Attempt at C# Functors

Functors.cs

    public static class Functors
    {
        public static IEnumerable<B> fmap<A,B>(this IEnumerable<A> functor, Func<A, B> f)
        {
            return functor.Select(f);
        }

        public static B? fmap<A, B>(this A? functor, Func<A, B> f) where A : struct where B : struct
        {
            return functor == null ? null : (B?) f(functor.Value);
        }
    }

    public class FunctorTests {
        [Test]
        public void Fmap_over_collection()
        {
            var collection = new[] { 1, 2, 3 };
            var result = collection.fmap(x => x+1);

            Assert.That(result, Is.EqualTo(new[] {2,3,4}));
        }

        [Test]
        public void Fmap_over_nullable()
        {
            int? justValue = 2;
            int? nothing = null;

            Assert.That(justValue.fmap(x => x + 1), Is.EqualTo(3));
            Assert.That(nothing.fmap(x => x + 1), Is.Null);
        }

        [Test]
        public void Fmap_over_string()
        {
            Assert.That("hello".fmap(Char.ToUpper), Is.EqualTo("HELLO"));
        }

        [Test]
        public void Fmap_converting_collection_to_different_type()
        {
            var collection = new[] { 1, 2, 3 };

            Assert.That(collection.fmap(x => x.ToString()), Is.EqualTo(new[] { "1", "2", "3" }));
        }
    }