Ejemplo de Comonad en Scala

Question

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

Answer 1

Bueno, mónadas permiten añadir valores a ellos, los cambia en base a un cálculo de un no-mónada a una mónada. Comonads le permite extraer valores de ellos y cambiarlos en función de un cálculo de comonad a non-comonad.

La intuición natural es que por lo general va aparecerá donde usted tiene un CM [A] y desea extraer A.

Ver this post muy interesante que toca comonads un poco por casualidad, pero, para mí en al menos, haciéndolos muy claros.

Answer 2

Lo que sigue es una traducción literal del código de this publicación en el 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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| | | | | | | | | | | | | | | | | | | |#| |#| | | | | | | | | | | | | |#| |#| 
| | | | | | | | | | | | | | | | | | | |#|#|#|#| | | | | | | | | | | | |#|#|#|#