In a couple of previous posts I’ve provided what I believe to be a good definition of a functional language (Functional Programming Definition) and why a functional model is ultimately required due to the constraints of reality (Everything is Functional).
However, I don’t know that I’ve been clear about exactly why functional programming is so beneficial. I think the benefits can be summed up in one word: predictability.
When I’m working with functional code, I know that I can lift any part from any where and get identical behavior given the same inputs wherever I may need it. When I know the language enforces these rules, I can be supremely confident in this quality of the code, even unfamiliar code.
When working in languages like C++, Java, and Python, I go to some lengths to try to maintain a functional quality to my code. It can be done with most languages. However, when veering into unknown code, my confidence in my ability to modify without odd unexpected side-effects drops dramatically, and it makes those changes much more of a slog. As bad a practice as it is generally recognized to be, I still bump into the occasional global mutable variable or static class member. It doesn’t take many of those to wreck your day.
I think a second closely-related benefit of the functional style is composability. When you have supreme confidence in your components, putting them together can be a snap. This benefit has a little more dependence on good architecture and design practices, but outright avoidance of the features that make code-reasoning difficult will certainly have an impact.
In reference to my earlier post, everything is functional if you’re willing to zoom out far enough. You could start up another process, container, VM, or cluster to restore your faith in the predictability of your code. Note that this approach has costs that could be fairly easily avoided by starting functionally from the start.
Disclaimer: it’s always possible to write bad code in any language or style. However, it can be argued that it’s much more difficult to reach the spaghetti-horizon where the functional style is enforced.