I just ran down a nasty bug related to this code below:

I did not realize that every time I was creating a new list with List(), I was getting the same instance of List. This seems counterintuitive in a library based on the idea of immutability.

Some code in the project was mutating an instance of an empty list which caused all subsequent newly minted lists to be non-empty. Obviously the code in my project was incorrect, but I came here to suggest not using a singleton pattern here regardless.