It’s still heavily object orientated. My preferred approach to applications these days is immutable core / imperative shell and Java makes that awkward.
Of course you can't have the pure/impure distinction enforced, but nothing that isn't a purely functional language is going to give you that.
The only really dumb limitation is that you can't have freestanding functions, but that's aesthetics more than substance.
https://www.destroyallsoftware.com/screencasts/catalog/funct...
Basically you have business domain objects that have value semantics and are immutable. You have functions that manipulate these objects. This makes the business domain unit-testable. Then on the edge of the application you have imperative code that manipulates databases, serves requests, etc. It’s like Haskell but as a design pattern rather than a language.
In the JVM world, Scala and Clojure communities are adopting this approach.
It is essentially straight forward to do. You look at a bunch of code in a module and determine which parts do these kinds of things:
- data integrity, validation
- parsing, transformation between formats
- domain logic, decision making
- algorithms and data structures
This is your functional core. Note it doesn't matter too much if you have local mutations as long as they don't leak out. What you want to achieve is to have an interface where you can ask questions and you get the same answer for a particular question. This is certainly more productive if your language affords you with functional constructs, but its not strictly necessary to get the benefits.
You code these parts in a way that they are data in, data out essentially. This leads to super easy and reliable unit/example tests and generative tests. And this part becomes very simple and easy to reason about.
Secondly you have the part of your code that actually _does_ stuff, affecting your environment in some way.
- basically any kind of I/O
- cross cutting concerns between other side effecting modules and services (think DI and such)
- error handling, error reporting and logging
- caching, throttling, filtering etc.
That's your imperative shell (I just call them modules). This part you write in an imperative/procedural style and you call the functional core parts from here, never the other way around. You coordinate effects so to speak.
It is clear that you test these with integration tests, where you need to setup specific program state (or use mocks, but I try to avoid that) in order to find different code paths. This is harder to test properly but at the same time you also get to see everything that actually happens in condensed form.
The degree to which you extract "pure" logic from your imperative shell is actually quite flexible. You can go as far as defining purely functional FSMs to extract much of the control for example. I think it really depends on code size and complexity as well as performance implications. The cool thing is that you can be quite pragmatic and iterative about it.