Clojure Command Line Arguments II



I wrote a small blog on parsing command line earlier this week. The only comment I got on it from punch was:

I'd like to learn from this post why command-line-args didn't work. What is it, actually, the command-line-args thingy, etc.?

Those are good questions. I didn't know answer to them. As I talk in post I wanted a simple solution to parsing args and another confusing experience brought me back to the same question, "Why args behave the way they do and What are *command-line-args*". I still don't have answer to them. In this post I am documenting two things, for my own better understanding. One around reproducing the issue of jar not able to work with *command-line-args*. Second one around limited features of sequence that are supported by args

Reproducing behaviour of jar (non)handling of *command-line-args*

We create a new project using lein:

$ lein new app cli-args
Generating a project called cli-args based on the 'app' template.
$ cd cli-args/
$ lein run
Hello, World!

We edit src/cli_args/core.clj to print args and *command-line-args*

cat <<EOF > src/cli_args/core.clj
(ns cli-args.core
  (:gen-class))

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (println "Printing args.." args)
  (println "Printing *command-line-args*" *command-line-args*))
EOF

$ lein run optional arguments

Now we create jar using lein uberjar

$ lein uberjar
Compiling cli-args.core
Created target/uberjar/cli-args-0.1.0-SNAPSHOT.jar
Created target/uberjar/cli-args-0.1.0-SNAPSHOT-standalone.jar
$ cd target/uberjar/
$ java -jar cli-args-0.1.0-SNAPSHOT-standalone.jar testing more optional arguments
Printing args.. (testing more optional arguments)
Printing *command-line-args* nil

Clojure is able to handle *command-line-args* but java is not. That narrows down the problem and can possibly lead to explanation on why it is happening(Maybe in another post).

Sequence features supported by args

I noticed another anomaly with args. I was passing couple of arguments and I noticed that it doesn't support get method.

cat <<EOF > src/cli_args/core.clj
(ns cli-args.core
  (:gen-class))

(defn -main
  "I don't do a whole lot ... yet."
  [& args]
  (println "first argument" (first args))
  (println "second argument" (second args))
  (println "third argument" (get args 3)))
EOF

This is what I noticed as I tried different inputs:

$ lein run
first argument nil
second argument nil
third argument nil
$ lein run hello world
first argument hello
second argument world
third argument nil
$ lein run hello world 3rd argument
first argument hello
second argument world
third argument nil

The get method doesn't work. I printed type for args and it is clojure.lang.ArraySeq. For my case, I "managed" by using last and that gave me what I wanted. Still, I am running out of options and I would have to either dig deeper to understand args or fall back to using a library(tools.cli).