Question

I’ve clojure running within Eclipse. I want to pass arguments to clojure when running it. In the below the arguments are available in the REPL but not in the script itself. What do I need to do such that in the below typing arg1 in the REPL will return the first argument?

Script:

(NS Test)
(def arg1 (nth *command-line-args* 0))

After clicking on the Eclipse “Run”…

Clojure 1.1.0
1:1 user=> #<Namespace test>
1:2 test=> arg1
nil
1:3 test=> *command-line-args*
("bird" "dog" "cat" "pig")
1:4 test=> (def arg2 (nth *command-line-args* 1))
#'test/arg2
1:5 test=>  arg2
"dog"
1:6 test=> 

Answer 1

Seems like maybe your arg1 is being defined before *command-line-args* gets a value. *command-line-args* is in clojure.core, so every namespace should be able to see it (unless you define a namespace and specifically tell it to exclude core or exclude that var). I don’t know how Eclipse launches a REPL or how/when it loads namespaces or user code, so I don’t know the problem.

But you could turn arg1 into a function, and then it should always return the current value of *command-line-args* (since it’ll be resolved at runtime, and *command-line-args* should have a value by the time you call the function).

(defn arg1 [] (nth *command-line-args* 0))

Better, if (nth *command-line-args* 0) is really that much to type itself (which I don’t think it is, really) you could write a better function:

(defn ARGV [n] (nth *command-line-args* n))

Then use (ARGV 0), (ARGV 1) etc. Keep in mind that vectors are themselves functions of their arguments, so you could just as easily do (*command-line-args* n) directly (once you’re sure *command-line-args* is not nil; otherwise you’ll get a NullPointerException.)

Using a lot of defs to assign names to things at the top-level is usually not idiomatic in Clojure. If you want to refer to command line arguments locally and just give them a shorter name for a while, there’s let:

(defn foo []
  (let [arg1 (nth *command-line-args* 0)]
    ...))

Again, this way arg1 should get its value at runtime (whenever you call foo) so it should work.