I would add that requiring modifications to dispatch to take effect immediately is one of those classic behaviors in dynamic languages that makes them hard/impossible to compile without heroic efforts (along with reified local scopes, the ability to add state to objects dynamically at any point, etc. – we should make a list some time). In Julia we’re nipping that one in the bud by defining the language to work in a way that can be implemented in the presence of compilation without insane contortions, namely with world ages that only take effect at the top-level, just the way that @kristoffer.carlsson described.
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