I have a dictionary (in RPython) that looks something like this: class MyDict(Obj): def add(self, k, v): newdict = self.dict.copy() newdict[k] = v self.dict = newdict def get(self, k): d = self.dict return MyDict._static_get(d, k) @staticmethod @purefunction def _static_get(d, k): if d not in k: return None return d[k] I'm trying to figure out the best way to optimize this. As you see, this structure is "append only". That is if mydict.get("foo") returns a value it will always return that value, for all time. In that sense, the function is pure. If however, the function returns None, then it could change in the future. This is for my Clojure on PyPy implementation. The idea is that types can be extended via protocols, at runtime, at any point in the program's execution. However, once a given type is extended to support a given interface (or protocol) it will never be able to be changed. That is, once we extend Foo so that it implements Foo.count() it will implement Foo.count() for all time. Any thoughts on how to optimize this further for the jit? I'd like to promote the value of get() to a constant, but I can only do that as long as it is not None. After Foo is extended, I'd like the jit to re-generate the jitted code to remove all guards, hopefully saving a few cycles. Timothy -- “One of the main causes of the fall of the Roman Empire was that–lacking zero–they had no way to indicate successful termination of their C programs.” (Robert Firth)