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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: If you create a directory called checkouts in your project root and symlink some other project roots into it, Leiningen will allow you to hack on them in parallel. That means changes in the dependency's source code will be visible in the main project without having to go through the whole install/switch-projects/deps/restart-repl cycle, and the copy in checkouts will take precedence over the dependency declared in project.clj. Note that this is not a replacement for listing the project in your main project's :dependencies; it simply supplements that for convenience. There is no special handling of subproject dependencies; if you change them you will need to lein install the subproject again.

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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