Functor in C#

Functor.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;

namespace FuncSharp
{
    public interface IFunctor<FA, FB, A, B>
    {
        FB Map(FA fa, Func<A, B> f);
    }

    public class OptionFunctor<A, B> : IFunctor<IOption<A>, IOption<B>, A, B>
    {
        public IOption<B> Map(IOption<A> fa, Func<A, B> f)
        {
            return fa.Map(f);
        }
    }

    public class TaskFunctor<A, B> : IFunctor<Task<A>, Task<B>, A, B>
    {
        public Task<B> Map(Task<A> fa, Func<A, B> f)
        {
            return fa.ContinueWith(task => f(task.Result));
        }
    }

    public class ListFunctor<A, B> : IFunctor<List<A>, List<B>, A, B>
    {
        public List<B> Map(List<A> fa, Func<A, B> f)
        {
            return fa.Select(f).ToList();
        }
    }

    public class FunctorTest
    {
        public void Run()
        {
            var o1 = 3.ToOption();
            var o2 = AddOne(o1, new OptionFunctor<int, int>());

            var t1 = Task.FromResult(3);
            var t2 = AddOne(t1, new TaskFunctor<int, int>());

            var l1 = new List<int> { 1, 2, 3, 4, 5 };
            var l2 = AddOne(l1, new ListFunctor<int, int>());
        }

        public FB AddOne<FA, FB>(FA fa, IFunctor<FA, FB, int, int> functor)
        {
            return functor.Map(fa, i => i + 1);
        }
    }
}