I would say there's two clear reasons functional languages are relatively rare in the professional world: 1) Functional languages are difficult to grasp. Mayb

I would say there's two clear reasons functional languages are relatively rare in the professional world: 1) Functional languages are difficult to grasp. Maybe it's the way CS is taught, but I've never heard someone say "Haskell just comes intuitively to me, but I just can't get how to work effectively in PHP". You hear the same statement the other way around all the time. The bottom line is if you have multiple tools to do the same job, and one has a huge learning curve and one has a tiny learning curve, the choice of tool is obvious. 2) Haskell doesn't have a clear application it's "good" at. If you learn PHP, Python, or Ruby, you can throw together a web application with little effort. If you learn Javascript, you can easily do front end web stuff, or if you're more adventurous, get into Node.js. If you learn the C family of languages you can write iOS apps, OS kernels, or games. What do you do with Haskell? I dunno.