Best practices in building and deploying Clojure applications: good tutorials?

Question

I am new to Clojure, and am beginning to experiment with building an application.

So far, everything I've seen about tutorials on compiling Clojure programs involves interactivity. For example, "load up the REPL and type (load-file "this-or-that") to run. This is fine, but it's not enough.

I am so used to the edit-compile-run idioms of languages like C or Delphi, that I am instinctively driven to make edits, then hit "M-x compile".

The problem is that "lein uberjar", which I understand is the equivalent to "make", is painfully slow to execute even for a hello world. So I'm going to have to figure out how this "interactive development" stuff works, stop using the uberjar like it's quick make, and save it only for the end of the day.

Another thing I noticed while building (using lein uberjar) is that the small GUI app I am working on pops up frames in the compilation process, as if they are executing while compiling. It just seems a bit counterintuitive to me; it is not quite as analogous to "make" as I had thought.

I know the Lisp way of developing things is interactively working in the REPL, and I am not trying to change that: I would like to adapt to this way of life. Unfortunately, I have seen little in the form of documentation on how to do so. For instance, how to reset the current state of the machine. It just seems kind of messy to just keep compiling individual snippets on the fly without being able to do some sort of reset.

Most tutorials I have seen on Clojure (and Lisp) in general seem to focus on hacking in the REPL. Best practices on the deployment of applications remains a mystery to me. My users are just going to be users; they are not going to be developers that are going to load files into a REPL.

So here is my question: any resources for good information or tutorials on the entire process of building a Clojure application, including deployment?

(Note: I have all of the prerequisites installed and working (e.g. Emacs, Slime, Leiningen, etc.), so this is not a question about that).

Answer 1

A couple of quick hints, then some links:

Don't use lein uberjar during development; prefer lein jar. The difference is that lein uberjar puts all your dependencies in the generated jar (including Clojure itself), so that your single jar is an entirely self contained package with your app inside; lein jar only jars your own code. The uberjar approach has obvious benefits for deployment, but for development, you should be able to just use the appropriate classpath when running your app, saving yourself the time necessary to prepare an uberjar. If you don't want to hand-manage the classpath for test runs, check out the lein run plugin.

Also, most likely the majority of your code should not actually be AOT compiled. AOT is necessary in some Java interop scenarios, but most of the time it brings one a slight boost in startup speed and annoying problems with binary compatibility with different releases of Clojure. I suppose the latter issue is not relevant to an uberjar-ed standalone app kind of project, but any library code at least should be left to be JIT-ed if at all possible. With Leiningen, you can put a :namespaces clause in the defproject form in project.clj to determine which namespaces are to be compiled; whatever you leave out will currently be JIT-ed by default. Older versions of Leiningen used to compile everything by default, which is actually a good reason to upgrade!

As for the windows popping out during compilation, I would guess that you're either running window-out-popping code during macro expansion time or outside of any function definition or similar construct. (Something like a (println "Foo!") at the top level.) That's just something you shouldn't do, I suppose -- unless you are planning to run your code as a script, anyway. To avoid the problem, wrap side-effecting code up in function definitions and provide an entry point to your application using the :main clause in project.clj. (If you say :main foo, then the -main function from the foo namespace will be used as the entry point to your app. That's the default, anyway, and at least the above mentioned lein run seems to have the name hardcoded -- not sure about lein itself.)

As for resetting the state of the REPL -- you can just restart it. With SLIME, M-x slime-restart-inferior-lisp will do just that while maintaining all other state of your Emacs session.

See also these discussions on the Clojure Google group:

1. Clojure for system administration
2. Prepping clojure for packaging (was: Re: Clojure for system administration)
3. Leiningen, Clojure and libraries: what am I missing?

Answer 2

No, you do not enter functions on the REPL.

You edit your source files, just as usual. The Lisp advantage is that you have the system running in the background at the same time, so you can compile individual functions from your source file and put them into the running system, or even replace them there.

If you use Slime, you press C-c C-c in your source file to compile and load the function at point. You can then switch to the REPL to test and explore, but anything you want to persist as source, you put into your source files.

Tutorials usually start by typing things on the REPL, because there is not much you need to set up for this, but serious development integrates the running system and source file management.

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Just to illustrate, my usual workflow (I am using Common Lisp, but Clojure is similar) is like this:

• Start Emacs
• M-x slime to start Slime, the Lisp system, and connect the two via Swank
• , (command) load-system foo to load the current project (compiling only if necessary) into the image
• C-x b switch to a source buffer
• C-c ~ make the source directory the current directory and the source package the current package of the REPL

Now, I'm set up with my system running in the background. Working is then:

• change or add a function or class definition
• C-c C-c to compile and load it into the image
• switch to REPL, test
• debug

There are no significant compilation pauses, because I never compile the whole thing at once, just individual definitions.