Paul Graham originally introduced me to the idea that programming in a niche language was a good idea. He makes great arguments for organizations adopting more powerful languages and hiring smaller teams of highly effective programmers that can harness the power of niche languages. Paul Graham made me feel good about working exclusively on Ruby projects.
Later I read (the fantastic) My Job Went to India by Chad Fowler. Chad also talks about how being proficient in a niche language can make you a more valuable asset. Again, I felt good about focusing so heavily on Ruby.
February marks 2 years of nothing but Ruby projects. I'm still happy with the decision, but choosing Ruby has certainly brought unexpected consequences. I've gained an enormous amount from my recent experience; however, this entry serves to highlight the sacrifices that are required when focusing on a niche language.
One undeniable drawback to focusing on a niche language is that your employer choice becomes severely limited. For example, there are Ruby jobs available in New York City; however, I only know of one organization in the entire city that would be a good fit for me personally if I decided to leave ThoughtWorks*. NYC isn't exactly small either, if I moved to a smaller town the chances are even higher that I would have a hard time finding an employer with values similar to mine and still be able to make a living as a Ruby programmer.
A great lesson from My Job Went to India is that you should strive to be the worst member of a band. If you haven't read the book (read it soon!), the lesson is essentially that you perform at a higher level when you work with others who are performing at a higher level. I agree with Chad's advice; however, it's harder to be the worst member of a band if there are limited numbers of musicians you can work with. Some of the greatest programmers I know are getting paid to deliver Ruby applications, but I still know significantly more great developers who are not. Sadly, when focusing on a niche language there are simply fewer mentors available to learn from.
Another constant pain point for niche languages is the lack of respect that they are given. In the enterprise world, Ruby is largely seen as a toy. As a result, Ruby programmers are seen as "hackers, script kiddies, immature and uneducated." I do believe this to be far from the truth, but that doesn't change the fact that it is the general attitude. This leads to unfriendly treatment at conferences, and worse it can also cause an interviewer to unfairly discredit a qualified candidate.
Niche languages also suffer from greater prejudice. No matter how many success stories emerge, it's still very common to hear that "Ruby doesn't scale, just look at Twitter". There's very little truth in the statement, and yet it still follows Ruby every where Ruby goes. Niche languages simply have more detractors and thus the propaganda emitted by the detractors reaches larger audiences.
Adoption leads to patterns and best practices. Obviously, greater adoption can lead to larger amounts patterns and best practices and lower adoption rates have less to offer in the way of established ideas. While developing an application with a niche language, it's not uncommon to be the first person to tackle a new problem. Blazing a new trail is often exciting, but it's rarely as efficient as following a proven path.
Vendor support is also often limited for niche languages. My first 3 Ruby projects were backed by an Oracle database. At the time, the Oracle driver for Ruby on Rails wasn't exactly what I'd consider to be stable. The driver has improved, but using Oracle is still a pain to this day. Currently, TextMate is the #1 choice for developers writing Ruby on Rails applications. TextMate runs exclusively on the Mac. Oracle not only does not run on (Intel) Macs, but doesn't even offer a (reasonable) way to connect to an Oracle database from a Mac. Therefore, it's (basically) impossible to run TextMate and connect to an Oracle database from the same computer.
To be clear, I'm very, very happy with my decision and the past 2 years of projects. I won't bother with writing up the merits of adopting a niche language, see Chad Fowler and Paul Graham's work for that. However, I do think it's important to note that adopting a niche language can carry some unfortunate consequences.
*I'm not leaving ThoughtWorks. I'm very happy at ThoughtWorks.