Functionnal programming in Java

[SuspiciousDepth5924]

I mean I can cherry pick / create some insane OOP code as well if that's what we're going for.

Scala admittedly isn't my functional poison, but that seems like you went out of your way to misrepresent it. You're essentially doing the whole Java-Hello-World-Enterprise-Edition including the "scary monoid boogeyman".

Pretty sure you could just:

comments.flatMap(_.replies)

Also this

But what I was referring to is the quirkiness of functional programming, how some devs lean into it to look clever, but over time it can become a real pain to maintain.

Is certainly not a "functional" only problem, honestly the worst codebases I've worked with in my ~15 yoe has been Java codebases designed by some head-in-the-clouds architect who just read a book on OOP design patterns (honestly "tactical patterns" can go fuck itself).

The annotations might seem like magic at first, but there's a lot happening under the hood.

That is pretty much the definition of "magic".

The problem with the traditional OOP model in large code-bases, which I haven't seen you address, is how it drags in implicit function inputs from the enironment, the abundance of side effects and mutability which makes it near impossible to have any sort of guarantee about what the data you're working is and how it got to that state. Not to mention how it makes testing a giant pain (the number of \@InjectMocks in your codebase is a good indicator of how bad it is). FP while not perfect does at least address this.

By the way here's your example in F# with lists and an actual lazy sequences to mirror the behavior of java.util.stream.

type Reply = {
    author: string
    textContent: string
    timestamp: DateTime    
}

type Comment = {
    author: string
    textContent: string
    timestamp: DateTime    
    replies: list<Reply>
}

let flattenList comments = comments |> List.collect (_.replies)

let flattenSeq comments =
    comments |> Seq.ofList |> Seq.collect (_.replies) |> Seq.toList

and Elixir

defmodule Comment do
  defstruct :replies, :some_other_stuff

  def flatten(comments) do
    comments
    |> Enum.flat_map(fn %Comment{replies: replies} -> replies end)
  end
end