Ask any programmer what his favorite language is good for and he'll yell, "Everything!" At least until his next favorite language comes along, which is also good for everything. The truth is: Any language that's Turing Complete and supports enough language features can solve any problem. The difference between languages and their usefulness is a matter of degrees of "yak shaving."
"Yak shaving" is a programmer's slang term for the distance between a task's start and completion and the tangential tasks between you and the solution. If you ever wanted to mail a letter, but couldn't find a stamp, and had to drive your car to get the stamp, but also needed to refill the tank with gas, which then let you get to the post office where you could buy a stamp to mail your letter—then you've done some yak shaving.
MORE ON PROS/CONS of DYNAMIC LANGUAGES
Some programming languages and some platforms (I'll make the distinction in a bit) minimize yak shaving. To create a simple language in Ruby, I have to find multiple undocumented libraries, install byzantine dependencies, track down build errors, find conflicts in things called "hoe" (which needs
rubyforge which needs
rake which needs Ruby), verify which version of Ruby I'm using, or try the undocumented
yacc integration, find out about Coco/R or other projects, install multiple non-Ruby packages, have the right libraries (again), talk with various people in IRC who don't want to talk to me—and then, after a few weeks, I might have just the harness done. Conversely, using ANTLR, without much fuss I can prototype an entire new language in a weekend and deploy it to just about any computer. It's not completely effortless; I can usually create a small parser for a protocol or mini-language in about a day or two using ANTLR and one book by Terrence Parr.
It's this distance between problem and solution that makes one language more suitable than another for a given task.
Aside: Sure, for the ultrageek in us, these yak-shaving expeditions can be fun, depending on the platform. I personally wouldn't do anything more complex than
ant buildon Java, but on Factor, Lua, Python or Ruby I'd gladly spend a weekend hacking at something just to prove I could do it (or to use it for something especially nerd powerful).
Why is this relevant to Ruby? Recently, there's been a huge push to take Ruby onto both the JVM and the CLR systems. Ruby's popularity means that companies like Sun, IBM and Microsoft want to add the language (and others, such as Python) to their platforms. This keeps programmers writing code for the vendors' platforms and keeps computers in the data center running their (expensive) software. Based on the adoption of Ruby on Rails I've seen for internal projects in 2008, this is a smart move.
What Ruby Makes Easy...and Hard
The original Ruby is called Matz Ruby Interpreter (MRI). It is a classic interpreter, similar to Perl, but it doesn't use any byte code as do Lua, Python and Java. MRI is written mostly in C (too much C for some) and is the gold standard in the Ruby world. With the MRI, some tasks are incredibly easy to do, and some are insanely difficult.
Systems scripting and automation. Ruby has the same capabilities as does Perl for regular expression processing, file crunching and system automation. Ruby has the added advantage in that its syntax is more impervious to obfuscation. That's not to say a rogue sysadmin couldn't craft something heinous, but you have to try to do something evil in Ruby. Perl developers have to try to write good, readable code. MRI also starts up quickly and uses fewer resources than JRuby so it's more suitable for short-lived cron jobs, long-running monitoring and other system-management tasks.
The caveat on Ruby for Web programming is that Rails is better suited for building Web applications. I've seen many projects attempt to create a WebDAV server or a content management system (CMS) with Rails and fail miserably. While you can do a CMS in Rails, there are much more efficient technologies for the task, such as Drupal and Django. (In fact, I'd say if you're looking at a Java Portal development effort, you should evaluate Drupal and Django for the task instead.)
Simplified APIs for nonexperts. Ruby's main strength is metaprogramming. You can make little languages (domain-specific languages, or DSLs) that don't look like Ruby. In the hands of an expert, this can turn future problems into simple implementations. (However, in the hands of an idiot, this can kill a project dead, so read my caveat against this later.) Ruby on Rails is in fact built like this, as more a DSL for doing Web applications.
Gluing C APIs together. MRI has a very good C binding API, and newer Ruby VMs like Rubinius will make this even easier. I've developed full wrappers to many complex C libraries in a few days. These C bindings have an object-oriented design and handle garbage collection with little fuss. It's trivial to take parts of a Ruby application that don't perform well and move them to a small C library, and using a system like Ruby2C or Ruby Inline, you can do this almost on the fly.
Prototyping network protocols. Ruby is a "good enough" language for writing simple servers and clients because of its simple threads, multiple I/O event libraries and protocol libraries for network protocols, including HTTP, SSH, SMTP and many others. It's also relatively easy—thanks to Ruby's very nice string handling—to create complete protocols in a short amount of time. When combined with advanced parsers like Ragel, you can make the protocols very robust for the majority of use cases, and then switch to C or another language for the heavy lifting.
