[malft]

The type looks a little insane, but it's just a combination of three mundane hacks that also exist in other languages.

The first: If you need a serializer/deserializer in a simple language like old-school Java, you can either write two methods for each class, or you can combine them into one method like this:

class HoveringSkull inmplements Streamable { String desc; float height; void stream(Stream c) { desc = c.stream(desc); height = c.stream(height); }

... and have two different implementations of Streamable. That's what the choice of (pro-)functor instance does -- you can swap out the implementation to get 'read' and 'write' (and 'deepClone' and 'equals' and 'hashCode' and 'defaultGui' and 'treeMatch' and ...)