“Functional programming” has a clear meaning, but

Question

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.

Answer 1

Among people who study programming languages for a living, "functional programming language" is a pretty weakly bound term. There is a strong consensus that:

• Any language that calls itself functional must support first-class, nested functions with lexical scoping rules.

A significant minority also reserve the term "functional language" for languages which are:

• Pure (or side-effect-free, referentially transparent, see also)

as in languages like Agda, Clean, Coq, and Haskell.

Beyond that, what's considered a functional programming language is often a matter of intent, that is, whether is designers want it to be called "functional".

Perl and Smalltalk are examples of languages that support first-class functions but whose designers don't call them functional. Objective Caml is an example of a language that is called functional even though it has a full object system with inheritance and everything.

Languages that are called "functional" will tend to have features like the following (taken from Defining point of functional programming):

• Anonymous functions (lambda expressions)
• Recursion (more prominent as a result of purity)
• Programming with expressions rather than statements (again, from purity)
• Closures
• Currying / partial application
• Lazy evaluation
• Algebraic data types and pattern matching
• Parametric polymorphism (a.k.a. generics)

The more a particular programming language has syntax and constructs tailored to making the various programming features listed above easy/painless to express & implement, the more likely someone will label it a "functional language".

Answer 2

I would say that a functional language is any language that allows functional programming without undue pain.

Answer 3

I wrote a (pretty long) analysis once on why the term 'functional programming language' is meaningless which also tries to explain why for instance 'functions' in Haskell are completely different from 'functions' in Lisp or Python: http://blog.nihilarchitect.net/archives/289/on-functional-programming/

Things like 'map' or 'filter' are for a large part also implementable in C for instance.

Answer 4

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.)

Answer 5

I think the same question can be asked about "OOP languages". After all, you can write object oriented programs in C (and it's not uncommon to do so). But C doesn't have any built-in language constructs to enable OOP. You have to do OOP "by hand" without much help from the compiler. That's why it's usually not considered an OOP language. I think this distinction can be applied to "functional languages", too: For example, it's not uncommon to write functional code in C++ (think about STL functions like std::count_if or std::transform). But C++ (for now) lacks built-in language features that enable functional programming, like lambdas. (Let's ignore boost::lambda for the sake of the argument.)

So, to answer your question, I'd say although it's possible to write function programs in each of these languages:

• C is not a functional language (no built-in functional language constructs)
• Scheme, Python and friends have functional constructs, so they're functional languages. But they also have imperative and OOP constructs, so they're usually referred to as "multi-paradigm" languages.

Answer 6

A language that makes it a lot harder to create functions with side effects than without side effects. The same counts for mutable/immutable data structures.