using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

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

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

​

        int? nullableTen = 10;

        int? nullableNull = null;

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

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

    }

​

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

        EnumerableApplicative.LiftA2(source, EnumerableApplicative.Pure(0), (a, _) => map(a));

​

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

        NullableApplicative.LiftA2(source, NullableApplicative.Pure(0), (a, _) => map(a));

}

​

public static class EnumerableApplicative

{

    public static IEnumerable<A> Pure<A>(A a) => new[] { a };

    public static IEnumerable<C> LiftA2<A, B, C>(IEnumerable<A> ta, IEnumerable<B> tb, Func<A, B, C> map2) =>

        ta.SelectMany(a => tb.Select(b => map2(a, b)));

}

​

public static class NullableApplicative

{

    public static A? Pure<A>(A a) where A : struct => a;

    public static C? LiftA2<A, B, C>(A? ta, B? tb, Func<A, B, C> map2) where A : struct where B : struct where C : struct =>

        (ta, tb) switch

        {

            (A a, B b) => map2(a, b),

            _ => default(C?)

        };

}

​