"Yet functional languages never really made it to the mainstream"<p>JavaScript development these days is getting pretty darn close to what I would consider 'mainstream' functional programming.<p>Heavy use of both first class and lambda functions, and closures have been standard practices for years now. And thanks to libraries like Underscore.js partial application and the use higher order functions like map/filter/etc are also becoming very common place.<p>I'll absolutely agree that what we see is not 'pure' functional programming but neither is a good deal of real world Common Lisp.<p>Anyone who has done a lot of programming in Lisp or Haskell has probably bemoaned the fact that many people are quick to dismiss so many great and useful tools these langauges provide. But I definitely think we're starting to see a trend where the most genuinely useful features of fp are becoming mainstream.