Clojure is pretty big in some places. The list here: <a href="http://clojure.org/Companies" rel="nofollow">http://clojure.org/Companies</a> includes Staples Sparx, Walmart Labs, Groupon, and several other well known names.
Jane Street is using OCAML. That or F# is my preferred choice.<p>Having said that, the tricky part is sticking to pure FP and coordinating data structures. Not impossible, but it's a foreign area for many shops.
I'd guess Scala is the most popular choice, but for nearly every language you'll find a company using it for at least parts of their products.
Clojure is gaining popularity. Staples uses it (their "SparX" group), and much more notably a Clojure service is used to process every single transaction in Walmart stores.
Scala is getting big in the enterprise. This list <a href="http://www.quora.com/What-startups-or-tech-companies-are-using-Scala" rel="nofollow">http://www.quora.com/What-startups-or-tech-companies-are-usi...</a> gives a small overview
Haskell usage by enterprises can be found here:
<a href="https://wiki.haskell.org/Haskell_in_industry" rel="nofollow">https://wiki.haskell.org/Haskell_in_industry</a>
Scala and F# are probably the most popular enterprise choices. Both connect well to existing infrastructures (JVM, .NET), are relatively easy to train new developers on, and have largish companies backing them.<p>Scala is probably the larger of the two options - big users notably include Twitter, Linkedin, Morgan Stanley, JP Morgan, Walmart and Reuters.<p>The use cases for F# tend to be smaller scale and more UI-oriented - internal reporting tools at a hedge fund, for example. That's not to say they are simple CRUD apps, merely that F# apps are not usually massive distributed systems.
Not a secret -- at Monsanto (Fortune 200) we use Scala heavily now, starting from one project in 2010 and now a dozen teams use it for many many production projects internally and in AWS. Certainly some in the almost 200k LOC range. Scala is replacing Java at Monsanto (and yes we're always hiring!)<p>NB: Our Climate Corporation teams are running Clojure.<p>"What is functional?" is a difficult question my partner and I ponder often. As someone else here said, simply having functions as first class values seems insufficient as that would include Perl, JavaScript, C (fxn pointers) and other langs as "functional." Saying supporting any mutability at all removes your status as "functional" means Lisps aren't functional, which seems to ignore the history of FP. Erlang doesn't have monadic IO but is highly regarded by many as functional. If our definition essentially includes only Haskell as "functional" then the term seems useless. So we define as functional any lang that supports functions as first class values AND significant enforced immutability.<p>Scala's first class functions, default immutable data structures and "val"s compile-time enforcing immutability without too much ceremony put Scala in the FP category for us, along with the Lisps, MLs, F# and Erlang.
Scala is gaining popularity because of Spark <a href="https://spark.apache.org/" rel="nofollow">https://spark.apache.org/</a>
If you consider Scala functional, I think it wins the popularity contest: used by startups and established companies (Linkedin, Twitter, Meetup, Verizon, Morgan Stanley, Autodesk, HuffPo, etc, etc...) and is not "we use it in a dark corner where nobody cares" and more like "betting the whole farm on it".
At my company (publicly traded; >200 engineers) I know that Scala has begun to be picked up by the Java people. We're also rolling out Storm (Clojure) but I don't know if there's anyone actually writing Clojure code.
For context I work for publicly traded multinational real estate portal with about 1000 employees.<p>The flow of dominant programming languages has been something like:<p>Perl -> Java,Ruby(Rails+Gems+APIs) -> Java,Scala,Javascript,Ruby<p>Not that those are the only ones floating around at our company, but Scala definitely has a leg up.
Scala, Clojure, and Erlang come to mind. F# is another possibility, since .NET/CLR is becoming increasingly popular in the enterprise (particularly in Windows-heavy datacenters), though I don't know of a whole lot of enterprise F# deployments off the top of my head.<p>Erlang's a bit of a special case; while you probably won't see many enterprises <i>consciously</i> using it (unless they use Jabber/XMPP via ejabberd, of course), it's rather abundant behind-the-scenes with communication infrastructure, and multiple large telecom providers and OEMs (including - obviously - Ericsson) use it for their equipment and infrastructure.
If you consider it functional, Common Lisp is still used by some very large and established companies: the Lisp HUG mailing list (LispWorks) will frequently have email signatures from some of them. There is some truth to the notion that CL is a competitive advantage and so its use tends not to be discussed in the open.
None, or possibly Haskell.<p>Scala, Clojure, F#, JavaScript aren't functional languages. Functional programming is programming with pure functions. It is not "first-class" functions. It is not closures. It is not static typing.
I guess you'd need to define what you mean by functional language. That term gets thrown around a lot, to the point of meaninglessness. If you mean referentially transparent then neither Scala nor F# are referentially transparent.
Javascript / Nodejs<p>it's multi-paradigm: scripting, object-oriented (prototype-based), imperative, <i>functional</i> ( source: <a href="https://en.wikipedia.org/wiki/JavaScript" rel="nofollow">https://en.wikipedia.org/wiki/JavaScript</a> )<p>Many languages known as functional (Scala, F#, Lisp, Scheme, OCaml) are multi-paradigm languages (check out Wikipedia!), purely functional programming language like Haskell are the exception (<a href="https://en.wikipedia.org/wiki/Purely_functional" rel="nofollow">https://en.wikipedia.org/wiki/Purely_functional</a> ). And the IO part is hardly functional.<p>@Down voter: care to explain?