Ejemplo de Comonad en Scala

monads scalaz

Question

Qué es Comonad, si es posible, descríbalo en la sintaxis de Scala. scalaz implementación de la biblioteca de scalaz , pero no está claro dónde puede ser útil.

Answer 1

Bueno, las mónadas le permiten agregar valores, cambiarlos en función de un cálculo de una mónada no a una mónada. Los Comonads le permiten extraer valores de ellos y cambiarlos en función de un cálculo de un Comonad a un no Comonad.

La intuición natural es que generalmente aparecerán donde tienes un CM [A] y quieres extraer A.

Vea this publicación muy interesante que toca comonads un poco de manera casual, pero, al menos para mí, dejándolos muy claros.

Answer 2

La biblioteca de Scalaz proporciona un ComonadStore que extiende la propiedad de Comonad . Se define así:

trait ComonadStore[F[_], S] extends Comonad[F] { self =>

  def pos[A](w: F[A]): S
  def peek[A](s: S, w: F[A]): A

  def peeks[A](s: S => S, w: F[A]): A =
  peek(s(pos(w)), w)

  def seek[A](s: S, w: F[A]): F[A] =
   peek(s, cojoin(w))

  def seeks[A](s: S => S, w: F[A]): F[A] =
   peeks(s, cojoin(w))

  def experiment[G[_], A](s: S => G[S], w: F[A])(implicit FG: Functor[G]): G[A] =
   FG.map(s(pos(w)))(peek(_, w))

}

Y una Store (que es análoga a (S => A, S) ) tiene una instancia de Comonad . Puedes ver esta question que explica qué es más específicamente.

También tienes los Coreader y Cowriter que son el doble de las Monads Reader y Writer , aquí hay una excelente blog en el blog que habla sobre esto en Scala.

Answer 3

Lo que sigue es una traducción literal de código de this publicación de blog.

case class U[X](left: Stream[X], center: X, right: Stream[X]) {
  def shiftRight = this match {
    case U(a, b, c #:: cs) => U(b #:: a, c, cs)
  }

  def shiftLeft = this match {
    case U(a #:: as, b, c) => U(as, a, b #:: c)
  }
}

// Not necessary, as Comonad also has fmap.
/*
implicit object uFunctor extends Functor[U] {
  def fmap[A, B](x: U[A], f: A => B): U[B] = U(x.left.map(f), f(x.center), x.right.map(f))
}
*/

implicit object uComonad extends Comonad[U] {
  def copure[A](u: U[A]): A = u.center
  def cojoin[A](a: U[A]): U[U[A]] = U(Stream.iterate(a)(_.shiftLeft).tail, a, Stream.iterate(a)(_.shiftRight).tail)
  def fmap[A, B](x: U[A], f: A => B): U[B] = U(x.left.map(f), x.center |> f, x.right.map(f))
}

def rule(u: U[Boolean]) = u match {
  case U(a #:: _, b, c #:: _) => !(a && b && !c || (a == b))
}

def shift[A](i: Int, u: U[A]) = {
  Stream.iterate(u)(x => if (i < 0) x.shiftLeft else x.shiftRight).apply(i.abs)
}

def half[A](u: U[A]) = u match {
  case U(_, b, c) => Stream(b) ++ c
}

def toList[A](i: Int, j: Int, u: U[A]) = half(shift(i, u)).take(j - i)

val u = U(Stream continually false, true, Stream continually false)

val s = Stream.iterate(u)(_ =>> rule)

val s0 = s.map(r => toList(-20, 20, r).map(x => if(x) ''#'' else '' ''))

val s1 = s.map(r => toList(-20, 20, r).map(x => if(x) ''#'' else '' '').mkString("|")).take(20).force.mkString("/n")

println(s1)

Salida:

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