"they need to require all other programmers to annotate their extensions with appropriate metadata."
If Nulan had multimethods, I'd probably just require programmers to add additional metadata to the object in addition to the %pattern-match gensym. But Nulan doesn't have linearization or multimethods, so I avoid that whole mess!
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"Whereas Magpie and many other multimethod systems make a big deal about subclasses"
And I have to agree: objects are amazing, but classes and subclasses are terrible. In fact, I'm of the opinion that probably all hierarchial relationships are too restrictive. Hence Nulan's object system which is completely flat, but has pretty much all the benefits of classes/prototypes.
Basically, the behavior that is common to all objects (of a particular kind) is put into a function, and behavior that is specific to a particular object is put into the object itself. And immutability gives you wonderfully clean semantics for prototype/class behavior, without all the complication and baggage.
