Why Erlang ?

Other functional languages such as Haskell or OCaml are for most people just a bit too academic. RubyOnRails is reaching end-of-hype, it's underlying language Ruby is dog slow and not scalable. Java is far too complicated for me and without blooooated tools such as eclipse or IDEA, it is a just mess to deal with, at least that was my painful experience I made recently: I developed a prototype of a Flash video streaming server in Erlang. There exists a more complete Java open source equivalent: Red5. I have a couple of years of experience as Java developer, so I thought the easiest was just to analyze the Java source code and port it to Erlang. Wrong. The Java code is so complex, that is was easier for me to reverse engineer the proprietary RTMP video streaming protocol by analyzing the TCP/IP packet flow of a running Red5 instance by using a network sniffer instead of reading its source code. Is there any other language I think one should not choose instead of Erlang ? Python. I actually had only heard good things about it, until I got to know about its dictated indentation style, which put an abrupt end for me to any further digging into that otherwise probably great language ...

Ok, end of ranting. Developers are a highly opinionated species. Sometimes discussions among developers about the right programming language take amusant, ridiculous and even religious dimensions, despite everybody trying to be objective (well, expect for his/her preferred language). So just use the right tool for the right job and don't listen to me... But listen to what the really smart and experienced people have to say. There is an interesting thread currently going on at the Erlang mailing list. A Java coder asks whether he should learn Erlang or OCaml next. Bob Ippolito (recently interviewed) says:

When I was evaluating Python alternatives for building the core
technology behind MochiAds I tried out a bunch of languages and Erlang
was the only one that was easy for me to learn and had the right
balance of features, performance, and reliability. A year later we
have about 16 machines running 80 Erlang nodes powering about 16
different "components" of our infrastructure and 4 people working on
it at the moment (originally it was just me). It worked out so well
that we rewrote the server component of our MochiBot service in Erlang
and we've been using it to build lots of internal tools such as our
monitoring software, our single sign-on service, etc. as well. None of
us had previous Erlang experience, but we're all very comfortable with
it now.

After about a year with Erlang, I'm not sure I could part with hot
code loading, light-weight processes, and multiplexed socket IO for
writing servers. Also, Mnesia has been really useful to us to
temporarily store "real-time" data (ram_copies) so that we don't have
to make users wait for it to get batched into the SQL databases. The
distribution stuff mostly Just Works once you figure out how to set it
up (though we did have one bad experience with a network partition due
to a switch acting up, it was recoverable manually).

O'Caml is a useful language too, but for writing a network app I can't
really imagine going with anything but Erlang if you're looking for
redundancy and scale. Unless you want to write your own half-baked
Erlang-like system before even trying to solve something a little
closer to your actual problem domain.

Joe Armstrong, one of the original authors of the Erlang language and Programming Erlang author says:

Erlang/OCaml/Haskell belong to the same language family - if you learn any one
of them then learning the next one in the family will be a lot easier
than starting from scratch.

These language differ - but have the same core concepts - the idea of immutable
state - programming with immutable state is the thing that you need to learn.
The details of how you do this vary from language to language (you can
use processes
with tail recursion to model state in Erlang, or monads in haskell, etc.).

I'd start with the language that most suits your problem domain - a
rough guess might
be to think of these languages as follows:

OCaml - use as a replacement for C - good for implementing virtual
machine emulators
tightly coded non-distributed applications.

Erlang - use as a replacement for Java - good for programming
distributed fault-tolerant
applications - good support for multicores/concurrency. Good as a glue
language to
glue together components co-ordinate activities on different machines etc.

Haskell - use for implementing domain specific languages, symbolic
computations etc.

And what has Erlang in the bag for Web developers ?

Not so much yet, if you look for an easy-to-learn, convention-over-configuration one-size-fits-all framework a la RubyOnRails. Web companies use Erlang today to overcome scalability problems for web based instant messaging (e.g.: ejabberd at twitter and Meebo). Among the few publicly known partially-to-mostly Erlang powered sites are MochiAds and Slideshare.

Here a little overview about some Erlang based web servers and frameworks:

• Yaws, the most popular Erlang web server. Active development since 2002. Many contributors, lots of add-ons. If you look for an Erlang-based, Apache-like web server, than yaws is the right thing for you.
  
• Erlyweb, by Yariv Sadan. The most popular Erlang MVC framework, built on top of yaws, enables you to easily do any-web-thing you can imagine, if you are a comfortable with Erlang and don' t mind to integrate yourself the AJAX toolkit of your choice. But don't expect a learning curve as with RubyOnRails, where you can start in the morning, without ever having heard anything about Ruby before, and at night you have your first simple web app running and have learnt Ruby without even noticing it.
  
• Tercio, by Eric Merrit. Different philosophy than Erlyweb, targeting AJAX apps which do most or even all rendering at client side.
  
• Mochiweb by Bob Ippolito. My preferred toolkit to easily build a custom HTTP server.
  
• And last and least, a shameless plug for my own upcoming web framework and service, which aims to lower the barrier to entry for individuals and companies doing utility computing based development and hosting of scalable AJAX / Comet web apps. You won't even need to know Erlang to start with, unless you want to customize the framework itself. More about this when I actually have something to show ...