I think one of the problems that people who get overexcited about functional languages and all the strong typing and such have is that they generally start out in a bad place. Imperative programs with every flaw in the book, threading by locks, for loops that try to modify the index mid-loop, mutation running rampant, all the bad things they complain about. I've worked in those code bases in industrial contexts, they can be nightmares for sure.

So they discover functional programming, and they just get hammered with so very many different ways of doing things. At first it all seems impossible, but then it slowly unlocks itself, and look! All the problems with imperative programs went away! You didn't even properly perceive them before, but they're gone.

You can't hear me, so my tone here is completely serious. I strongly recommend any professional programmer spend some time with a language like this and attain enough fluency to write real, non-trivial code in it, not just map a few lists and maybe use a monad or two. Get something that hits the network or something.

And the problem is, too many of them stop here. Conventional programming sucks. Functional programming rules. Anyone who doesn't use functional programming languages is a loser still wandering blind in the wilderness.

However, there are in fact a lot of practical problems with functional programming languages too, many in the original blog post, others expressed elsewhere. It can be as simple as, that critical library I need is not available in my obscure niche functional programming language, but alas, in the real world, that's enough to be a determining factor.

Where the religious-level advocates of FP lose contact with reality is that there's a third option: You take what you learned in the functional programming world and come back into conventional languages. And I don't mean "use a few maps and jam a monad in to your language even though it completely fails to fit"; that's actually still completely missing the point of functional programming. I mean, you start writing "conventional" code except you pay attention to mutation. You may not write completely pure code everywhere, but the more you mix in the more characteristics of pure code, the more of the benefits you get. (I do think there's an interesting discontinuity at 100% pure, when you can 100% count on it and then build further on that ability to be sure all code is pure, but you do still get a gradient of benefits the more purity you put into your conventional code.) You pay attention to side effects and start isolating them into units instead of mixing them in. You learn how to multithread with messages instead of memory sharing and locks. You don't drag in inappropriate APIs from a foreign paradigm; you take the fact your eyes were opened, and you write code with those now-opened eyes.

And it is not perfect. You will still occasionally have the original imperative problems. But you will have radically, radically fewer of them, so few that the cost/benefit analysis of using the super-strong stuff becomes very difficult to justify, especially over the advantages of being able to use that library you really need. (And see the library has a mutation problem and wrap it in a way that solves it for you, instead of letting it drag the rest of your code base down, etc.)

I have ridden the mighty moonworm... errr... I have fiddled with the Haskell type system and done some interesting things with it. But by and large they really weren't worth it, not in the sense that they don't solve some problem, but in the sense that back in the conventional programming world, I really don't have those problems anymore. No credit for solving problems I don't have.

This is where I diverge with people complaining about not using functional programming. They are comparing writing imperative crap with writing pristine functional code. In this context I don't deny I'd take the functional code too. But I am comparing writing eyes-open conventional code with writing normal functional code. In this context the advantages are a great deal more muted and it isn't anywhere near the day-or-night level of difference... and I gotta say, pitching me back on full-on functional programming by claiming I just don't get it and I just want to write bad code and enable bad code and in general be lazy and bad is bad advocacy in almost every possible sense of that term.