Yes this. With I often think about questions like:

- Is this code useful in other contexts? Are there other contexts where I might want this? (Or someone else might want this?)

- Modules have a clear API boundary

- Modules can be described independently of the code which uses them

- Modules can (and should) have their own testing suite, independent of the test suite of the containing module

There's lots of examples; but you can kind of spot code like this in your projects. It has a property of being disentangled. "This just solves this one specific problem I have with strings / event emitters / random numbers / my database, separate from anything else I'm trying to do". "I kinda want to just document this code separate from the documentation of everything else". "I don't want to pull that whole library in, but can I just steal these 3 functions for this other project?"

> This is a common problem with improper application of the idea of modularity as well as OO concepts (in particular, breaking things into classes and thinking it creates a module).

Exactly! The class is not the smallest unit of modularity. You unionize several pieces of mutable state with several functions (aka methods) in class based programming and if your unionization was a design flaw you're pretty much screwed. To make your code truly modular you need to break it down further. Separate state and function. To go even deeper make the function pure and make state immutable. Abstract mutability to a small unsafe portion of your code.

>The reality is that there are two (at least) kinds of modularity. "Syntactic" (I need a better name for this) modularity like Java classes, or C's translation units (why I need a better name). These act as modules, but don't necessarily define a real, proper module. And then there are your real modules which are comprised of the things that must exist together (like in the earlier example), regardless of the project structure or language's notion of a module.

The only true logic module is a stateless function that is independent of all context. Think on that, that is literally the smallest unit of logic that can be moved around anywhere.

The problem here is how do you program in a way such that even for these functions are easily de-composable and rearranged without necessitating a rewrite in the case where you find the logic needs massive changes?

Use point free programming with pure non-procedural functions solves this. The problem is mutability must happen somewhere and usually this is abstracted to a very small section of your code.

>I had a team try to convince me their code was "modular" but their GUI portion was directly tied to the DB portion and other logic. Nothing could be instantiated separately, so it wasn't really modular, it was just using the language and build system's notions of modules to create the illusion of modularity (C++ and VS in their case). In contrast, another team had developed a collection of libraries (C# and VS again) that were used in multiple applications. That was real modularity.

GUI programming is inherently hard as the state of the GUI is by nature muteable and it's hard to write modular code as a result. However modern programming frameworks generally get around this using the technique I outlined above, see React redux and MVU. https://guide.elm-lang.org/architecture/