Clojure symbol evaluation error - clojure

Question

So I currently have this code:
(ns contact-form.core
(:gen-class))
(def foo "Hello World!")
(defn some-func [a-symbol]
(println (str a-symbol "'s value is: " (eval a-symbol))))
(defn -main [& args]
(some-func 'foo))
After I do C-c C-k in Emacs, I get the following output:
contact-form.core> (-main)
foo's value is: Hello World!
nil
But when I do lein uberjar and run the resulting jar file, I get an error:
Exception in thread "main" java.lang.RuntimeException: Unable to resolve symbol: foo in this context, compiling:(NO_SOURCE_PATH:0)
at clojure.lang.Compiler.analyze(Compiler.java:6235)
at clojure.lang.Compiler.analyze(Compiler.java:6177)
at clojure.lang.Compiler.eval(Compiler.java:6469)
at clojure.lang.Compiler.eval(Compiler.java:6431)
at clojure.core$eval.invoke(core.clj:2795)
at contact_form.core$some_func.invoke(core.clj:7)
at contact_form.core$_main.doInvoke(core.clj:10)
at clojure.lang.RestFn.invoke(RestFn.java:397)
at clojure.lang.AFn.applyToHelper(AFn.java:159)
at clojure.lang.RestFn.applyTo(RestFn.java:132)
at contact_form.core.main(Unknown Source)
Caused by: java.lang.RuntimeException: Unable to resolve symbol: foo in this context
at clojure.lang.Util.runtimeException(Util.java:156)
at clojure.lang.Compiler.resolveIn(Compiler.java:6720)
at clojure.lang.Compiler.resolve(Compiler.java:6664)
at clojure.lang.Compiler.analyzeSymbol(Compiler.java:6625)
at clojure.lang.Compiler.analyze(Compiler.java:6198)
... 10 more
So I have two questions:
Why does the uberjar not function exactly the same as the REPL?
What can I do to fix this problem?

Answer 1

1. Why does uberjar function differently to the REPL?
A cause of the error "NO_SOURCE_PATH" is that you are not presently in the namespace that defined 'foo.
To illustrate: if I evaluate your code in my REPL and execute it, it places me in the contact-form.core namespace as you would expect because (ns contact-form.core) is evaluated by the REPL, but if I switch to the user namespace and call -main I can produce the same error:
contact-form.core=> (-main)
foo's value is: Hello World!
nil
contact-form.core=> (ns user)
nil
user=> (contact-form.core/-main)
CompilerException java.lang.RuntimeException: Unable to resolve symbol: foo in this context, compiling:(NO_SOURCE_PATH:120)
user=>
So, by specifying your entry point to main for standalone uberjar execution (outside the REPL), it is equivalent to calling (contact-form.core/-main) from the default namespace in your jar which is clojure.core, because (ns contact-form.core) has not been evaluated. Result: main can be executed with a fully qualified (namespaced) path to the function, but none of the symbols from contact-form.core are available in the current default namespace.
2. The fix
The solution would be to explicitly switch to your namespace first.:
(defn -main [& args]
(use 'contact-form.core)
(some-func 'foo))