It’s funny how over time the meaning of a technical word will converge to something halfway between what the experts intended and some fuzzy notion consisting of the most easily graspable components of that idea. In this inevitable process an idea is stripped of all of its flavor and is reduced to a set of bullet points graspable in an hour long presentation. Over the last few years this has happened to functional programming, right along with its popularization.
- First-class and higher-order functions
- Pure functions
Now that almost every language has tacked-on “functional features”, the functional party is over. The term has become just as perverted as Object-Oriented is to its original idea. It seems as though these days all it takes is lambda expressions and a higher order functions library to claim your language supports functional programming. Most of these languages don’t even bother to include any kind of proper support for simple tail recursion, much less efficient co-recursion or function composition. Oh, and any kind of inclination toward even encouraging purity? You wish.
But this isn’t necessarily a bad thing. The term functional isn’t at all evocative of the actual properties that make functional languages so wonderful. The term we should have been using all along is Expression-Oriented Programming. It’s the composition of expressions, the building of programs by sticking together little modular pieces, that makes functional languages great and first class functions are just a small part of enabling that. Expression-Oriented Programming tends towards first classing everything.
However, even the term first class is too weak to pin down this concept. All first class means is “as good as most other things” and this can still imply a really awful lowest common denominator. Just take a look at Microsoft’s C#. Sure, functions are first class, but it’s still a pathetic attempt at emulating what is possible in functional programming languages because the rest of the language isn’t.
Let’s end with a simple example to drive home the point. In C#, because the switch statement doesn’t produce an expression, you can’t assign its result to a variable. You even get an error that tells you so.
However, F# does a much better job of supporting Expression-Oriented Programming as almost every language construct outside of type definitions is itself an expression.
Expression-Oriented programming a simple idea, just programming with little composable parts, but it leads to beautiful and expressive code. It is at the core of why programs in functional languages are small, simple and less error prone. We should give credit where the credit is due, not to just the functions who are but one small player in the Expression-Oriented story.