Compiling SelectMany Linq without System.Linq. Also a List Monad.

gistfile1.cs

using System;
using System.Collections.Generic;

public static class ExtensionMethods
{
    public static IEnumerable<T> ToList<T>(this T value) { 
         yield return value;
     } 
    
    public static IEnumerable<U> SelectMany<T, U>(this IEnumerable<T> m, Func<T, IEnumerable<U>> k)
    {
        foreach (var x in m)
            foreach (var y in k(x))
                yield return y;
    }
    
    public static IEnumerable<V> SelectMany<T, U, V>(this IEnumerable<T> id, Func<T, IEnumerable<U>> k, Func<T,U,V> s)
    {
        return id.SelectMany(x => k(x).SelectMany(y => s(x, y).ToList()));
    }
}

public class ListMonad
{
    public static void WithExtensions() 
    {
        IEnumerable<string> list1 = new string[] {"1", "2", "3"};
        IEnumerable<string> list2 = new string[] {"c", "d"};

        var r = list1.SelectMany( 
                    x => list2, (x, y) => x + y); 

        foreach (var i in r)
            Console.WriteLine(i);
    }

    public static void WithLinq()
    {
        IEnumerable<string> list1 = new string[] {"1", "2", "3"};   
        IEnumerable<string> list2 = new string[] {"c", "d"};   
        
        var r = from x in list1
                from y in list2
                select x + y;
        
        foreach (var i in r)
            Console.WriteLine(i);
    }


    public static void Main() 
    {
        Console.WriteLine("With Extension Methods");
        WithExtensions();

        Console.WriteLine("");

        Console.WriteLine("With Linq");
        WithLinq();
    }
}