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I am learning Clojure and I really am loving some of its features. The time is coming to think of some real "pet projects" and I realize I'm not sure how to actually use Clojure.
I see many web and templating frameworks (e.g. Compojure), but somehow I'm in doubt on whether it's worth it. I feel that in the long run it can't serve the needs of real world applications which you address with Spring, Hibernate and some pieces of the Java EE stack.
On the other hand, I see great potential in the concurrency features but I'm short on ideas on how to really use them.
Enough background, my questions are:
Edit: Thanks for all the great answers. They're all really inspiring. So if you have anything else to add, go ahead and don't be put off by the fact that one has been accepted.
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In answer to the "background" part of the question:
I think you should read Jörg W. Mittag's answer to an SO question entitled "Real world Haskell programming". He makes a number of excellent points. Read on for my take on the FP in the real world issue; scroll past the horizontal line for answers to the two actual questions.
There's a number of FP-centric companies which seem to be really good at what they're doing; for some examples, google Jane Street (OCaml), Galois (Haskell), FlightCaster (Clojure for backend heavy lifting; I seem to remember reading that their frontend is currently done in Rails). Supposedly automated trading strategies are often coded in FP-oriented languages; that would indeed make perfect sense, although I have no inside data to confirm this. For additional examples to do with Clojure, see the list of companies on the success stories page.
Some people seem to be enjoying a degree of success in addressing the needs of real world applications in Rails, Django etc. It would appear that they feel no need to touch J2EE & friends. Not that these have much to do with FP, but they are like FP in that they're nothing like the "Enterprise Languages" of the present.
As for the two actual questions:
Why not just pick up whatever it is you've last been thinking to do and do it in Clojure? Obviously anything can be done in Java (and most things probably have been), but a leaner language might make the product cleaner, the experience more pleasant and less time consuming etc.
About mixing Clojure and Java -- I've seen a decent amount of Clojure code using a couple of classes coded directly in Java (for whatever reason). I've tried going the other way around myself and it's a bit of a pain in that it's much simpler to work with interface inheritance than class inheritance in Clojure, unexpected coupling in the Java code can seriously interfere with the ability of the Clojure code to do things in the most natural way etc. Still, it's entirely possible to extend a Java programme in Clojure and it seems like a particularly safe & sane way of experimenting with it for the worried Java developer.
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Functional programming can be applied to almost any task. Web applications, scientific applications, games, you name it.
It is very common to mix Clojure and Java, since Clojure does not have many dedicated libraries for things like I/O or networking.
Organizations that already have a lot of Java code can use Clojure for small subsections of their Java projects.
For new projects, it is usually more effective to use Clojure as the high-level driver language, calling Java libraries where necessary.
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I have been working on a small web application using Clojure, and while there is nothing special about the application that could not have been done in a different language, the experience of writing it has been completely different. I have written web apps using ASP.net and moving to Clojure was less about learning the different syntax and more about learning a different way to think and program. Having to learn a different way to think will occur regardless of the project you choose to work on, so I would worry less about finding the perfect functional project and more about finding something you just want to work on.
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I think the answer to this has a great deal to do with the context your project is embedded in, and the constraints that imposes on you. Absent social factors I think Clojure is likely at least as "good" a language as Java is for any problem, with the possible exception of cases where you need the last bit of performance. And even in those cases things are not nearly as simple as they seem. For one thing some future version of Clojure can probably, in the theoretical limit, be compiled to bytecode that is as "fast" as what Java is compiled into (assuming a bit more work from the programmer at bottlenecks.) More importantly, optimization is a multi-factorial problem, and one in which programmer productivity and the flexibility of code factors heavily. So while there is a sense in which it would be accurate to say that Clojure is slower than Java, that sense might not be the important one when discussing the performance of a particular application.
So I'd say that if you disregard social factors Clojure's use cases are close to a superset of Java's. I wouldn't try to write a Linux kernel module in clojure though...
Of course, it's true that not all problems have equally natural solutions in functional languages. But people have come up with some interesting ways of dealing with some of the cases where FP seems to map badly to the domain, and anyway Clojure actually offers you enough escape hatches from pure FP that if you really feel the need to write part of your program in an imperative style you can (though of course you give up some of the benefits of Clojure in that case.) In the worst case you could use Clojure to drive the Java library in much the same way that you would in Java... it's hard to imagine a case where that would be a good idea, but in most cases that would not be markedly inferior to just using Java, and in many it might be better.
I'm still a neophyte at Clojure, though I've been programming in CL and scheme for a long time, and I spent about five years writing Java for a living. But I would probably prefer Clojure to Java for just about anything even without knowing it quite as well, as long as there were no social factors involved.
It would be a mistake to dismiss social factors though. I've been a Lisp programmer long enough to have a finely honed instinct for how well a Lisp will work in a given context. I've introduced Lisp to commercial settings where it has been a big win, and I've introduced it to settings where it really wasn't. I'd think long and hard about staking your career on successfully transitioning a team of programmers to any Lisp, Clojure included, particularly if they are not too keen on the idea.
Just to give you an idea of what I think Clojure might be useful for, I am currently writing a lot of poker-related code in Clojure. Some of it is pretty simple stuff (finding the best five card hand you can make from seven cards) and some of it is a bit more interesting (looking at someone's playing history and extracting meaningful trends from it using a few heuristics and some basic statistics.) None of it requires much in the way of Clojure's sophisticated concurrency mechanisms, but it is still much nicer (for me at least) in Clojure than it would be in, say, Java.
There are certainly some other cases that someone might describe where Clojure wins big because of its sophisticated mechanisms for managing concurrency, etc. I am aiming at something more modest- I am just pointing out that even if you don't need those mechanisms you might find Clojure a very congenial language for general purpose programming, albeit one that requires you to rethink how you abstract things if you're coming from an imperative/OO background. And hey, if you need the concurrency mechanisms (as you might, the way things are going), at least you already know Clojure.
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I like writing game programs when I learn a new language.
I am in the process of learning Clojure and started writing a Spider solitaire player. If you have never played Spider, don't start; it is very additive :-). See http://www.spidersolitaire.org/.
In writing this game, I am getting to use several things that I want to learn: functional programming, concurrency, Java-interop (for Swing), etc.
I have also started writing a Bejeweled player (http://www.popcap.com/games/free/bejeweled2), but have run into a problem finding the definitive rules for scoring the game.
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