using System;

​

public delegate T IMaybe<out T>();

​

public static class Maybe

{

    public static IMaybe<T> Return<T>(T x)

    {

        return () => x;

    }

​

    public static IMaybe<T> Nothing<T>()

    {

        return () =>

        {

            throw new InvalidOperationException("Нельзя получить значение");

        }

​

        ;

    }

​

    public static IMaybe<TResult> Bind<TArg, TResult>(this IMaybe<TArg> maybe, Func<TArg, IMaybe<TResult>> func)

    {

        return () => func(maybe())();

    }

​

    public static IMaybe<TResult> Select<TArg, TResult>(this IMaybe<TArg> maybe, Func<TArg, TResult> func)

    {

        return maybe.Bind(value => Return(func(value)));

    }

​

    public static IMaybe<T> Where<T>(this IMaybe<T> maybe, Func<T, bool> predicate)

    {

        return maybe.Bind(x => predicate(x) ? Return(x) : Nothing<T>());

    }

​

    public static IMaybe<TC> SelectMany<TA, TB, TC>(this IMaybe<TA> ma, Func<TA, IMaybe<TB>> maybeSelector, Func<TA, TB, TC> resultSelector)

    {

        return ma.Bind(a => maybeSelector(a).Select(b => resultSelector(a, b)));

    }

}

​

public class Program

{

    public static void Main()

    {

        try

        {

            var one = Maybe.Return(1);

            var nothing = Maybe.Nothing<int>();

            var nothing2 =

                from ax in one

                from ay in nothing

                select ax + ay;

            var two = one.Where(z => z > 0).Select(z => z + 1);

            Console.WriteLine(one());

            Console.WriteLine(two());

            Console.WriteLine(nothing2());

        }

        catch (Exception e)

        {

            Console.WriteLine(e);

        }

    }

}