I am suspicious of claims that procedural, imperative programming is “easier” than functional programming: in terms of how quickly one can learn it, how accurately one can predict what a program does by reading it, and how accurately one can make a modification. Assuming two developers both of equal experience in their preferred paradigms and languages I expect the functional programmer will score higher in these cases.

The cost of functional programming is that it does take some training to write programs in this style that many people are not familiar with. And I think it’s that lack of familiarity that people are referring to when they say that it is more “difficult.”

Personally I can’t disagree that it is harder to learn if you are already familiar with procedural programming. It took me quite a few months of dedicated work to get productive with Haskell. It was a painful, as I recall, because I had to throw out all of the accumulated experience I had with a handful of other languages.

Now that I am productive with it though I don’t find it any more “difficult” than programming in C or JavaScript or Python. In fact I find it easier: I can rely on the type system and reason about my programs and how they compose instead of stepping through their execution. Functional programming demands that I debug my thinking rather than my program most of the time. The former takes less time in my experience and becomes less frequent the more experienced you become.