Monoid and Foldable for C#

Monoid.cs

    public interface Monoid<A>
    {
        public A Empty => default(A);
        A Append(A x, A y);
    }

    public static class Foldable
    {
        public static A Fold<A>(this IEnumerable<A> list, Monoid<A> M) =>
            list.FoldMap(x => x, M);

        public static A FoldMap<A, B>(this IEnumerable<B> list, Func<B, A> f, Monoid<A> M) =>
            list.Aggregate(M.Empty, (p, c) => M.Append(p, f(c)));
    }

    public class SumDecimal : Monoid<decimal>
    {
        public decimal Append(decimal x, decimal y) => Decimal.Add(x, y);
    }


var people = new List<Person> 
{
    new Person { Name = "Reid", Age = 36 },
    new Person { Name = "Miles", Age = 6 }
};
var totalAge = people.FoldMap(x => x.Age, new SumDecimal());
//totalAge = 42