatly simplified using the new polymorphic fns?
• Undo the commit after 12c6a3e that reverts all the erased stuff