You're moving goalposts a bit here, I think. Or maybe it's my fault, I should have defined 'in-depth' more precisely. Anyway, I specifically mentioned knowing

You're moving goalposts a bit here, I think. Or maybe it's my fault, I should have defined 'in-depth' more precisely. Anyway, I specifically mentioned knowing "some facts about the implementation" instead of "knowing the implementation inside-out". The same is true for tools, libraries, and frameworks - there is no need to remember every function/method signature and every class name in a library - as long as you know the most important parts and can quickly and accurately find the relevant documentation for the rest. To me, 'knowing in depth' (as a language user) means that no matter the question, you know where to search for the answers. There's no need to remember the answers themselves, although it kind of happens naturally with repetition anyway. On the other hand, it's also important to know which questions are not worth answering. It's exactly as you say: > Really learning what you can safely ignore in whatever context is the most important part So, to sum it up: if you can do both these things, then, to me, you have in-depth knowledge about the language (or actually any other field). The next step - ie. actually knowing all the answers by heart - is mostly useless for language users and only matters for (and is best left to) language implementers/advocates/nerds. > A language like Java has a giant and evolving standard library plus multiple compilers etc. And that’s ignoring any simply popular libraries etc. Java stdlib is only "huge" in comparison with C, where stdlib is non-existent. The popular libraries may be ported from other languages or even directly called via FFI. And if the library is really good, it will be ported to other languages, too, which means you'll have an easier time using these languages in the future. Even without all that, though, if you know more-or-less what's where and can quickly search for the details - you're good. It's not that easy to learn this - you need to remember the structure of the docs at least - but it's much easier than actually learning and remembering all of the stdlib, etc.