Projection

Projection is a StaticStage for extracting a value from a container type. When the expected value is present it returns Yield.Some; when it is absent it returns Yield.None with Status.Success, preserving the overall pipeline status without signalling an error.

Two convenience members simplify process implementations: some(out) produces StaticYield.Some(out, Status.Success) and none is a pre-built StaticYield.None(Status.Success) — both with this as the implicit evolution.

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

object OptionGet extends Projection[Option[Int], Int] {
  override protected def process(in: Option[Int]): StaticYield[Int, Nothing] =
    in match {
      case Some(v) => some(v)
      case None    => none
    }
}

OptionGet(Some(7))
// res0: Yield[Option[Int], Int, Nothing] = Some(
//   out = 7,
//   status = Success,
//   evolution = <function1>
// )
OptionGet(None)
// res1: Yield[Option[Int], Int, Nothing] = None(
//   status = Success,
//   evolution = <function1>
// )

For covariant binary type constructors C[+_, +_], LeftProjection[C] and RightProjection[C] provide reusable singleton projections. Each is defined over the widened type C[Any, ?] or C[?, Any] and exposed as a concrete Projection[C[T, ?], T] via a typed apply[T] method. The cast is sound because C is covariant in both positions, so a C[Any, ?] can be read safely as a C[T, ?] for any T.