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Q: How do you pronounce Leiningen?
A: It's LINE-ing-en. ['laɪnɪŋən]

Q: What's a group ID? How do snapshots work?
A: See the tutorial for background.

Q: How should I pick my version numbers?
A: Use semantic versioning.

Q: What if my project depends on jars that aren't in any repository?
A: The best thing to do is to get them in a repository. The deploy guide explains how to set up a private repository. In general it's easiest to deploy them to a static HTTP server or a private S3 bucket with the s3-wagon-private plugin. If you are just doing exploratory coding and not collaborating with a team you could install locally.

Q: I want to hack two projects in parallel, but it's annoying to switch between them.
A: Leiningen provides a feature called checkout dependencies. See the tutorial to learn more.

Q: Is it possible to exclude indirect dependencies?
A: Yes. Some libraries, such as log4j, depend on projects that are not included in public repositories and unnecessary for basic functionality. Projects listed as :dependencies may exclude any of their dependencies by using the :exclusions key. See lein help sample for details.

Q: Why doesn't deps task populate the lib directory in version 2?
A: The only reason version 1 copied the jars around in the first place was to support existing tooling that needed a cheap way to calculate a project's classpath. Now that Leiningen has a mature plugin ecosystem, this is no longer needed; jars can be referenced directly out of the ~/.m2/repository directory. If you need to see a listing of all the dependencies that will be used and their versions, use lein deps :tree. To get the classpath use lein classpath.

Q: What does java.lang.NoSuchMethodError: clojure.lang.RestFn.<init>(I)V mean?
A: It means you have some code that was AOT (ahead-of-time) compiled with a different version of Clojure than the one you're currently using. If it persists after running lein clean then it is a problem with your dependencies. Note that for your own project that AOT compilation in Clojure is much less important than it is in other languages. There are a few language-level features that must be AOT-compiled to work, generally for Java interop. If you are not using any of these features, you should not AOT-compile your project if other projects may depend upon it.

Q: I specified a dependency on version X but am getting version Y; what's up?
A: One of your dependencies' dependencies has declared a dependency on a hard version range, which overrides your "soft" declaration. If you change yours to a hard version range, it will refuse to function due to conflicts, so it's best to find the dependency that's at fault via the lein-pedantic plugin and add an :exclusions clause to it. See lein help sample for how exclusions work. You may also want to report a bug with the dependency that uses hard version ranges as they cause all kinds of problems and exhibit unintuitive behaviour.

Q: I'm behind an HTTP proxy; how can I fetch my dependencies?
A: Set the $http_proxy environment variable in Leiningen 2.x. You can also set $http_no_proxy for a list of hosts that should be reached directly, bypassing the proxy. This is a list of patterns separated by | and may start or end with a * for wildcard, e.g. localhost|* For Leiningen 1.x versions, see the instructions for configuring a Maven proxy using ~/.m2/settings.xml.

Q: What can be done to speed up launch?
A: The main delay involved in Leiningen comes from starting two JVMs: one for your project and one for Leiningen itself. Most people use a development cycle that involves keeping a single project REPL process running for as long as they're working on that project. Depending on your editor you may be able to do this via its Clojure integration. (See nrepl.el or foreplay, for example.) Otherwise you can use the basic lein repl.

Q: Still too slow; what else can make startup faster?
A: The wiki has a page covering ways to improve startup time.

Q: It looks like a different set of dependencies are being used in the repl vs other tasks.
A: The repl needs to add a few extra implicit dependencies in order to function. For instance, it needs to load an nREPL server, and it loads a client for fetching ClojureDocs examples, both which have dependencies that could interfere with the dependencies declared in your own project. You can declare a :leiningen/reply profile to override the clojuredocs client dependencies. While you can't run the repl task without nREPL in the dependencies, specifying your own version of it will override the version Leiningen adds. Another way around the problem would be to load a bare repl by invoking lein trampoline run -m clojure.main/repl, though you may want to use rlwrap on that to get proper key bindings and history.

Q: What does "Unrecognized VM option 'TieredStopAtLevel=1'" mean?
A: Old versions of the JVM do not support the directives Leiningen uses for tiered compilation which allow the JVM to boot more quickly. You can disable this behaviour with export LEIN_JVM_OPTS= or upgrade your JVM to something more recent. (newer than b25 of Java 6)

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