Stagnation

Stagnation is an Evolution mixin that seals evolve to unconditionally return this, regardless of the status, and makes dispose a no-op. Mixing it into a stage expresses that the stage is always its own continuation.

Stagnation has a self-type of Stage[I, O, E], so it can only be mixed into things that are already stages.

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

object Counter extends Stage[Unit, Int, Nothing] with Stagnation[Unit, Int, Nothing] {
  private var n = 0

  override def apply(in: Unit): Yield[Unit, Int, Nothing] = {
    n += 1
    Yield.Some(n, Status.Success, this)
  }

  override def skip(): Evolution[Unit, Int, Nothing] = this
}

Counter(())
// res0: Yield[Unit, Int, Nothing] = Some(
//   out = 1,
//   status = Success,
//   evolution = <function1>
// )
Counter(())
// res1: Yield[Unit, Int, Nothing] = Some(
//   out = 2,
//   status = Success,
//   evolution = <function1>
// )
Counter.skip()
// res2: Evolution[Unit, Int, Nothing] = <function1>

Stagnation seals evolve but only provides a default no-op dispose — dispose remains open for override, so a stage can still release resources when it is torn down. The skip method stays open as well, giving the stage full control over what evolution it exposes when bypassed. In practice, most stateless stages use this for skip too — which is exactly what the next layer provides.