Clojure and Leiningen to manage Clojure projects
I would like to introduce you to Clojure. I am fascinated by how it works and it is also known to sharpen your thinking as a programmer. In this article, we'll do a basic setup and start programming in Clojure!
What is Clojure ?
Daniel Higginbotham in his book Clojure for the Brave and True has humorously defined Clojure as
an alloy of Lisp, functional programming and a lock of Rich Hickey's own epic hair
Leiningen is a useful tool to automate and manage your Clojure projects. Leiningen and Clojure require Java. Since, we are focusing on the JVM implementation of Clojure, OpenJDK will be required. You must have
> 1.6 java version installed.
Follow the steps on leiningen.org to download Leiningen. It will automatically download Clojure compiler,
Creating a new project
lein new app clojure-project
After installing you would find the following files in the directory.
project.clj(like package.json of npm) contains project dependencies.
resourcesfolder to save assets (like images).
src/clojure-project/core.cljwill be where you write your code.
Running the project
(ns clojure-project.core (:gen-class)) (defn -main "I don't do a whole lot...yet." [& args] (println "Hello, World!"))
This code will be already present in the
src/clojure-project/core.clj file. If you have ever used C, C++ you would know namespace and the main function. Well, the
defn -main is the starting point of your program.
cd into the clojure-project and run:
You should see
Hello, World! as the output!
Building the project
Clojure works even with environment change. It is not necessary that Leiningen is required to run the project elsewhere. To allow code shareability, we can create a stand-alone file that works where Java is installed.
After running this command a stand-alone file would be created in a target/uberjar/ folder (same directory level as src). This jar file can be distributed on any platform.
REPL - read, evaluate and print loop. It is a tool that takes single line and executes the code as soon as it evaluates (just like Console in web developer tools or python prompt).
To start Clojure repl
The prompt would look something like this:
nREPL server started on port 56969 on host 127.0.0.1 - nrepl://127.0.0.1:56969 REPL-y 0.4.4, nREPL 0.7.0 Clojure 1.10.1 OpenJDK 64-Bit Server VM 1.8.0_242-b08 Docs: (doc function-name-here) (find-doc "part-of-name-here") Source: (source function-name-here) Javadoc: (javadoc java-object-or-class-here) Exit: Control+D or (exit) or (quit) Results: Stored in vars *1, *2, *3, an exception in clojure-project.core=>
Clojure follows prefix notation and everything resides in matching parentheses.
Samples to try out:
(+ 1 2 3 4 5)should add up all numbers and return 15
( str "hello world" )should print
( str "Hi! " "John " "Doe" )should concatenate all the strings
( * 1 2 3 4 5 )should return multiplied value 120