Kyo (Alt. Encoding Explorations)

kyo.scala

package zero

import izumi.reflect.Tag
import Kyo.*
import scala.collection.View.FlatMap

type Id[T]    = T
type Const[T] = [U] =>> T
type MX[T]    = Any

//////////////
// KYO CORE //
//////////////

opaque type <[+T, -S] >: T = T | Kyo[T, S]

implicit inline def convert[T, S](zero: Kyo[T, S]): T < S = zero

extension [T, S](self: T < S) //
  inline def map[U, S2](f: T => U < S2): U < (S & S2) = flatMap(f)

  def flatMap[U, S2](k: T => U < S2): U < (S & S2) =
    def flatMapLoop(v: T < S): U < (S & S2) =
      v match
        case kyo: Suspend[MX, Any, T, S] @unchecked =>
          new Continue[MX, Any, U, S & S2](kyo):
            def apply(v: Any) = flatMapLoop(kyo(v))
        case v: T @unchecked =>
          k(v)
    flatMapLoop(self)

  def when(cond: Boolean): Any < S =
    if cond then self else ()

sealed abstract class Kyo[+T, -S]

object Kyo:
  sealed abstract class Suspend[Command[_], T, U, S] extends Kyo[U, S] with Function1[T, U < S]:
    def command: Command[T]
    def tag: Tag[?]
    def isRoot: Boolean
    def apply(v: T): U < S
  end Suspend

  class Root[Command[_], T, E](
      val command: Command[T],
      val tag: Tag[E]
  ) extends Suspend[Command, T, T, E]:
    def isRoot: Boolean    = true
    def apply(v: T): T < E = v
  end Root

  sealed abstract class Continue[Command[_], T, U, S](
      s: Suspend[Command, T, ?, ?]
  ) extends Suspend[Command, T, U, S]:
    val command = s.command
    val tag     = s.tag
    def isRoot  = false
  end Continue
end Kyo

abstract class Handler[Command[_], Result[_], E](using val tag: Tag[E]):
  def pure[T](v: T): Result[T]
  def apply[T, U, S](command: Command[T], k: T => U < (E & S)): Result[U] < S

  def handle[T, S](value: T < (E & S)): Result[T] < S =
    value match
      case suspend: Suspend[Command, Any, T, S] @unchecked if suspend.tag.tag == tag.tag =>
        apply(suspend.command, suspend.apply)

      case suspend: Suspend[MX, Any, T, S] @unchecked =>
        new Continue[MX, Any, Result[T], S](suspend):
          def apply(v: Any) =
            handle(suspend(v))

      case v: T @unchecked =>
        pure(v)
end Handler

abstract class SimpleHandler[Command[_], E](using override val tag: Tag[E]) extends Handler[Command, Id, E]:
  def pure[T](v: T): Id[T] = v
  def apply[T, U, S](command: Command[T], k: T => U < (E & S)): U < S
end SimpleHandler

abstract class Effect[E](using val tag: Tag[E]):
  type Command[_]
  def suspend[T](command: Command[T]): T < E = Root(command, tag)
end Effect

////////////
// ABORTS //
////////////

class Aborts[E: Tag] extends Effect[Aborts[E]]:
  override type Command[T] = Left[E, Nothing]

object Aborts:

  def abort[E: Tag](value: E): Nothing < Aborts[E] =
    Aborts[E].suspend(Left(value))

  def fromEither[E: Tag, T, S](either: Either[E, T] < S): T < (Aborts[E] & S) =
    either.map {
      case Right(value) => value
      case Left(e)      => abort(e)
    }

  def handler[E: Tag] = new Handler[Const[Left[E, Nothing]], Either[E, *], Aborts[E]]:
    def pure[T](v: T): Either[E, T] = Right(v)

    def apply[T, U, S](command: Left[E, Nothing], k: T => U < (Aborts[E] & S)): Either[E, U] < S =
      command

  def run[E: Tag, T, S](value: T < (Aborts[E] & S)): Either[E, T] < S =
    handler[E].handle(value)

//////////
// ENVS //
//////////

class Envs[R: Tag] extends Effect[Envs[R]]:
  override type Command[T] = Envs.Input[T]

object Envs:
  object Input
  type Input[T] = Input.type

  def get[R: Tag]: R < Envs[R] = Envs[R].suspend(Input)

  def run[R: Tag, T, S](env: R)(value: T < (Envs[R] & S)): T < S =
    val handler = new SimpleHandler[Input, Envs[R]]:
      def apply[T, U, S](command: Input[T], k: T => U < (Envs[R] & S)): U < S =
        this.handle(k(env.asInstanceOf[T]))

    handler.handle(value)

/////////////
// EXAMPLE //
/////////////

object Example extends App:

  val program: String < (Envs[Int] & Aborts[String] & Envs[String]) =
    for
      number <- Envs.get[Int]
      _      <- Aborts.abort(s"DON'T BE SO NEGATIVE! ${number}").when(number < 0)
      string <- Envs.get[String]
    yield string * number

  val result: Either[String, String] < Any =
    Aborts.run(Envs.run("LO")(Envs.run(-11)(program)))

  println(result)