using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        var range = Enumerable.Range(1, 10);

        Console.WriteLine(EnumerableFunctor.Map(range, x => x).SequenceEqual(range)); // выведет True

​

        int? nullableTen = 10;

        int? nullableNull = null;

        Console.WriteLine(NullableFunctor.Map(nullableTen, x => x) == nullableTen); // выведет True

        Console.WriteLine(NullableFunctor.Map(nullableNull, x => x) == nullableNull); // выведет True

    }

}

​

public static class EnumerableFunctor

{

    public static IEnumerable<B> Map<A, B>(IEnumerable<A> source, Func<A, B> map) =>

        source.Select(map);

}

​

public static class NullableFunctor

{

    public static B? Map<A, B>(A? source, Func<A, B> map) where A : struct where B : struct =>

        source is A notNullSource ? map(notNullSource) : default(B?);

}