using System;

using System.Collections.Generic;

using System.Linq;

///Practical functional programing in C#

//functional programming in modern Web Application development using Asp.Net Core and 

//Domain Driven Design with the language-extfunctional library in C#

//https://leanpub.com/practical-functional-CSharp       

​

public abstract class Maybe<T>

    {

        public Maybe<T1> Map<T1>(Func<T, T1> f) =>

         this switch

         {

             None<T>() => new None<T1>(),

             Some<T>(var v) => new Some<T1>(f(v)),

             _ => throw new NotImplementedException(),

         }; 

    }

    public class None<T> : Maybe<T>

    {

        public None() { }

        public void Deconstruct() { }

    }

​

    public class Some<T> : Maybe<T>

    {

        private readonly T value;

        public Some(T value) => this.value = value;

        public void Deconstruct(out T value) => value = this.value;

    }

 public static  class FunctionalExtensions

    {

   public static Maybe<T> FirstOrNone<T>(this List<T> @source, Func<T, bool> predicate)

        {

            var firstOrDefault = @source.FirstOrDefault(predicate);

            if (firstOrDefault != null)

                return new Some<T>(firstOrDefault);

            else

                return new None<T>();

        } 

    }

public class Program

{ 

    public static void Main()

    {

​

         string result = new None<int>()

            .Map(x => x + 1)

            switch

            {

                None<int>() => "nothing",

                Some<int>(var x) => x.ToString(),

                _ => throw new NotImplementedException(),

            };

​

​

        Console.WriteLine(result);

    }

​

​

    public class MockClientRepository

    {

        List<Client> clients = new List<Client>{

                    new  Client{Id=1, Name="Jim"},

                    new  Client{Id=2, Name="John"}

                };

​

        public Maybe<Client> GetById(int id) => clients.FirstOrNone(x => x.Id == id);

    }

    public class Client

    {

        public int Id { get; set; }

        public string Name { get; set; }

        public int EmployeeId { get; set; }

    }

​

​

​

}