Getting started

Get up and running with Clojure


John Stevenson helps us take our first steps with the functional and dynamic JVM language, in this taster of his Developer.Press title

John Stevenson helps us take our
first steps with the functional and dynamic JVM language, in this
taster of his

There are many reasons
why I really enjoy Clojure as a software development language. The
most important of these is simplicity. Simplicity shines through in
the syntax of the language as well as the overall design approach
to software.

Clojure encourages a
modular approach to software, breaking down complexity to clearly
define data structures and composable behaviour. If you think of
small packets of behaviour working together to produce something
much larger than itself; this is how Clojure libraries and
applications are designed.

Constructing things
with Clojure reminds me of the Unix design; each aspect of it does
one thing really well allowing you to chain them all together
easily. The nature of the language encourages developers to create
Clojure libraries and applications with a highly modular approach,
providing a great way to build complex systems simply.

Functional design is

In Clojure, any
function you call returns a value when evaluated, even if that
value is null. This means that functions can be used as arguments
to other functions, helping you to design some very elegant and
modular solutions.

Functions calling
functions give the developer a way of dealing with complexity in a
simple way. This is achieved by breaking down behaviour into
effortless components that work cleanly together. Functions and data structures are represented by the same
syntax (homo-iconicity), this helps make your code simpler, more
concise and it encourages developers to become productive

Dynamic duo

Clojure comes with a
dynamic runtime environment called the REPL. This dynamic
environment allows you to define and run code as you write it,
making the language even easier to learn and experiment with.

Coupled with the
dynamic type nature of Clojure language, this makes development
really fly. You can quickly define and redefine your functions in
the REPL and add them to your codebase if valuable. The REPL can
load in your project code and help you discover ways to extend and
improve your code quickly without going through a lengthy build

Leveraging the Java

Having a functional
language on the Java Virtual Machine (JVM) get projects delivered
quickly by using a huge amount of libraries developers are already
familiar with.

Clojure does not need
to be a huge language because it can use so many tried and tested
libraries. This ranges from its own growing set of libraries to the
thousands of existing Java, Groovy and JRuby libraries.

Because of the language design, it is often easier to use
many Java libraries from Clojure. For example, when doing swing
programming, it is simpler using the

in Clojure
to chain method calls, avoiding lots of plumbing

After nearly two
decades of compiler and runtime optimisation by some very smart
people (including Martin Odersky of Scala and Java Collections
fame), the JVM is a very fast environment. In terms of performance,
Clojure is up there with a typical C/C++ application, with no
noticeable performance difference for most classes of

Deploying Clojure is
just as easy as deploying Java. Everything is compiled down to
byte-code onto the JVM. You create jar or war files and drop them
in to the environment just like any other Java bundles. If your
environment is not set up for Clojure, then you can create an
uberjar, a jar containing your Clojure application and the Clojure
libraries themselves. As Clojure is quite small, the uberjars
remain a manageable size.

First Steps in

Getting started with
Clojure can be as simple as firing up your browser and pointing it
to the Try Clojure website, as
shown in Figure 1. This website gives you a
dynamic runtime environment where you can start writing Clojure
straight away. Any code entered is evaluated immediately, giving
you instant results from your code. This gives us a very rapid way
of learning the language syntax.

Figure 1: repl in
a web page

But wait, we don’t
know any Clojure! We will cover the syntax of the language soon,
but if you want a taster you can follow the simple tutorial on All you have to do it to type
tutorial in
the window and follow the instructions. [Editor’s Note – soon means in the Developer.Press

As is a project on Github, you can get a
copy and run the website locally. The project is written using a
Clojure framework called Noir
(which in turn uses other Clojure frameworks called Ring, Compojure and
Hiccup). It
gives you an example of a Clojure web project,
which can guide you as you create your own web apps.

Your own Clojure
development environment is a great learning tool
where you can experiment with the syntax of the Clojure language.
To write your own applications though, you will want to set up a
Clojure development environment on your own machine. Arguably the
easiest and most humane way to work with Clojure is to use a tool
called Leiningen (pronounced Line – ing – en).

Leiningen is a build tool designed
especially for Clojure, allowing you to define your project tasks
in Clojure. Absolutely no more files full of XML anywhere!
Leiningen also manages all your project dependencies, just like
Maven and Ivy. Leiningen is also used to deploy your Clojure
project in a live environment, creating standard Java jar files. If
the environment does not have Clojure already available, Leiningen
can create an UberJar containing your application and Clojure.

Installing Leiningen

Download the latest
install script for Leiningen,
lein, from the website.

lein install
script will download a number of jar files that make up the
Leiningen project. First add the
lein install script to the
execution path of your operating system and then run it to download
the latest version.

Linux/MacOSX install

Assuming the lein script is
in your Downloads folder, move the lein script
into a folder called bin in your home directory (create
the bin folder if necessary):


~/Downloads/lein ~/bin

Make the lein script executable:

chmod +X

If this is the first time you have
created a bin directory for the login account you are using, the
environment settings need to be reloaded. Either logout and login
again or type the command:


Run the lein script when you
are connected to the Internet so it can download the required files

Running Leiningen once installed

The same script to install Leiningen
also runs it. When you run the lein script with no
arguments it shows you a list of tasks you can perform.



Build jar and deploy to remote repository 

deps Show details about
help Display a list of tasks or help for
a given task

Install current project to the local repository

jar Package up all the project’s files
into a jar file
new Generate project scaffolding based
on a template
pom Write a pom.xml file to disk for
Maven interoperability

 Start a repl session either with the current project or

run Run the project’s -main
test Run the project’s tests.
trampoline Run a task without nesting the
project’s JVM inside Leiningen’s
uberjar Package up the project files and
all dependencies into a jar file

 Upgrade Leiningen to specified version or latest

version Print version for Leiningen and the
current JVM 
with-profile  Apply the given task with the
profile(s) specified

Now you have Leiningen installed, you can use it to create your
own Clojure projects. You can also use Leiningen to run the REPL to
give you the dynamic coding environment. Next we will cover Clojure
development with Leiningen.

