World Age for beginners

How a dynamic language gets to compile anyway, explained from the question every newcomer asks.

For a long time “world age” was, to me, just the name of an error I got when I used eval inside a function and then tried to call what I had defined. The manual calls it an advanced topic most people never need, so I never looked further. This Discourse post changed that. It starts from a simpler question, why dynamic languages are usually slow, and only names world age once you already understand the problem it solves.

The point is that a compiler can only optimize what it can assume. Julia lets you redefine almost anything, so it promises one thing in return: a running call keeps seeing the world as it was when it started, and new definitions show up on the next call. That single rule lets Julia compile and cache code ahead of each call, and it explains why invokelatest exists. If you want the formal version, the post points to Belyakova et al. (OOPSLA 2020).

World Age for beginners: one way to compile a dynamic language, Julia Discourse.