Code And Cocktails

Clojure Project 'Reloaded' Pattern Gotcha

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Ensure that user.clj does not contain dependencies (even transitive) upon code that needs to be compiled. This file is loaded well before Leiningen is started enough to compile files.

Problem Description

In my latest learning project with Clojure I decided to try Stuart Sierra’s Reloaded project pattern1. I liked the idea of this project pattern because it promised to make the REPL driven development on a web application smoother. Things were going smoothly as I worked on simple dummy pages for my little application. However when I connected the data file parsing code with its custom exception to the application I started getting compilation and class not found errors related to the custom exception. The reason for this is a rather simple one - but was difficult for me to find information on or figure out.

First an aside about custom exceptions. It seems that in the Clojure community that custom exceptions are avoided2. Either one of the built-in Java exceptions, ex-info3 or Slingshot4 is used instead. However it had been my favored approach to create a domain-specific exception for my application’s needed, so I stumbled forward and learned how to do it. It was relatively simple by using the :gen-class option of ns and the :aot feature of Leiningen. And it worked well… until I connected the code using the exception with the web application that was implemented in the reloaded pattern.

A big part of the reloaded pattern is the use of functions in user.clj to start and stop the system. This file is loaded by Clojure whenever it is started5 so it is perfect for functionality wanted in a REPL. This file is kept in a directory which is only included in the class path when the dev profile is used (the repl and compile tasks use the dev profile by default).

Everything is fine until the code in user.clj depends upon (even transitively) code which must be compiled (such as custom exceptions). Then we hit a annoying chicken-and-egg problem6 wherein Leiningen when trying to compile (or launch the REPL) naturally starts Clojure, which in turn loads user.clj which in turn depends upon code that needs to be compiled. The error that is reported says that the compiled class cannot be found.

The Confusion

This error led me to first think that my custom exception was not written properly and thus wasn’t being compiled, then I thought it was a problem of the :aot feature in Leiningen and interaction with the dev profile. But the problem was more fundamental. It was just my sort of luck that kept me from finding the answer until I spent hours debugging and researching. Now it is easy to find several reports of this problem7. It is not a Leiningen problem, not a Clojure problem, not a reloaded pattern problem, but an annoyingly unfortunate interaction between them.

The Workaround

Luckily, once the problem was identified, there is a relatively easy workaround. I took the parts of user.clj which depended upon the custom exception and moved those to a new file reloaded.clj which is then loaded when the REPL starts by using the :repl-options configuration in project.clj. :repl-options as a :init configuration which can contain an expression which is evaluated when the REPL is starting. I set it to (load "reloaded").