Trying out the typeclass encoding proposed in Classes for the masses

GenericFib.cs

using System;
using System.Linq;
using System.Collections.Generic;

namespace ClassesForTheMasses
{
    public interface Zero<A> { A Zero(); }
    public interface One<A> { A One(); }
    public interface Add<A> { A Add(A x, A y); }

    public interface From<A, TFrom> { A From(TFrom x); }

    public interface Num<A> : Zero<A>, One<A>, Add<A>, From<A, int> {};

    public struct NumInt : Num<int> {
        public int Zero() => 0;
        public int One() => 1;
        public int Add(int x, int y) => x + y;

        public int From(int x) => x;
    }

    public struct NumLong : Num<long> {
        public long Zero() => 0;
        public long One() => 1;
        public long Add(long x, long y) => x + y;

        public long From(int x) => x;
    }

    public struct NumDouble : Num<double> {
        public double Zero() => 0;
        public double One() => 1;
        public double Add(double x, double y) => x + y;

        public double From(int x) => x;
    }

    public static class GenericMath {
        public static IEnumerable<T> Fib<T, TNum>() where TNum : struct, Num<T> {
            var num = default(TNum);
            var prev_1 = num.One(); 
            var prev_2 = num.One(); 

            yield return prev_1;
            yield return prev_2;

            while (true) {
                var next = num.Add(prev_1, prev_2);
                yield return next;
                prev_2 = prev_1;
                prev_1 = next;
            }
        }
    }

    public class Program
    {
        public static void Main(string[] args)
        {
            var fibs = GenericMath.Fib<int, NumInt>().Take(25).ToList();
            Console.WriteLine("[{0}]", String.Join(", ", fibs));
        }
    }
}