Free Monad as Functor

.cs

    public abstract class Free<T>
    {
        public abstract Free<T1> Map<T1>(Func<T, T1> map);

    }

    public class Pure<T> : Free<T>
    {
        private T source;
        public Pure(T v) => source = v;
        public override Free<T1> Map<T1>(Func<T, T1> map) => new Pure<T1>(map(source));
    }

    public class Roll<T, F> : Free<T> where F : IFunctor<Free<T>>
    {
        private F source;
        public Roll(F v) => source = v;
        public override Free<T1> Map<T1>(Func<T, T1> f) => new Roll<T1, IFunctor<Free<T1>>>(source.Map(x => x.Map(f)));
    }