FP is easier to reason about because humans have a hard time with implicit state. Just because you don't know it doesn't mean its hard. That's why functional-ish things like Redux are taking off, and every new language except Go has incorporated significant functional features.
> Just because you don't know it doesn't mean its hard
I spent a long time learning Haskell and have written enough Clojure especially to safely call myself an expert in it. I obsess over languages (not proud of that btw). FP is hard. It's too hard for most programmers. A lot of people avoid saying this but it's true. It's hard in a way that you can't fix with experience.
That's why the tooling for it sucks. Suppose you want to make an IDE or even an API for your product. Do you want a lot of people to use it? Then you use ${BLUB}. Heavily abstract stuff dies. Look at Tensorflow vs. Pytorch. Tensorflow 1.0 had this wonderful functional, declarative, immutable way of writing neural networks, but people protested because they couldn't mutate variables in a for loop and have it automagically autodifferentiate. Abstraction kills network effects.
> every new language except Go has incorporated significant functional features
None of them come close to the semantics of FP (except Scala, but its negative adoption rate kind of proves my point). They all allow side effects which breaks the first rule.
You can say they stole some of the convenient syntactic sugar from FP, like pattern matching and lambdas (e.g., Kotlin and Rust). Great. So what advantage do FP langs have now? Making it hard to do I/O? Category theory? Having unmaintained compilers?
