...and Common Lisp and Smalltalk take it a step further. You can go ahead and change the data structure in a way that's semantically incompatible. The runtime will notice that the definition has changed and either automatically reinitialize any existing instances to conform to the new definition, or, if it doesn't know how to do that, pose a UI asking you to tell it how.
And when control reaches a function that expects the old kind of object and runs into an error, it'll enter a breakloop that you can use to modify the now-out-of-date function to handle the new definition properly. Once you've redefined the function you can then tell the runtime to resume and it will proceed as if the function you called had originally had the new definition.
And when control reaches a function that expects the old kind of object and runs into an error, it'll enter a breakloop that you can use to modify the now-out-of-date function to handle the new definition properly. Once you've redefined the function you can then tell the runtime to resume and it will proceed as if the function you called had originally had the new definition.