[drnewman]

I'd think the preferred way would be to implement them as a separate class. That'd be more in line with the design of other core classes (like TrueClass & FalseClass). It'd also have created the opportunity to create a "Callable" protocol early in the design which would have been great, and would harmonize with the design of Ruby's many other protocols Enumerable, Comparable, Range-compatible values, etc.