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FAQ

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 deploy guide explains how to set up a private repository. If you are not sharing them with a team you could also just install locally. In general it's easiest to deploy them to a private S3 bucket with the s3-wagon-private plugin.

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.

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 lein deps :tree 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|*.mydomain.com. 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 the JVM. Most people use a development cycle that involves keeping a single REPL process running for as long as you're working on that project. Depending on your editor you may be able to do this via its Clojure integration. (See swank-clojure or VimClojure, for example.) Otherwise you can use the basic lein repl.

Q: Still too slow; what else can make startup faster?
A: The trampoline task uses Leiningen to calculate the command needed to launch your project and then allows Leiningen's JVM to exit before launching your project. Exporting the $LEIN_FAST_TRAMPOLINE environment variable allows the command to be cached, making it possible to skip launching the Leiningen JVM entirely. You can set this on a per-project variable by creating a .lein-fast-trampoline file in the project root. In addition, you can usually set :bootclasspath true in project.clj to speed up project launches, though there are compatibility issues with some libraries.

Q: Why is Leiningen 2 still in a preview release?
A: As of the preview3 release, Leiningen 2 is very stable and recommended for general use. The main thing keeping it from a final release is the fact that the current Clojars repository mingles snapshots with releases, which is undesirable. Since switching the default repositories to a releases-only Clojars (which is still in development) would be a breaking change, a series of previews is being released in the mean time.

Q: I don't have access to stdin inside my project.
A: This is a limitation of the JVM's process-handling methods; none of them expose stdin correctly. This means that functions like read-line will not work as expected in most contexts, though the repl task necessarily includes a workaround. You can also use the trampoline task to launch your project's JVM after Leiningen's has exited rather than launching it as a subprocess.

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