David R. MacIver

I try to stay out of language wars these days. I find the whole endeavour incredibly tedious. I don’t really feel like arguing whether OCaml is better than Ruby is better than Scala is better than brainfuck is better than C. I like them all (ok maybe not brainfuck), and there are valid arguments for and against each of them.

But one thing I have a lot of trouble with is bad arguments. Not “arguments I disagree with”, but arguments which are simply outright bad.

Robert Fischer has a post on his blog: Scala is not a functional language. It’s not the first time this idea has come up. It’s not an idea entirely without merit. Scala’s functional features are certainly not as seamless to use as one might hope.

The problem is that while it is not an idea without merit, its presentation is certainly one without content. As per usual, every time this comes up, no one is actually willing to say what on earth they mean by “functional programming language”. This problem is ubiquitous. Consider the following wikipedia entry:

In computer science, functional programming is a programming paradigm that treats computation as the evaluation of mathematical functions and avoids state and mutable data. It emphasizes the application of functions, in contrast to the imperative programming style, which emphasizes changes in state. Functional programming has its roots in the lambda calculus, a formal system developed in the 1930s to investigate function definition, function application, and recursion. Many functional programming languages can be viewed as embellishments to the lambda calculus.

Notice the bait and switch there? It defines “functional programming” and then talks about “functional programming languages“. Everyone does this. The only unambiguous definition you find which includes the term “functional programming language” is that of Purely functional language. This is much less ambiguous, but it’s also the case that the vast majority of people arguing about whether Scala (or other language of choice) is functional are comparing it to a decidedly impure language. Certainly neither Clojure nor OCaml are purely functional.

On the other hand, if you choose “functional programming language” to simply mean “supports functional programming” then suddenly you have to acknowledge various languages like Ruby as functional and for some reason this makes people uncomfortable. So instead we end up with all sorts of hand waving and mumbling and no one is able to have a useful discussion because everyone is too busy making assertions which can’t be argued with because they don’t mean anything.

So no one defines the term, but everyone seems to “know what it means”. And what it inevitably means is “shares features with this language which I use and like and consider to be a functional language”.

For a particular extreme example, consider the following conversation on reddit from the last time this subject came up:

vagif:
Well, here’s my understanding of this disagreement.
I think many people (me included) do not feel that Scala is functional enough. It is not because of the mutable variables. It all boils down to simple syntax. That’s why completely imperative and old fashioned language CL (i use sbcl) feels to me much more functional than Scala will ever be with its modern functional features like pattern matching, type inference and list comprehensions.
“Functional” for me means simple small core of orthogonal features.
“Functional” for me means – List processing.
“Functional” for me means no premature optimization (deciding to use arrays instead of lists because they are faster etc.)
Obviously, trying to accommodate java OO model, makes Scala way to complicated, arcane, baroque in its syntax, to feel functional enough.

DRMacIver:
I enjoy the fact that the word “function” does not appear in your definition of functional…

vagif:
And for a good reason. Functions are part of any imperative language. That does not make them more functional. It is all that infrastructure, that allows for easy juggling those functions, that makes a language functional. Passing them as parameters and return values, anonymous functions, closures, polymorphic functions (be it a result of type inference or dynamic nature of language). It is this small core of orthogonal features, all geared up for list processing, that truly creates a functional feeling in a programmer.

This sort of woolly thinking is endemic in these arguments. Please try to avoid it. If you’re not willing to define “functional programming language” in a way that is at least moderately unambiguous and doesn’t involve arguments about “feelings” or simply feature comparisons with some language which you state as an example of functional programming, don’t bother making arguments about whether a language is functional or not.

Of course, once you start defining the term people will start arguing about the definitions. This is pretty tedious, I know. But as tedious as arguing about definitions is, it can’t hold a candle to arguing without definitions.