using System;

using System.Collections;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var eta = Enumerable.Range(3, 2);

        var etb = Enumerable.Range(15, 4);

​

        int? nta = 10;

        int? ntb = null;

​

        Print(EnumerableApplicative.LiftA2(eta, etb, (a, b) => (a, b)));  // Combine

        Print(NullableApplicative.LiftA2(nta, nta, (a, b) => (a, b)));  // Combine

        Print(NullableApplicative.LiftA2(nta, ntb, (a, b) => (a, b)));  // Combine

        Print(ZipListApplicative.LiftA2(new ZipList<int>(eta), new ZipList<int>(etb), (a, b) => (a, b)));  // Combine

    }

​

    private static void Print<T>(T value) where T : struct

    {

        Console.WriteLine(value);

    }

​

    private static void Print<T>(T? value) where T :struct

    {

        Console.WriteLine(value?.ToString() ?? "Null");

    }

​

    private static void Print<T>(IEnumerable<T> source)

    {

        Console.WriteLine($"[{string.Join(", ", source)}]");

    }

}

​

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?)

        };

}

​

public static class ZipListApplicative

{

    public static IEnumerable<A> Pure<A>(A a) =>

        Enumerable.Repeat(a, int.MaxValue); // вообще тут должен быть бесконечный генератор

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

        new ZipList<C>(ta.Zip(tb, map2));

}

​

public class ZipList<T> : IEnumerable<T>

{

    private readonly IEnumerable<T> _inner;

​

    public ZipList(IEnumerable<T> inner)

    {

        _inner = inner;

    }

​

    public IEnumerator<T> GetEnumerator()

    {

        return _inner.GetEnumerator();

    }

​

    IEnumerator IEnumerable.GetEnumerator()

    {

        return ((IEnumerable)_inner).GetEnumerator();

    }

}