Extensions to Join all the elements in the collection calling the functor and using separator

Fluid enumerable join to string.cs

        public static string Join<T>(this IEnumerable<T> collection, char separator, Func<T, string> fn)
        {
            return collection
                .Head()
                .Aggregate(new StringBuilder(), (b, x) =>
                                                    {
                                                        b.Append(fn(x));
                                                        b.Append(separator);
                                                        return b;
                                                    })
                .Append(collection.Last())
                .ToString();
        }