Scala中的Comonad示例

Question

什麼是Comonad，如果可以用Scala語法描述的話。 我發現scalaz庫的實現，但它不清楚它可以用在哪裏。

Answer 1

好，單子讓你值添加到他們，改變他們根據從非單子到單子計算。 Comonads允許您從中提取值，並根據從comonad到非comonad的計算來更改它們。

自然的直覺是，他們通常會在出現在你有一個CM [A]和要提取A.

見this非常有趣的帖子上comonads倒是有點漫不經心，但對我來說至少，使他們非常清楚。

Answer 2

接下來是從this博客文章的字面翻譯代碼。

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) 

輸出：

| | | | | | | | | | | | | | | | | | | |#| | | | | | | | | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#|#| | | | | | | | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#| |#| | | | | | | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#|#|#|#| | | | | | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#| | | |#| | | | | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#|#| | |#|#| | | | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#| |#| |#| |#| | | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#|#|#|#|#|#|#|#| | | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#| | | | | | | |#| | | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#|#| | | | | | |#|#| | | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#| |#| | | | | |#| |#| | | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#|#|#|#| | | | |#|#|#|#| | | | | | | | 
| | | | | | | | | | | | | | | | | | | |#| | | |#| | | |#| | | |#| | | | | | | 
| | | | | | | | | | | | | | | | | | | |#|#| | |#|#| | |#|#| | |#|#| | | | | | 
| | | | | | | | | | | | | | | | | | | |#| |#| |#| |#| |#| |#| |#| |#| | | | | 
| | | | | | | | | | | | | | | | | | | |#|#|#|#|#|#|#|#|#|#|#|#|#|#|#|#| | | | 
| | | | | | | | | | | | | | | | | | | |#| | | | | | | | | | | | | | | |#| | | 
| | | | | | | | | | | | | | | | | | | |#|#| | | | | | | | | | | | | | |#|#| | 
| | | | | | | | | | | | | | | | | | | |#| |#| | | | | | | | | | | | | |#| |#| 
| | | | | | | | | | | | | | | | | | | |#|#|#|#| | | | | | | | | | | | |#|#|#|#