During my first Introduction to Programming course at university, I was taught Java

The language is the easy part. Getting tool support for your language change is the hard part. Getting the library ecosystem to adopt it is even harder. I think that's why extremely flexible languages have seen limited adoption - if your language is more of a language construction kit where everyone can implement their own functionality, everyone has to implement their own tool support (or, more likely, live without any) and there's a limit to how far you can go with that. The best languages find the sweet spot where they give you enough flexibility to implement most reasonable programs, but are still constrained enough that tools can understand and work with all possible code.

Too much change isn't good though. There's value in consistent basics. I've seen people doing things like: #define BEGIN { #define END } because they liked Pascal, and that way lies madness.

> I also remember the TA saying "why would you want that?" Is a typical response of someone without the background and without the imagination. It may well be, that doing Java-only for too long robs one of both. An alternative response could have been: "What a fascinating idea! How would you apply that? / What would you do with that?" I am happy for you, that you found the exceptions and that your picture of the computer programming world is not as incomplete and bleak as that of the TA back then.

There is metaprogramming support in Java, but it's not as inviting and friendly as hygienic macros or Ruby patching. The obvious example is reflection, with which you can do a lot of bizarre things, some of which are useful sometimes. Another is annotations, which is heavily used by libraries in a way similar to how macros are used in certain Lisp like languages. https://www.baeldung.com/java-reflection https://www.baeldung.com/java-annotation-processing-builder Then you've got the byte code itself, and there be dragons: < https://aphyr.com/posts/341-hexing-the-technical-interview > While you rarely see byte code shenanigans in Java code bases, it's how some other languages on the JVM achieve things like runtime metaprogramming.

The reason “custom” programming languages (sometimes called macros) are not popular is that (statistically) no one wants to learn a custom language for each project. People want to learn the basics as few times as possible. Orgs prefer to standardize, most businesses are not snowflakes. It can be done, but it is not economical, and therefore not practical.