Monad in c#

IfNotNull.cs

using System;

void Main()
{
    string foo = "Hello";

    // a.then(f).then(g).then(j)
    foo.Then(x => x.Trim().Ret())
        .Then(x => x.Substring(0, 5).Ret())
        .Then(x => x.Length.Ret())
        .Then(x => (x * x).Ret())
        .Value
        .Dump(); // Dump is basically a chained 'Console.WriteLine' call.
}

// Nil or also called Identity
public struct Identity<T> 
{
    public Identity(T value) => Value = value;

    public T Value { get; set; }
    public bool HasValue => Value != null;
}

public static class Extensions
{
    // Abbreviation for 'To Nil' or ToNil. Can be called Return or ret in other languages
    public static Nil<T> Ret<T>(this T value) => new Nil<T>(value);

    // Bind is also known as 'then' or 'ContinueWith' in other programming languages
    // a.then(f).then(g).then(j)
    public static Nil<TOut> Then<TIn, TOut>(this TIn @this, Func<TIn, Nil<TOut>> fn)
        => @this.Ret().Then(fn);

    // f(a).then(g).then(j)
    public static Nil<TOut> Then<TIn, TOut>(this Nil<TIn> @this, Func<TIn, Nil<TOut>> fn)
        => @this.HasValue ? fn(@this.Value) : new Nil<TOut>();
}

IfNotNullNothing.cs

using System;

void Main()
{
    string foo = "Hello";

    foo.Bind(x => x.Trim().Nil())
        .Bind(x => x.Substring(0, 5).Nil())
        .Bind(x => x.Length.Nil())
        .Bind(x => (x * x).Nil())
        .Value
        .Dump();
}

public struct Nil<T> 
{
    public Nil(T value) => Value = value;
    protected Nil() { }

    public T Value { get; set; }
    public virtual bool HasValue => true;
    public static Nil<T> Nothing = new Nothing<T>();
}

public struct Nothing<T> : Nil<T>
{
    public override bool HasValue => false;
}

public static class Extensions
{
    // // Abbreviation for 'To Nil' or ToNil. Also called Return or ret in other languages.
    public static Nil<T> Nil<T>(this T value) => new Nil<T>(value);

    // Bind is also known as 'then' or 'ContinueWith' in other programming languages
    // a.then(f).then(g).then(j)
    public static Nil<C> Bind<B, C>(this B b, Func<B, Nil<C>> fn)
        => b.Ret().Bind(fn);

    public static Nil<C> Bind<B, C>(this Nil<B> b, Func<B, Nil<C>> fn)
        => b.HasValue ? fn(b.Value) : Nil<C>.Nothing;
}