REPL up!

Before you set up your Clojure environment, the Java Development Kit (JDK) is necessary. The JDK version should be 1.6 or later. Throughout the book, we will use JDK 1.8 to develop and test the code.

This is how the command-line result will look, once you type java -version:

Leiningen is a standard build tool for Clojure. It simplifies the Clojure development, including setting up your project, compiling and testing code, and creating libraries for deployment.

It's easy to set up a Clojure environment using Leiningen. There are only a few steps before you can enjoy Clojure in REPL!

Here are the steps we need to perform to run Clojure REPL:

Here is a very simple code to demonstrate how REPL works. This code simply loops forever with read, eval, and println functions:

user=> (defn simple-repl [] 
  #_=>   (try 
  #_=>     (while true 
  #_=>       (println (eval (read)))  
  #_=>       ) 
  #_=>     (catch Exception e (println "exited..")) 
  #_=>     ) 
  #_=>   ) 
#'user/simple-repl 
user=> (simple-repl) 
(+ 1 1) 
2 
(defn hello [s] (println "Hello world " s)) 
#'user/hello 
(hello "Makoto") 
Hello world  Makoto 
nil 
exited.. 
nil 
user=> 

You can exit simple-repl by entering ^D (Ctrl + D).

Leiningen is a very powerful tool for Clojure developers. The lein new living-clojure command generates the following directory structure:

Let's pick up project.clj, which defines the project:

(defproject living-clojure "0.1.0-SNAPSHOT" 
  :description "FIXME: write description" 
  :url "http://example.com/FIXME" 
  :license {:name "Eclipse Public License" 
            :url "http://www.eclipse.org/legal/epl-v10.html"} 
  :dependencies [[org.clojure/clojure "1.8.0"]]) 

In project.clj, the :dependencies section declares the libraries used by your project.

Leiningen internally uses Maven to solve the dependencies of libraries. However, declaring libraries in project.clj is much simpler than doing it in the pom.xml file of Maven.

To use other libraries for your project, add them to the dependency section. We will review how to do this in a later recipe. In the preceding project.clj file, the Clojure library named org.clojure/clojure is declared and automatically downloads in the maven directory. This is the reason why you don't need to download and set up the Clojure library explicitly.

Without Leiningen, you have to do it in a more native way. Here are the steps:

REPL supports the command-line editing feature, like Linux bash shell does, but the preceding way does not. Another difference is that REPL solves library dependency problems in project.clj, but using the native way, you can solve them by yourself.

Please see related web sites as follows: