tl;dr: How to get a Clojure project's classpath without the 'test' dependencies, from the REPL.

## Context

Leiningen uses the Maven library under the hood, to manage dependencies. As a consequence, the dependency scope mechanism is used to affect the classpath.

The default, compile, is never explicitly specified.

The two mainly used scopes are provided (for the famous javax.servlet/servlet-api when working with Java EE servers...) and test, in order to point out that the dependency is for tests only.

Neither is transitive (brings no other deps), and the test one is not required for normal use of the application.

I'd like to get my project's classpath, the one required at runtime.

And I find annoying that 'test dependencies' are contained in this path.

## First try

I start a REPL with custom profiles: lein with-profiles +local,+cljs,+om repl

Let's see what (System/getProperty "java.class.path") gives us.

omg! It returns all dependencies, including 'test' ones, and also those of every profiles used to launch the REPL...

What a mess!

Several solutions equivalent to this one hang out in internet:

(seq (.. Thread currentThread getContextClassLoader getURLs))

but with recent lein versions, it displays nil...

## Using lein

Outside of the REPL, there is a lein classpath task.

This task simply wraps a function located in the leiningen.core.classpath namespace.

As with the other tries, it also gives the full classpath with tests dependencies. But it reads dependencies from the project.clj file, so there is no interference with optional profiles nor plugins.

So, we'll filter dependencies before passing the project's map to this function.

### Step 1: leiningen-core

Add a dependency to your project's map file: project.clj, If your *read-eval* is true you might use the following notation:

   :dependencies [...
[leiningen-core #=(leiningen.core.main/leiningen-version)]]


Thus the same version of Leningen than the one used to start the REPL will be used.

If that does not work, go back to basics:

   :dependencies [...
[leiningen-core "2.6.1"]]


### Step 2: require

In the REPL session, type: (require '[leiningen.core.classpath :as lcc]) to be able to use the get-classpath function.

Also (require '[leiningen.core.main :refer [leiningen-version]]) to let the leiningen-version seen above return the correct number.

### Step 3: dependency notation

A dependency, as added to any project file, is a vector with at least the artifact name (group-id/name coordinate) and its version (version will become optional with managed-dependencies, see last paragraph). Some modifiers might be added, such as :exclusions, :classifier, or :scope, as pairs of key/value.

Examples:

:dependencies [[org.clojure/clojure              "1.8.0"]

[leiningen-core
#=(leiningen.core.main/leiningen-version)]

[clojurewerkz/propertied          "1.2.0"
:exclusions [[clojurewerkz/support]]]

[midje/midje                      "1.8.3" :scope "test"]]


### Step 4: assembly

If the value of the scope modifier is "test", we will remove the dependency.

; read project's file and call get-classpath with updated dependencies
(defn classpath-without-test-deps []
(leiningen.core.classpath/get-classpath
; filter project's dependencies before getting classpath.
(update pmap
:dependencies
(fn [deps]
(filter (fn [dep]
(as-> dep $$(drop-while #(not= :scope %)$$)
(second $$) (or$$ "")
(name $$) (not= "test"$$))) deps))))))


### Step 5: cleaning

It is not desirable to permanently change the dependencies of a project to add leiningen-core for this very specific need.

Several solutions are possible:

1. add in the project file an entry of the type:
:profiles {
:dev {:dependencies [[leiningen-core "2.6.1"]]}

} 

This will set a profile used only in development mode, which will not pollute the standard project dependencies. We can take the opportunity to add a few plugins also, like michaelblume/lein-marginalia to generate a nice project documentation.

1. create a profile common for all projects (user-wide or system-wide) in the profiles.clj configuration file. In this case, the profile should not be called dev.
2. use a packaged library such as repl-tasks, and inject useful functions in the clojure-core namespace. See here.

## Managed dependencies

Leiningen 2.6.2 will bring a very interesting new feature: managed dependencies. They allow to declare the version of some dependencies in a 'parent' project, and to propagate automatically in child projects. Those coming from Java and Maven's world remember about parent pom and cie.

Small problem, the scope should be set in these managed-dependencies. So to benefit from this new feature, we'll have to look at both the project deps and those of its optional parent to be able to detect a 'test' scope.

So stay tuned for an update to this post, as soon as lein 2.6.2 is available!