在斯卡拉：如何遞歸地將函數作爲參數傳遞給它自己的函數調用

Question

我試圖重構一些代碼並使用更高階的函數。但由於某種原因，當我將該函數作爲參數傳遞給它自己的函數時。我收到錯誤無法解析引用「權重」與這樣的簽名。

這裏是我的代碼：

abstract class CodeTree 
case class Fork(left: CodeTree, right: CodeTree, chars: List[Char], weight: Int) extends CodeTree 
case class Leaf(char: Char, weight: Int) extends CodeTree 

def walk[T](t: CodeTree, op: CodeTree => T, cmb: (T,T) => T) = t match { 
    case Fork(l,r,_,_) => cmb(op(l), op(r)) 
    case Leaf(_,x)  => x 
} 

def weight(tree: CodeTree): Int = walk[Int](tree, weight, _ ++ _) 

def chars(tree: CodeTree): List[Char] = tree match { 
    case Fork(l,r,x,_) => chars(l) ++ chars(r) 
    case Leaf(x,_)  => List(x) 
} 

Answer 1

我猜走法應該返回T和++應該+，這算什麼，你想幹什麼？

abstract class CodeTree 
case class Fork(left: CodeTree, right: CodeTree, chars: List[Char], weight: Int) extends CodeTree 
case class Leaf(char: Char, weight: Int) extends CodeTree 

def walk[T](t: CodeTree, op: CodeTree => T, cmb: (T,T) => T): T = t match { 
    case Fork(l,r,_,_) => cmb(op(l), op(r)) 
    case t: Leaf  => op(t) 
} 

def weight(tree: CodeTree): Int = walk[Int](tree, weight, _ + _) 

def chars(tree: CodeTree): List[Char] = tree match { 
    case Fork(l,r,x,_) => chars(l) ++ chars(r) 
    case Leaf(x,_)  => List(x) 
}