leiningen/doc/FAQ.md

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 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|*.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 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 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: Leiningen can't find some jars that the preview version could!
A: As of 2.0.0-RC1, Leiningen no longer checks the Clojars classic repository by default, replacing it with the Clojars releases repository. This speeds up dependency resolution and allows for more secure builds which enforce artifact signatures, but many libraries haven't been promoted to the releases repository yet. If you need such a library (or any snapshot version), you can add the old repo to your :repositories in project.clj: :repositories [["clojars-classic" "https://clojars.org/repo"]]