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](https://github.com/technomancy/leiningen/blob/preview/doc/TUTORIAL.md)
  for background.

**Q:** How should I pick my version numbers?  
**A:** Use [semantic versioning](http://semver.org).

**Q:** What if my project depends on jars that aren't in any repository?  
**A:** The [deploy guide](https://github.com/technomancy/leiningen/blob/preview/doc/DEPLOY.md)
  explains how to set up a private repository. If you are not sharing
  them with a team you could also just
  [install locally](https://github.com/kumarshantanu/lein-localrepo).
  In general it's easiest to deploy them to a private S3 bucket with
  the [s3-wagon-private](https://github.com/technomancy/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](http://maven.apache.org/guides/mini/guide-proxies.html)
  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](http://github.com/technomancy/swank-clojure) or
  [VimClojure](https://bitbucket.org/kotarak/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](https://github.com/ato/clojars-web/issues/24),
  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.