Question

I’m trying out clojure on my second day and I don’t understand almost anything yet. I am working with the Programming Clojure 2nd ed. and I am stuck with libraries.

I have Leiningen and have the REPL running. The book first tells the reader to run a simple

(require 'clojure.java.io)

which works just fine (I get a nil). Then it wants to load a file called introduction.clj by running another simple

(require 'examples.introduction)

where I get an error message

FileNotFoundException Could not locate clojure/java/introduction__init.class
or clojure/java/introduction.clj on classpath:   clojure.lang.RT.load (RT.java:432)

I downloaded the introduction.clj file and looked where should I place it. The error and the book says the command will search in my classpath, but I have no idea where or what that is (after 1h of searching and reading I still don’t get it, sorry). I ran a few commands and I had many classpaths listed (from which none contain a clojure/java/io.clj).

So I tried another approach – find the io.clj file on my disk and simply copy the file there and run it with a command

(require 'clojure.java.introduction)

This doesn’t seem to work either. By the way, the io.clj file I found was in “C:\Program Files\clojure\src\clj\clojure\java”. I tried running several other .clj files from the java folder as well from the clojure folder, like javadoc.clj or inspector.clj and all seem to work just fine with the above mentioned command. Only the new file doesn’t seem to load this way.

Any help appreciated 🙂

Answer 1

Clojure runs on the Java Virtual Machine, so you will need to learn a bit about PATH and CLASSPATH concepts:

See: http://docs.oracle.com/javase/tutorial/essential/environment/paths.html

Regarding the error message, the Clojure runtime is expecting to find introduction.clj in the directory clojure\java\example\introduction.clj (not where it really should be – see below).

The convention for Clojure namespaces is that the last component is the file name, while any previous components are parent directories. So

clojure.java.introduction

would have to be in the directory (relative to your source “root” or classpath)

clojure\java\introduction.clj

(The lein REPL automatically adds your source root to the classpath).

Another concept you need to understand is where the “root” of your source code is located. For Leiningen (the build tool you are using) the default is either “src” or “src/main/clojure” – as documented in the Leiningen sample project file on GitHub).

Finally, if you get really stuck, it seems the complete project for the book is available on GitHub.

Looking at the project, I see that you should actually be placing the file under src\examples\introduction.clj