Why functional languages?

Functional languages use a different paradigm than imperative and object-oriented languages. They use side-effect-free functions as a basic building block in the language. This enables lots of things and makes a lot of things more difficult (or in most cases different from what people are used to).

One of the biggest advantages with functional programming is that the order of execution of side-effect-free functions is not important. For example, in Erlang this is used to enable concurrency in a very transparent way. And because functions in functional languages behave very similar to mathematical functions it's easy to translate those into functional languages. In some cases, this can make code more readable.

Traditionally, one of the big disadvantages of functional programming was also the lack of side effects. It's very difficult to write useful software without IO, but IO is hard to implement without side effects in functions. So most people never got more out of functional programming than calculating a single output from a single input. In modern mixed-paradigm languages like F# or Scala this is easier.

Lots of modern languages have elements from functional programming languages. C# 3.0 has a lot functional programming features and you can do functional programming in Python too. I think the reasons for the popularity of functional programming is mostly because of two reasons: Concurrency is getting to be a real problem in normal programming because we're getting more and more multiprocessor computers; and the languages are getting more accessible.

I don't think that there's any question about the functional approach to programming "catching on", because it's been in use (as a style of programming) for about 40 years. Whenever an OO programmer writes clean code that favors immutable objects, that code is borrowing functional concepts.

However, languages that enforce a functional style are getting lots of virtual ink these days, and whether those languages will become dominant in the future is an open question. My own suspicion is that hybrid, multi-paradigm languages such as Scala or OCaml will likely dominate over "purist" functional languages in the same way that pure OO language (Smalltalk, Beta, etc.) have influenced mainstream programming but haven't ended up as the most widely-used notations.

Finally, I can't resist pointing out that your comments re FP are highly parallel to the remarks I heard from procedural programmers not that many years ago:

• The (mythical, IMHO) "average" programmer doesn't understand it.
• It's not widely taught.
• Any program you can write with it can be written another way with current techniques.

Just as graphical user interfaces and "code as a model of the business" were concepts that helped OO become more widely appreciated, I believe that increased use of immutability and simpler (massive) parallelism will help more programmers see the benefits that the functional approach offers. But as much as we've learned in the past 50 or so years that make up the entire history of digital computer programming, I think we still have much to learn. Twenty years from now, programmers will look back in amazement at the primitive nature of the tools we're currently using, including the now-popular OO and FP languages.

The main plus for me is its inherent parallelism, especially as we are now moving away from more MHz and towards more and more cores.

I don't think it will become the next programming paradigm and completely replace OO type methods, but I do think we will get to the point that we need to either write some of our code in a functional language, or our general purpose languages will grow to include more functional constructs.