StaticStage and StaticYield

StaticStage seals apply as well as skip, and delegates processing to a single abstract method process(in: I): StaticYield[O, E]. The sealed apply wraps the result into a full Yield with this as the evolution — the implementor never has to write that part.

StaticYield mirrors Yield but omits the Evolution field, since StaticStage always supplies this as the continuation. It has two variants: StaticYield.Some(out, status) when a value is produced, and StaticYield.None(status) when the stage deliberately emits nothing.

import h8io.stages.*
import h8io.stages.base.*

object IsEven extends StaticStage[Int, String, Nothing] {
  override protected def process(in: Int): StaticYield[String, Nothing] =
    if (in % 2 == 0) StaticYield.Some(s"$in is even", Status.Success)
    else             StaticYield.None(Status.Success)
}

IsEven(4)
// res0: Yield[Int, String, Nothing] = Some(
//   out = "4 is even",
//   status = Success,
//   evolution = <function1>
// )
IsEven(7)
// res1: Yield[Int, String, Nothing] = None(
//   status = Success,
//   evolution = <function1>
// )

Use StaticStage when the stage may or may not produce a value depending on the input. When output is always guaranteed, Fn is more specific.