什么是catamorphism并且可以在C#3.0中实现？

在第二个示例中,另一个树的重建方式不同:

class Example
{
    // original version recreates entire tree, yuck 
    public static Tree Change5to0(Tree tree)
    {
        return Tree.FoldTree((int x, Tree l, Tree r) => Tree.Node(x == 5 ? 0 : x, l, r), null, tree);
    }

    // here it is with XFold - same as original, only with Xs 
    public static Tree XChange5to0(Tree tree)
    {
        return Tree.XFoldTree((int x, Tree l, Tree r, Tree orig) =>
            Tree.XNode(x == 5 ? 0 : x, l, r)(orig), _ => null, tree);
    }
}

在第三个例子中,折叠树用于绘图:

class MyWPFWindow : Window 
{
    void Draw(Canvas canvas, Tree> tree)
    {
        // assumes canvas is normalized to 1.0 x 1.0 
        Tree.FoldTree((KeyValuePair kvp, Func l, Func r) => trans =>
        {
            // current node in top half, centered left-to-right 
            var tb = new TextBox();
            tb.Width = 100.0; 
            tb.Height = 100.0;
            tb.FontSize = 70.0;
                // the tree is a "diff tree" where the bool represents 
                // "ReferenceEquals" differences, so color diffs Red 
            tb.Foreground = (kvp.Value ? Brushes.Black : Brushes.Red);
            tb.HorizontalContentAlignment = HorizontalAlignment.Center;
            tb.VerticalContentAlignment = VerticalAlignment.Center;
            tb.RenderTransform = AddT(trans, TranslateT(0.25, 0.0, ScaleT(0.005, 0.005, new TransformGroup())));
            tb.Text = kvp.Key.ToString();
            canvas.Children.Add(tb);
            // left child in bottom-left quadrant 
            l(AddT(trans, TranslateT(0.0, 0.5, ScaleT(0.5, 0.5, new TransformGroup()))));
            // right child in bottom-right quadrant 
            r(AddT(trans, TranslateT(0.5, 0.5, ScaleT(0.5, 0.5, new TransformGroup()))));
            return null;
        }, _ => null, tree)(new TransformGroup());
    }

    public MyWPFWindow(Tree> tree)
    {
        var canvas = new Canvas();
        canvas.Width=1.0;
        canvas.Height=1.0;
        canvas.Background = Brushes.Blue;
        canvas.LayoutTransform=new ScaleTransform(200.0, 200.0);
        Draw(canvas, tree);
        this.Content = canvas;
        this.Title = "MyWPFWindow";
        this.SizeToContent = SizeToContent.WidthAndHeight;
    }
    TransformGroup AddT(Transform t, TransformGroup tg) { tg.Children.Add(t); return tg; }
    TransformGroup ScaleT(double x, double y, TransformGroup tg) { tg.Children.Add(new ScaleTransform(x,y)); return tg; }
    TransformGroup TranslateT(double x, double y, TransformGroup tg) { tg.Children.Add(new TranslateTransform(x,y)); return tg; }

    [STAThread]
    static void Main(string[] args)
    {
        var app = new Application();
        //app.Run(new MyWPFWindow(Tree.DiffTree(Tree.Tree7,Example.Change5to0(Tree.Tree7))));
        app.Run(new MyWPFWindow(Tree.DiffTree(Tree.Tree7, Example.XChange5to0(Tree.Tree7))));
    }
}    

我之前的印象是它提到了一种创建可以概括不同折叠的函数的方法,因此它可以折叠树和列表.实际上可能还有这样的东西,某种类型的仿函数或箭头,但现在这超出了我的理解水平.

Brian在第一段中的答案是正确的。但是他的代码示例并未真正反映出如何以C＃样式解决类似问题。考虑一个简单的类node：

class Node {
  public Node Left;
  public Node Right;
  public int value;
  public Node(int v = 0, Node left = null, Node right = null) {
    value = v;
    Left = left;
    Right = right;
  }
}

这样我们可以在main中创建一棵树：

var Tree = 
    new Node(4,
      new Node(2, 
        new Node(1),
        new Node(3)
      ),
      new Node(6,
        new Node(5),
        new Node(7)
      )
    );

我们在Node的命名空间中定义了通用的fold函数：

public static R fold(
  Func combine,
  R leaf_value,
  Node tree) {

  if (tree == null) return leaf_value;

  return 
    combine(
      tree.value, 
      fold(combine, leaf_value, tree.Left),
      fold(combine, leaf_value, tree.Right)
    );
}

对于变形，我们应指定数据状态，节点可以为空或有子级。通用参数决定了我们在这两种情况下的操作。注意迭代策略（在这种情况下为递归）隐藏在fold函数中。

现在不用写：

public static int Sum_Tree(Node tree){
  if (tree == null) return 0;
  var accumulated = tree.value;
  accumulated += Sum_Tree(tree.Left);
  accumulated += Sum_Tree(tree.Right);
  return accumulated; 
}

我们可以写

public static int sum_tree_fold(Node tree) {
  return Node.fold(
    (x, l, r) => x + l + r,
    0,
    tree
  );
}

优雅，简单，经过类型检查，可维护等。易于使用Console.WriteLine(Node.Sum_Tree(Tree));。

添加新功能很容易：

public static List In_Order_fold(Node tree) {
  return Node.fold(
    (x, l, r) => {
      var tree_list = new List();
      tree_list.Add(x);
      tree_list.InsertRange(0, l);
      tree_list.AddRange(r);
      return tree_list;
    },
    new List(),
    tree
  );
}
public static int Height_fold(Node tree) {
  return Node.fold(
    (x, l, r) => 1 + Math.Max(l, r),
    0,
    tree
  );
}

F＃在“简洁”类别中获胜，In_Order_fold但是当该语言提供了用于构造和使用列表的专用运算符时，这是可以预期的。

C＃和F＃之间的巨大差异似乎是由于F＃使用闭包来充当隐式数据结构来触发尾部调用优化。Brian的答案中的示例还考虑了F＃中的优化，以躲避重构树。我不确定C＃是否支持尾部调用优化，也许In_Order_fold可以写得更好，但是在讨论处理这些Catamorphism时C＃的表现力如何时，这些要点都不重要。

在语言之间翻译代码时，您需要了解该技术的核心思想，然后根据语言的原语来实现该思想。

也许现在您可以说服您的C＃同事更加认真地对待折叠。