實現摺疊與理解

Question

在斯卡拉如何將fold作爲for-comprehension實現？我看到唯一的方法是使用一些遞歸調用？這是一個失敗的嘗試，不知道如何做到這一點？什麼是執行fold爲for-comprehension

val nums = List(1,2,3)       
    nums.fold(0)(_+_)        
    def recFold(acc: Int = 0): Int = { 
    (for { 
     a <- nums 
     b = recFold(a + acc) 
    } yield b).head 
    }            
    recFold(0) //Stack overflow 

Answer 1

如果你真的想使用for，你不需要遞歸，但你需要一個可變變量：

val nums = List(1,2,3) 

def recFold(zero: Int)(op: (Int, Int) => Int): Int = { 
    var result: Int = zero 
    for { a <- nums } result = op(result, a) 
    result 
} 

recFold(0)(_ + _) // 6 

這是非常相似，如何foldLeft在TraversableOnce實際執行：

def foldLeft[B](z: B)(op: (B, A) => B): B = { 
    var result = z 
    this foreach (x => result = op(result, x)) 
    result 
} 

Answer 2

折可以實現從右到左或從左到右兩種方式的最佳途徑。無需使用for加遞歸。遞歸就夠了。

def foldRight[A, B](as: List[A], z: B)(f: (A, B) => B): B = { 
    as match { 
     case Nil => z 
     case x :: xs => f(x, foldRight(xs, z)(f)) 
    } 
    } 

    @annotation.tailrec 
    def foldLeft[A, B](as: List[A], z: B)(f: (A, B) => B): B = { 
    as match { 
     case Nil => z 
     case x :: xs => foldLeft(xs, f(x, z))(f) 
    } 
    }