Leiningen checkout dependencies are a useful feature. Checkout dependencies allow you to work on a library and consuming project at the same time. By setting up checkout dependencies you can skip running lein install in the library project; it appears on the classpath of the consuming project. An example of what this looks like can be found in the Leiningen documentation or in a previous post of mine.

By default, Leiningen adds the :source-paths, :test-paths, :resource-paths, and :compile-path directories of the checkout projects to your consuming project’s classpath. It also recurses and adds the checkouts of your checkouts (and keeps recursing).

You can override what gets added to your classpath by :checkout-deps-shares to your project.clj. This key’s value should be a vector of functions that when applied to your checkouts' project map return the paths that should be included on the classpath. The default values can be found here and an example of overriding the default behavior can be found in the sample.project.clj.

I ran into a situation this week where having my checkouts' :test-paths on the classpath caused issues my consuming project. My first pass at fixing this problem was to add :checkout-deps-shares [:source-paths :resource-paths :compile-path] to my project.clj. This didn’t work. My project.clj looked like below.

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(defproject example "1.2.3-SNAPSHOT"
  :dependencies [[library "1.2.2"]
                 [org.clojure/clojure "1.6.0"]]
  :checkout-deps-shares [:source-paths :resource-paths :compile-path])

Why didn’t it work? It didn’t work because of how Leiningen merges duplicate keys in the project map. When Leiningen merges the various configuration maps (from merging profiles, merging defaults, etc) and it encounters values that are collections it combines them (more details found in documentation). Using lein pprint :checkout-deps-shares shows what we end up with.

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$ lein pprint :checkout-deps-shares
(:source-paths
 :resource-paths
 :compile-path
 :source-paths
 :test-paths
 :resource-paths
 :compile-path
 #<Var@43e3a075:
   #<classpath$checkout_deps_paths leiningen.core.classpath$checkout_deps_paths@6761b44b>>)

We’ve ended up with the default values and the values we specified in the project.clj. This isn’t hard to fix. To tell Leiningen to replace the value instead of merging you add the ^:replace metadata to the value. Below is the same project.clj but with ^:replace added.

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(defproject example "1.2.3-SNAPSHOT"
  :dependencies [[library "1.2.2"]
                 [org.clojure/clojure "1.6.0"]]
  :checkout-deps-shares ^:replace [:source-paths :resource-paths :compile-path])

This solves the problem of :test-paths showing up on the classpath but it introduces another problem. Checkouts' checkout dependencies no longer show up on the classpath. This is because leiningen.core.classpath/checkout-deps-paths is no longer applied to the checkouts.

Without leiningen.core.classpath/checkout-deps-paths Leiningen stops recursing and, as a result, no longer picks up checkouts' checkout dependencies. My first attempt at fixing this was to modify my project.clj so the :checkout-deps-shares section looked like below.

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:checkout-deps-shares ^:replace [:source-paths :resource-paths :compile-path
                                 leiningen.core.classpath/checkout-deps-paths]

The above fails. It runs but doesn’t actually add the correct directories to the classpath. The next attempt is below.

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:checkout-deps-shares ^:replace [:source-paths :resource-paths :compile-path
                                 #'leiningen.core.classpath/checkout-deps-paths]

This attempt failed quicker. Now an exception is thrown when trying to run Leiningen tasks.

The next one works. It takes advantage of dynamic eval through read-eval syntax. With the below snippet the checkouts' checkouts are added to the classpath.

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:checkout-deps-shares ^:replace [:source-paths :resource-paths :compile-path
                                 #=(eval leiningen.core.classpath/checkout-deps-paths)]

Hopefully this is useful to someone else. It took a bit of digging to figure it out and many incorrect attempts to get correct. The full example project.clj is below.

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(defproject example "1.2.3-SNAPSHOT"
  :dependencies [[library "1.2.2"]
                 [org.clojure/clojure "1.6.0"]]
  :checkout-deps-shares ^:replace [:source-paths :resource-paths :compile-path
                                   #=(eval leiningen.core.classpath/checkout-deps-paths)])