Functional Programming

I've just read a really good blog on functional programming by defmacro.org (I couldn't find his/her name on the blog anywhere). I didn't know that Alan Turing, Kurt Godel and John von Neumann all worked together at Princeton. I guess it makes sense that those three guys would know each other, but just imagine being able to listen in to their conversations. Hmm, probably completely over my head:) Anyway, there was a fourth guy, who I'd not heard of before, Alonzo Church. He invented an alternative to the Turing machine called lambda calculus. Of course the Turing machine went on to be the basis of pretty much all modern computers, but Alonzo wasn't entirely forgotten. In the late 50's LISP was invented as a programming language that implemented lambda calculus and is still with us. It's never really been mainstream, but the Emacs text editor is written in LISP, for example. One think that I've read in several places about LISP is that it makes some things, like creating domain specific languages much easier. So why is functional programming interesting? And what can you do that you can't do with imperative programming languages like the C* family? Well, it all goes back to the lambda calculus. I don't want to repeat defmacro's excellent article, but you can do all kinds of cool stuff, like mathematically redictable unit testing, simple parallel processing with no chance of race conditions, and even hot deployment where you can drop in code changes to a running program. Also because of the nature of functional programs (they are, in effect, a single function) you can use the lambda calculus to do automated optimisations to your program or do lazy execution. Also, because a functional language's state is always represented by its stack, unlike an imperative language where the state is a combination of the stack and the heap, you can use 'continuations' to halt and restart your program, or even pass your program's state to a separate program (it's also what allows for hot deployment). Now that C# is begining to get some funcional programming constructs (lambda expressions, expression trees, delayed execution), functional programming is going to become a mainstream concern for dot net developers. Joel Spolsky, has a nice post, Can your programming language do this? It shows some stuff that becomes much easier once functions become first class citizens, and there's a post about it on the powershell blog, showing some of the powerfull functional features of that language. If you look at everyone's current favorite language, Ruby, it already does all this stuff, as well as providing a full compliment of object oriented features. Unfortunately, some of the really cool stuff I mentioned above, like parallelisation or automated optimisations, can only be done with 'pure' functional language like Haskell or Erlang. It's making me think that I'd like to dig into a good Haskell book sometime.