However, Ruby's I/O, garbage collection (GC) and threads suffer from serious design flaws and performance problems.
Deploying Ruby in any IT department is also a painful experience because of its open-source nature, but see below for why that's more of a social problem.
Web application testing. Among Ruby's strengths are the tools that help a development team make sure the software runs as designed. Two Ruby libraries, Watir and Selenium, automate testing a site's user interaction. Combined with a test automation system like RSpec, Watir can produce complete reports of each test case and failure output, and write the tests in a nice readable format.
Telephony applications. Jay Philips has a framework called Adhearsion that gives you an amazingly slick syntax for the Asterix telephony platform. Using Adhearsion, you can write very complex yet clean telephone-based applications; you also can connect them to Web applications, databases and anything else Ruby can talk to. If you're doing any telephone work, it's worth the few days' time to try out Asterix and Adhearsion.
JRuby: First, the Pluses
JRuby is another open-source version of Ruby that is sponsored by Sun Microsystems and runs on the JVM. If you are an all-Java shop but want to try out the new hotness in languages, then JRuby is perfect for you. It allows programmers to write code in Ruby, to dip seamlessly into Java where needed, to write the hard parts in Java for speed and then deploy to the same infrastructure as a Struts or JSP deployment would. It even supports Swing. Several libraries make Swing even easier (one written by me). In its current state, I'd use JRuby for the following:
Breathing new life into tired old Java APIs. JRuby's tight integration with Java libraries is so seamless that you can code against a Java API in a way that looks and feels like Ruby. It's amazing how many times the same exact code—but done in Ruby with "less syntax"—seems easier to write. Syntactically, a Java implementation would be almost identical, but the Ruby code is looser and more fun. The Java purists will toss this as a "loser's argument," but sometimes letting tired, beaten Java programmers use a simple fun language to write their code can give the project a morale boost.
Gluing together Java libraries. Java programmers love to combine various elements of a "stack" to create the perfect framework. This usually results in a "Frankenstack" consisting of mostly off-the-shelf components, open-source libraries, lots of XML and some strange special sauce the star developer created (just before she left).
Ruby can work as a better glue between all the stack components since its more friendly syntax and easier unit testing frameworks promote readability and ease of use. However, even the worst Architecture Astronaut can turn Ruby into something horrible, as I've witnessed on many "professional" Ruby on Rails projects.
Rapid prototyping and experimentation. JRuby lets programmers prototype and interact with Java libraries dynamically, the same way they would in a scripting language. It dynamically converts types and gives them the feel of "just talking to the computer." To find out if a solution is possible (something we call a "spike"), JRuby can turn the test into a single-day job rather than a two-week trip to Frankenstack City. It also means you can try out new APIs quickly to evaluate whether they're worth using in a project; that can help when you make purchasing decisions.
Enterprise application integration. EAI is that constant dirty job somebody has to do, and it's never been pretty, ever. When you do most EAI tasks in a strict language like Java, you end up with monumental crystal palaces nobody can maintain. If you do them in a dirty, loose language like Perl, you end up with giant balls of duct-taped twine nobody can maintain. With JRuby, the parts that are best done with strict processing can be done in Java (or even just Java style), and the parts that are dirty and require
regex wizardry can be done in Ruby. You get the best of both platforms' network libraries, string processing, file handling and design philosophies when you try to make multiple different systems talk.
Web programming but with the Java platform. You can host Ruby on Rails applications in standard Java application servers like Tomcat, Jetty, Glassfish or Resin. Rails applications simply need to be packaged into Web application archives (WARs) and hooked in, using a simple set of libraries from the Goldspike or Warbler projects. Since Ruby has no legacy notion of complex (and useless) Java technologies like JNDI, JMX, JTA, servlets, application servers or session migration, you don't need to purchase insanely expensive products that don't meet your needs. Simply start with Tomcat. And, when you absolutely need it, buy (or use open-source versions of) the components you need.
Swing or SWT GUI development. Traditionally, developing a Swing or SWT application was a painful experience, involving gigantic books detailing every arbitrary bit of trivia about library specifics that were mostly legacy cruft. With JRuby's metaprogramming and dynamic nature, it's possible to build full complete professional user interfaces using SWT or Swing in a very short time. The results are usually the same in performance, and any parts that aren't fast enough can be redone in Java where needed. There are also several libraries like Profligacy (mine) and Cheri.
But that's just the advantages...
And on the Downside...
I don't recommend you use either JRuby or Ruby for: