Well, OOP had some help in the form of C++. It was easy to sneak in because it didn't strictly require people who were used to C to change their habits. Folks

Well, OOP had some help in the form of C++. It was easy to sneak in because it didn't strictly require people who were used to C to change their habits. Folks could, in effect, switch compilers first and then learn the language at their leisure. Of course, you could also make the argument (and I would) that C++ wasn't really an object-oriented language so much as a structured language with classes thrown in. Most of the popular functional languages, on the other hand, require folks to do a whole lot of mental gearshifting before they can be productive in the new language. That's a natural impediment to adoption. I suspect, though, that functional programming has finally found its C++, in the form of more recent versions of C#. Like with C++, it doesn't require users to immediately switch programming styles to be able to get the job done. Also like C++, though, you really can't use the language to its full capacity without getting very comfortable with a lot of functional programming concepts. Pretty much all of the neat new stuff in the .NET 4 class libraries, for example, is built around higher-order programming. Most of it will also be useless (or at least severely crippled) for someone who can't get past relying on mutable state.