Setting Clojure "constants" at runtime

Question

I have a Clojure program that I build as a JAR file using Maven. Embedded in the JAR Manifest is a build-version number, including the build timestamp.

I can easily read this at runtime from the JAR Manifest using the following code:

(defn set-version
  "Set the version variable to the build number."
  []
  (def version
    (-> (str "jar:" (-> my.ns.name (.getProtectionDomain)
                                   (.getCodeSource)
                                   (.getLocation))
                    "!/META-INF/MANIFEST.MF")
      (URL.)
      (.openStream)
      (Manifest.)
      (.. getMainAttributes)
      (.getValue "Build-number"))))

but I've been told that it is bad karma to use def inside defn.

What is the Clojure-idiomatic way to set a constant at runtime? I obviously do not have the build-version information to embed in my code as a def, but I would like it set once (and for all) from the main function when the program starts. It should then be available as a def to the rest of the running code.

UPDATE: BTW, Clojure has to be one of the coolest languages I have come across in quite a while. Kudos to Rich Hickey!

Answer 1

I hope i dont miss something this time.

If version is a constant, it's going to be defined one time and is not going to be changed you can simple remove the defn and keep the (def version ... ) alone. I suppose you dont want this for some reason.

If you want to change global variables in a fn i think the more idiomatic way is to use some of concurrency constructions to store the data and access and change it in a secure way For example:

(def *version* (atom ""))

(defn set-version! [] (swap! *version* ...))

Answer 2

You could use dynamic binding.

(declare *version*)

(defn start-my-program []
  (binding [*version* (read-version-from-file)]
    (main))

Now main and every function it calls will see the value of *version*.