Not at all trying to diss the OP, who wrote an interesting article, but if anything, this has put me off FP a bit. It really does seem like a lot of effort to go through for unclear benefits.
I have no doubt that learning FP will make me a better programmer (so perhaps it is worth it for that alone) but it seems to me I should rather spend those hours learning more data structures, or algorithms or machine learning.
Not sure I get it.
edit: it appears partially to be a definition problem. Exactly what is meant by FP and in what language, is a large part of the question.
I'm not sure what part of reducing a codebase by 2/3rds but with double flexibility is an "unclear benefit". Imagine if you could do that with a C# or Java library, people would be freaking out. Or getting almost all the safety of unit tests without writing and maintaining unit tests! That's awesome!
You want to be a lot better programmer? Go through SICP! It'll cover FP, data structures, interpreters, algorithms, and OO. You think you know OO now? I'd wager the chapter on OO will blow your socks off with awesome stuff you can use right now. And you'll learn FP enough to give you a taste of what's possible in the more powerful languages like Haskell.
Rather than sit around trying to figure out if it'll be worth it, just do it. I've never heard a programmer who has learned it who has said it was a waste of time. So then, what are you waiting for? No study will ever prove its better, just like no study proved Java was better, or C++ was better, or C was better. It's impossible to prove. Was each objectively better? In some ways. Is Haskell objectively better than all of them? Yup. There's all the proof you're going to get: opinions of those who know all of them. You either trust that, or you stay comfortable and fall behind.
I have no doubt that learning FP will make me a better programmer (so perhaps it is worth it for that alone) but it seems to me I should rather spend those hours learning more data structures, or algorithms or machine learning.
Not sure I get it.
edit: it appears partially to be a definition problem. Exactly what is meant by FP and in what language, is a large part of the question.