ClojureScript lows as low as the highs

Dependency resolution quirks bring progress to a halt, temporarily.

I started rolling all the code from the testbed into my actual application today. getting started was easy enough, just copy in the project.clj and some folders, fix up some paths and names. Porting my old JQuery code was easy, as expected, and I was able to structure the code much nicer.

Once I had things rearranged and got everything to build I remembered one thing I didn't try in the testbed. In order for my Python code to interact with the Javascript VM I need to be able to call js functions. In advanced compilation mode though all the functions and var names are mangled. There are two ways to expose ClojureScript symbols in the js runtime:

;; Export interop symbols on js global object (window)
(set! js/set_history set-history)

;; Add the :export metadata to the functions you want to access
(defn ^:export set-history
  "Set this page's filter history"
  (reset! history (js->clj new-hist)))

The second method is recommended, but the names exported must be fully qualified: lister_cljs.filter.set_history. Exporting on the window risks name collisions, but it aligns closer to how I had things before. To test that all works though I had to build everything together and ensure the symbols really exist. That is where I ran into trouble.

When I tried to build with advanced compilation I got some warning, and then a big green message saying the code was successfully compiled. When I check in the app though I can't see the symbol I exported. I messed around for a while trying to get anything to show up, until I noticed that none of my code was running. In fact the body of the compiled Javascript file was an empty function!

You may have guessed, the warning I ignored was actually important. It was actually an error reported by the Google Closure compiler. And while the compiler died with the error the lein process continued to completion and reported "success". The error code was ERROR: JSC_CONSTANT_REASSIGNED_VALUE_ERROR. constant reactDOMserver assigned a value more than once. which was weird since I wasn't using any react in my code.

What followed was successively banging my head against the cljs compiler docs, the lein-cljsbuild docs, the Google Closure compiler docs, and searching high and low for similar problems. I did eventually find similar problems, but for some reason Google was no help. I figured out the problem was a dependency collision, and the only things I was using that involved React were Figwheel and Devcards. Since I had the latest versions of both, and there were no documented incompatibilities, I resorted to reading through old Github issues.

I found that Devcards had similar issues for other people, and confirmed that removing my Devcards dep allowed the build to complete. The odd thing though was that I don't use any of the Devcards code in my dev or min build profiles. I expected the Google Closure compiler to just drop the unused code, but it seemed to be loading all the test code too.

My first attempted solution was to separate the test code from the normal code, and only add the test code path to the testcards build: :source-paths ["src" "test"]. This had no effect. Because I had Devcards in my project-wide dependencies map the offending code was loaded into every build. In the end the final solution was so simple, but so painful to come up with - Remove Devcards from the project deps and add it only to the testcards build:

{:id "test"
 :source-paths ["src" "test"]
 :dependencies [[devcards "0.2.1-7"]] ;<-- This Here!
 :figwheel {:devcards true}
 :compiler {:main lister-cljs.test.core
            :asset-path "lib"
            :output-to "testcards/main.js"
            :output-dir "testcards/lib"
            :source-map-timestamp true}}

Edit: Nope, even harder than that

The next morning I am working on building out my system when I ran into an odd error. I try to fire up Figwheel and start writing some tests, and it doesn't work. Figwheel complains that I have an invalid key :dependencies in my config. Which I do.

The code up above is in :cljsbuild {:builds {...}}, and cljsbuild doesn't deal with dependencies, Lein does. To make Devcards an optional dependency I need that code under the project.clj :profiles key:

:profiles {:test {:dependencies [[devcards "0.2.1-7"]]}}

:cljsbuild {:builds
            [{:id "test" ...

Then I need to run lein with-profile test figwheel test to activate that profile and have the Devcards jar added to my classpath.