Question

I’m trying to learn about catamorphisms and I’ve read the Wikipedia article and the first couple posts in the series of the topic for F# on the Inside F# blog.

I understand that it’s a generalization of folds (i.e., mapping a structure of many values to one value, including a list of values to another list). And I gather that the fold-list and fold-tree is a canonical example.

Can this be shown to be done in C#, using LINQ’s Aggregate operator or some other higher-order method?

Answer 1

LINQ’s Aggregate() is just for IEnumerables. Catamorphisms in general refer to the pattern of folding for an arbitrary data type. So Aggregate() is to IEnumerables what FoldTree (below) is to Trees (below); both are catamorphisms for their respective data types.

I translated some of the code in part 4 of the series into C#. The code is below. Note that the equivalent F# used three less-than characters (for generic type parameter annotations), whereas this C# code uses more than 60. This is evidence why no one writes such code in C# – there are too many type annotations. I present the code in case it helps people who know C# but not F# play with this. But the code is so dense in C#, it’s very hard to make sense of.

Given the following definition for a binary tree:

using System; using System.Collections.Generic; using System.Windows; using System.Windows.Controls; using System.Windows.Input; using System.Windows.Media; using System.Windows.Shapes;  class Tree<T>   // use null for Leaf {     public T Data { get; private set; }     public Tree<T> Left { get; private set; }     public Tree<T> Right { get; private set; }     public Tree(T data, Tree<T> left, Tree<T> rright)     {         this.Data = data;         this.Left = left;         this.Right = right;     }      public static Tree<T> Node<T>(T data, Tree<T> left, Tree<T> right)     {         return new Tree<T>(data, left, right);     } } 

One can fold trees and e.g. measure if two trees have different nodes:

class Tree {     public static Tree<int> Tree7 =         Node(4, Node(2, Node(1, null, null), Node(3, null, null)),                 Node(6, Node(5, null, null), Node(7, null, null)));      public static R XFoldTree<A, R>(Func<A, R, R, Tree<A>, R> nodeF, Func<Tree<A>, R> leafV, Tree<A> tree)     {         return Loop(nodeF, leafV, tree, x => x);     }      public static R Loop<A, R>(Func<A, R, R, Tree<A>, R> nodeF, Func<Tree<A>, R> leafV, Tree<A> t, Func<R, R> cont)     {         if (t == null)             return cont(leafV(t));         else             return Loop(nodeF, leafV, t.Left, lacc =>                    Loop(nodeF, leafV, t.Right, racc =>                    cont(nodeF(t.Data, lacc, racc, t))));     }      public static R FoldTree<A, R>(Func<A, R, R, R> nodeF, R leafV, Tree<A> tree)     {         return XFoldTree((x, l, r, _) => nodeF(x, l, r), _ => leafV, tree);     }      public static Func<Tree<A>, Tree<A>> XNode<A>(A x, Tree<A> l, Tree<A> r)     {         return (Tree<A> t) => x.Equals(t.Data) && l == t.Left && r == t.Right ? t : Node(x, l, r);     }      // DiffTree: Tree<'a> * Tree<'a> -> Tree<'a * bool>      // return second tree with extra bool      // the bool signifies whether the Node 'ReferenceEquals' the first tree      public static Tree<KeyValuePair<A, bool>> DiffTree<A>(Tree<A> tree, Tree<A> tree2)     {         return XFoldTree((A x, Func<Tree<A>, Tree<KeyValuePair<A, bool>>> l, Func<Tree<A>, Tree<KeyValuePair<A, bool>>> r, Tree<A> t) => (Tree<A> t2) =>             Node(new KeyValuePair<A, bool>(t2.Data, object.ReferenceEquals(t, t2)),                  l(t2.Left), r(t2.Right)),             x => y => null, tree)(tree2);     } } 

In this second example, another tree is reconstructed differently:

class Example {     // original version recreates entire tree, yuck      public static Tree<int> Change5to0(Tree<int> tree)     {         return Tree.FoldTree((int x, Tree<int> l, Tree<int> 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<int> XChange5to0(Tree<int> tree)     {         return Tree.XFoldTree((int x, Tree<int> l, Tree<int> r, Tree<int> orig) =>             Tree.XNode(x == 5 ? 0 : x, l, r)(orig), _ => null, tree);     } } 

And in this third example, folding a tree is used for drawing:

class MyWPFWindow : Window  {     void Draw(Canvas canvas, Tree<KeyValuePair<int, bool>> tree)     {         // assumes canvas is normalized to 1.0 x 1.0          Tree.FoldTree((KeyValuePair<int, bool> kvp, Func<Transform, Transform> l, Func<Transform, Transform> 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<KeyValuePair<int, bool>> 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))));     } }