DNEAVES

To answer the headline: Functional languages are as useful in pretty much any context non-functional ones are, they just need to be handled a little different.

Functional languages’ goal tend to lean towards static or predictable typing and reduction of clandestine mutable state changes. State has to change in a program, it’s just about how its handled, and this is more about reducing referential update behavior.

Like in Python, you can have a list, pass that to a function that returns None, but deep inside that function appends to the list, and you may be surprised that your list is different as a result. This would (typically) not happen in an FP language, you would have to explicitly return the changed list up the stack of functions, and as a dev you could see from a glance that the list could change as a result of the function, leading to less surprises.

“IO” is of course an outlier, as it’s leaving the safety bubble the language provided. I could read the same database row twice in 60 seconds and get a different result, because someone else could make a transaction in that time. But once I have read the row and parsed it into a data-model, I can expect that my model in-code won’t change unless I tell it to.

I think the shallowness of understanding also stems from the codebase itself, and moreso that than the Ai finding where something is.

A poorly organized codebase, dynamic file imports lsp’s can’t follow, lack of annotations on unclearly-named arguments, etc. are way worse for understanding a codebase than Ai saying “the thing you need is this method”, which Ai could also explain how it got there at least. At that point, Ai is a better than what they would have without it.

Write clean, understandable code. Document it, too. Everyone will thank you

Functional language developers are obsessed with side effects but I rarely ever see problems with it in real code. Like I can remember once in maybe 10 years setting a developer write a “get” method that also wrote to a file. Caught during code review, rewritten easily.

While, sure, get methods writing files could happen in side-effectful code, the side-effect concern is less about that and more of expectations. With OOP languages, class methods can create a dichotomy of expected returns: Using a list reversal as an example, will a list.reverse(some_list) function reverse the list in-place, or return a copy of the reversed list and leave the original in-tact? To know the answer, you have to check the docs or the type signature (which, yes, is easy, but the answer isn’t obvious). Which, the main concern is trying to debug things “magically changing”, since nested function calls and inherited function calls can modify something from layers upon layers of abstraction away.

FP languages tend to not have this dichotomy, with very few exceptions like IO in Haskell, ports in Elm and Gren, external functions in Gleam, or FFI in Haskell, to name some ways from languages I know, all of which usually move the control of the program outside the language’s “default area”, anyway. Outside the exceptions, to reverse a list a reverse function needs to be given that list explicitly, and it also will be returning a copy of the list reversed, only and always