Programming at the REPL: Troubleshooting

Calling a Var that has not been defined


user=> my-nonexistent-name
CompilerException java.lang.RuntimeException: Unable to resolve symbol: my-nonexistent-name in this context, compiling:(NO_SOURCE_PATH:0:0)


  • Maybe you made a typo in the Var name, e.g you should have written my-non-existent-name instead of my-nonexistent-name.

  • Maybe you forgot to define my-nonexistent-name in the REPL (using e.g def or defn): this could happen if you wrote a (def my-nonexistent-name …​) in your code file but forgot to evaluate it in the REPL.

  • Maybe you defined my-non-existent-name, but in another namespace. You should either write myapp.ns23/my-non-existent-name or switch to namespace myapp.ns23/my-non-existent-name.

Using a missing namespace alias


user=> ns3/foo-bar
CompilerException java.lang.RuntimeException: No such namespace: ns3, compiling:(NO_SOURCE_PATH:0:0)


  • Maybe you made a typo in the namespace alias, e.g you should have written n3/foo-bar instead of ns3/foo-bar

  • Maybe you have never defined the ns3 alias in the current namespace: you can fix that by evaluating (require '[myapp.ns3 :as ns3])

  • Maybe you have defined the ns3 alias, but in a different namespace that the one your REPL is in at the moment.

Referring to a namespace that has not been loaded


$ clj
Clojure 1.9.0
user=> clojure.set/union
CompilerException java.lang.ClassNotFoundException: clojure.set, compiling:(NO_SOURCE_PATH:0:0)


You are using a namespace that has yet not been created in the REPL. Note that the error message (‘ClassNotFoundException’) is particularly confusing in this case: the reason is that the Clojure compiler, having found no loaded namespace named clojure.set, is trying to interpret clojure.set as a Java class. The solution is to make sure the clojure.set lib has been loaded.

Trying to require a namespace that does not exist


user=> (require '[a.b.c])
FileNotFoundException Could not locate a/b/c__init.class or a/b/c.clj on classpath.  clojure.lang.RT.load (

Explanations: Clojure did not find any existing namespace named a.b.c, then searched the classpath for a lib that would define it, eventualling failing with the above Exception. The cause may vary:

  • maybe you just made a typo in the namespace name

  • maybe you forgot to add the lib to your classpath, which is usually done by adding it to the configuration of your project’s dependencies (e.g in the deps.edn or project.clj file), and sometimes (depending on the project management tool) running an installation command (e.g lein deps).

  • maybe you did add the library to the classpath, but that was after you started the REPL: in this case, you should re-start the REPL.