REPL driven development with Leiningen

The REPL is a full Clojure environment where you can write code
and get instant feedback on what it does. There is no need to fire
off a separate compiler or run a build tool and there is no need to
wait. Any correct Clojure code you write will run in the REPL.

Using the REPL gives you an opportunity to experiment and test
out your assumptions quickly. Developers continue to use the REPL
as part of their developer process as it’s a highly productive

Once you have Leiningen installed, you can run the REPL using:
lein repl as shown in Figure 2.


Figure 2: Starting up the REPL from

When the REPL starts, it loads several Clojure libraries, so you
can already use the core of the Clojure language. The REPL also
loads in the basic Java libraries so you can use some familiar
objects and API calls.

Once you have the REPL up and running you can call existing
functions in the Clojure language as well as define your own. You
can also define your own data structures that functions will act

Everything in the REPL is kept in memory
until you end your REPL session by calling the Clojure function

Writing Clojure code in the REPL

You can type Clojure code directly into the REPL and it will be
evaluated once you create a complete expression. An expression is a
piece of Clojure code defined within a balanced set of parentheses
– round brackets. A really simple example is shown in
Figure 3:

Figure 3: Simple adding in

Here we are using the + function to
add up several numbers and when we press return the expression is
evaluated. The result of the function is displayed on the line
following the code we have entered. We are then prompted to enter
more Clojure code.

Getting work done by calling

In Clojure you get the impression that everything is a function,
even data structures act like functions. This is a great thing
about the language, but can be a source of confusion at first. As
we have seen in the previous example, calling a function is simply
a matter of putting it in between parenthesis as the first item.
Here are some more examples to try. A familiar way to print strings
is to use the println command. When used in the REPL,
println will output in the REPL window, as shown in
Figure 4.

Figure 4: Printing to standard

Notice that after the string is printed, the value nil is
returned. All functions return a value when evaluated. If no value
is generated by the function, then a nil value is returned by
default. The list function generates an ordered list of
numbers from the arguments given, like in Figure

Figure 5: A sample list
data structure

Functions can be part of the arguments to another function (and
arguments to themselves as well). In the next code example
(Figure 6), we first create a list which is
ordered but is not arranged in numeric order. By wrapping the
list function in a sort function, the value
returned from list is sorted before being returned.

Figure 6: Examples of using the
list function

There are many functions that work on lists and other
collections of data. For example, you can use the function
max to find the largest numerical value from a list of
numbers, as shown in Figure 7.

Figure 7: Finding the maximum

You also have functions that explicitly use other functions to
process a collection of data, for example map and

map will
take the function it has been given and run it with each of the
elements in a collection (in this case an array, see

Figure 8). So each number in the array has the
inc function
mapped to it, incrementing each number in turn before returning the

Figure 8: Using the map

Show me the documentation

If you want to understand what a function
does, you can either search for it on, or look at the
built-in documentation as
shown in

Using the doc function you can read the documentation
for any function you specify as an argument, even (doc

Figure 9: In-built documentation
for the map function 

As you are still discovering the wide range of functions in
Clojure, you probably don’t know the name of all of them. Using the
function find-doc (Figure 10) you can
search through all the functions available from the REPL. This is
also handy if you are programming on the move and don’t have an
Internet connection.

Figure 10: Documentation for the
find-doc function

So if we want to use a function that iterates over a data
structure, we can use find-doc to list those functions
that may help us (see Figure 11). The
find-doc function returns the descriptions of three
functions that relate to the concept of iterate.

Figure 11: Finding the in-built
documentation for functions that iterate 

Missing some functions

If the function doc does not work, then it could be
that the library it is held in a Clojure namespace and has not been
added to your environment. To use the doc function and
several other useful functions, add the library
clojure.repl to your namespace with the function use
(use ‘clojure.repl)

Show me the source code

To get inspiration as you are learning Clojure and to see common
coding approaches, including documentation styles, you can use the
source function to look at the Clojure code for any function the
environment knows about, like in Figure 12.

Figure 12: Viewing the source code
for a function

In this example, the function documentation also includes a map
that defines when the documentation was added to the Clojure
language. You can also go to the Clojure Docs website for access to
definitions and source code

Author Bio: John is a Community Developer
at Heroku & Salesforce and an active organiser of several
communities in London. He runs workshops and speaks on technical
topics including open source projects, developer tools, continuous
delivery, TDD & BDD practices and declarative (functional)
programming (Clojure). John also presents on kanban as a way to
help individuals, teams and organisations become more effective. He
is a strong advocate of group learning and encourages others to get
involved with the community for their own personal growth. In his
spare time, John likes to cycle quite a lot (200-400km) on a
weekend if he is not too busy learning.

This is an excerpt from John’s
Developer.Press title ‘Clojure
Made Simple
‘, priced at $3.99. Developer.Press offers
digital format books on Java, PHP, mobile technologies and more at
low prices – find out more at the Developer.Press website and
check out the new releases.

John is the UK ambassador for Atlassian and active organiser of several communities in London. He runs workshops and speaks on technical topics including open source projects, developer tools, continuous delivery, TDD & BDD practices and declarative (functional) programming (Clojure). John also presents on kanban as a way to help individuals, teams and organisations become more effective. He is a strong advocate of group learning and encourages others to get involved with the community for their own personal growth.
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