In C#, What is a monad?

Question

There is a lot of talk about monads these days. I have read a few articles / blog posts, but I can\'t go far enough with their examples to fully grasp the concept. The reaso

Answer 1

See my answer to "What is a monad?"

It begins with a motivating example, works through the example, derives an example of a monad, and formally defines "monad".

It assumes no knowledge of functional programming and it uses pseudocode with function(argument) := expression syntax with the simplest possible expressions.

This C# program is an implementation of the pseudocode monad. (For reference: M is the type constructor, feed is the "bind" operation, and wrap is the "return" operation.)

using System.IO;
using System;

class Program
{
    public class M
    {
        public A val;
        public string messages;
    }

    public static M feed(Func> f, M x)
    {
        M m = f(x.val);
        m.messages = x.messages + m.messages;
        return m;
    }

    public static M wrap(A x)
    {
        M m = new M();
        m.val = x;
        m.messages = "";
        return m;
    }

    public class T {};
    public class U {};
    public class V {};

    public static M g(V x)
    {
        M m = new M();
        m.messages = "called g.\n";
        return m;
    }

    public static M f(U x)
    {
        M m = new M();
        m.messages = "called f.\n";
        return m;
    }

    static void Main()
    {
        V x = new V();
        M m = feed(f, feed(g, wrap(x)));
        Console.Write(m.messages);
    }
}