using System;

using System.Collections.Generic;

using System.Linq;

using System.Threading.Tasks;

​

public class Program

{

    public static async Task Main()

    {

        Console.WriteLine(await FutureApplicative.LiftA2(Task.FromResult(10), FutureApplicative.Pure(20), (a, b) => a*b)); // выведет 200

    }

}

​

public static class FutureApplicative

{

    public static Task<A> Pure<A>(A a) => Task.FromResult(a);

​

    public static async Task<C> LiftA2<A, B, C>(Task<A> ta, Task<B> tb, Func<A, B, C> map2)

    {

        await Task.WhenAll(ta, tb);

        return map2(ta.Result, tb.Result);

    }

}