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"Functional programming" has a clear meaning, but does "functional language"?

Asked 2010-02-18T19:54:37.433
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I understand very clearly the difference between functional and imperative programming techniques. But there's a widespread tendency to talk of "functional languages", and this really confuses me.

Of course some languages like Haskell are more hospitable to functional programming than other languages like C. But even the former does I/O (it just keeps it in a ghetto). And you can write functional programs in C (it's just absurdly harder). So maybe it's just a matter of degree.

Still, even as a matter of degree, what does it mean when someone calls Scheme a "functional language"? Most Scheme code I see is imperative. Is it just that Scheme makes it easy to write in a functional style if you want to? So too do Lua and Python. Are they "functional languages" too?

I'm (really) not trying to be a language cop. If this is just a loose way of talking, that's fine. I'm just trying to figure out whether it does have some definite meaning (even if it's a matter-of-degree meaning) that I'm not seeing.

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Jane Street's Brian Hurt wrote a very good article on this a while back. The basic definition he arrived at is that a functional programming language is a language that models the lambda calculus. Think about what languages are widely agreed to be functional and you'll see that this is a very practical definition.

Lisp was a primitive attempt to model the lambda calculus, so it fits this definition — though since most implementations don't stick very closely to the ideas of lambda calculus, they're generally considered to be mixed-paradigm or at best weakly functional.

This is also why a lot of people bristle at languages like Python being called functional. Python's general philosophy is unrelated to lambda calculus — it doesn't encourage this way of thinking at all — so it's not a functional language. It's a Turing machine with first-class functions. You can do functional-style programming in Python, yes, but the language does not have its roots in the same math that functional languages do. (Incidentally, Guido van Rossum himself agrees with this description of the language.)

answered 2010-02-18T20:51:47.770

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