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<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8"/>
<title>Clojure - Functional Programming for the JVM</title>
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<body>
<div style="text-align: right">
<a href="http://www.ociweb.com">Home</a> |
<a href="http://www.ociweb.com/articles/publications/jnb.html/">Java News Brief Archive</a> |
<a href="http://www.ociweb.com/education/">OCI Educational Services</a>
</div>
<table width="100%" border="0" cellpadding="0" cellspacing="0">
<tr>
<td>
<img alt="" src="images/OCILogo.png" width="180" height="120"/>
</td>
<td>
<img alt="" src="images/MarchJavaNewsBriefLeft.png" width="10" height="58"/>
</td>
<td style="width:100%">
<img alt="" src="images/MarchJavaNewsBriefMiddle.png" width="100%" height="58"/>
</td>
<td>
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</td>
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</table>
<div>
<hr />
<img alt="" src="images/MarchTechnicalInsightoftheMonth.png" width="377" height="34"/>
</div>
<h1>Clojure - Functional Programming for the JVM</h1>
<p class="author">
by<br />
<a href="http://www.ociweb.com/mark/" onclick="window.open(this.href,'_blank');return false;" title="Author Bio">R. Mark Volkmann</a>,
Partner <br />Object Computing, Inc. (OCI)<br />
last updated on 6/2/13
</p>
<p style="text-align:center">
<img src="images/clojure.png" alt="Clojure logo" width="400px"/>
</p>
<h2>Contents</h2>
<table border="1">
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Intro">Introduction</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#ConditionalProcessing">Conditional Processing</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#ReferenceTypes">Reference Types</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#FP">Functional Programming</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Iteration">Iteration</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Compiling">Compiling</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Overview">Clojure Overview</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Recursion">Recursion</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Testing">Automated Testing</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#GettingStarted">Getting Started</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Predicates">Predicates</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#EditorsIDEs">Editors and IDEs</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Syntax">Clojure Syntax</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Sequences">Sequences</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#DesktopApps">Desktop Applications</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#REPL">REPL</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#IO">Input/Output</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#WebApps">Web Applications</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Vars">Vars</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Destructuring">Destructuring</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Databases">Databases</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Collections">Collections</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Namespaces">Namespaces</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Libraries">Libraries</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#StructMaps">StructMaps</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Metadata">Metadata</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Conclusion">Conclusion</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#DefiningFunctions">Defining Functions</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Macros">Macros</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#References">References</a>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#JavaInterop">Java Interoperability</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px">
<a href="#Concurrency">Concurrency</a>
</td>
<td colspan="1" rowspan="1" style="padding:5px"> </td>
</tr>
</table>
<h2><a name="Intro">Introduction</a></h2>
<p>
The goal of this article is to provide a fairly comprehensive
introduction to the Clojure programming language.
A large number of features are covered, each in a fairly brief manner.
Feel free to skip around to the sections of most interest.
The section names in the table of contents are hyperlinks
to make this easier when reading on-line.
</p>
<p>
Please send feedback on errors and ways to improve explanations to
<a href="mailto:mark@ociweb.com">mark@ociweb.com</a>, or fork
<a href="https://github.com/mvolkmann/Clojure-Article">the repository</a>
and send a pull-request.
I'm especially interested in feedback such as:
</p>
<ul>
<li>You said X, but the correct thing to say is Y.</li>
<li>You said X, but it would be more clear if you said Y.</li>
<li>You didn't discuss X and I think it is an important topic.</li>
</ul>
<p>
Updates to this article that indicate the "last updated" date
and provide a dated list of changes will be provided at
<a href="http://www.ociweb.com/mark/clojure/">http://www.ociweb.com/mark/clojure/</a>.
Also see my article on software transactional memory
and the Clojure implementation of it at
<a href="http://www.ociweb.com/mark/stm/">http://www.ociweb.com/mark/stm/</a>.
</p>
<p>
Code examples in this article often show
the return value of a function call or its output
in a line comment (begins with a semicolon)
followed by "->" and the result.
For example:
</p>
<div class="code">
<pre xml:space="preserve">
(+ 1 2) ; showing return value -> 3
(println "Hello") ; return value is nil, showing output -> Hello
</pre>
</div>
<h2><a name="FP">Functional Programming</a></h2>
<p>
<a href="http://en.wikipedia.org/wiki/Functional_programming">Functional programming</a>
is a style of programming that emphasizes
"first-class" functions that are "pure".
It was inspired by ideas from the
<a href="http://en.wikipedia.org/wiki/Lambda_calculus">lambda calculus</a>.
</p>
<p>
"Pure functions" are functions that
always return the same result when passed the same arguments,
as opposed to depending on state that can change with time.
This makes them much easier to understand, debug and test.
They have no side effects such as changing global state or
performing any kind of I/O, including file I/O and database updates.
State is maintained in the values of function parameters
saved on the stack (often placed there by recursive calls)
rather than in global variables saved on the heap.
This allows functions to be executed repeatedly
without affecting global state (an important characteristic
to consider when transactions are discussed later).
It also opens the door for smart compilers to improve performance
by automatically reordering and parallelizing code,
although the latter is not yet common.
</p>
<p>
In practice, applications need to have some side effects.
Simon Peyton-Jones, a major contributor to the
functional programming language Haskell, said the following:
"In the end, any program must manipulate state.
A program that has no side effects whatsoever is a kind of black box.
All you can tell is that the box gets hotter."
(<a href="http://www.youtube.com/watch?v=iSmkqocn0oQ&t=3m20s">http://www.youtube.com/watch?v=iSmkqocn0oQ&t=3m20s</a>)
The key is to limit side effects, clearly identify them,
and avoid scattering them throughout the code.
</p>
<p>
Languages that support "first-class functions" allow functions to be
held in variables, passed to other functions and returned from them.
The ability to return a function supports
selection of behavior to be executed later.
Functions that accept other functions as arguments are called
"higher-order functions". In a sense, their operation is
configured by the functions that are passed to them.
The functions passed in can be executed any number of times,
including not at all.
</p>
<p>
Data in functional programming languages is typically immutable.
This allows data to be accessed concurrently
from multiple threads without locking.
There's no need to lock data that can't be changed.
With multicore processors becoming prevalent,
this simplification of programming for concurrency
is perhaps the biggest benefit of functional programming.
</p>
<p>
If all of this sounds intriguing and
you're ready to try functional programming,
be prepared for a sizable learning curve.
Many claim that functional programming isn't more difficult
than object-oriented programming, it's just different.
Taking the time to learn this style of programming
is a worthwhile investment in order to
obtain the benefits described above.
</p>
<p>
Popular functional programming languages include
<a href="http://clojure.org/">Clojure</a>,
<a href="http://en.wikipedia.org/wiki/Common_Lisp">Common Lisp</a>,
<a href="http://erlang.org/">Erlang</a>,
<a href="http://research.microsoft.com/en-us/um/cambridge/projects/fsharp/default.aspx">F#</a>,
<a href="http://www.haskell.org/">Haskell</a>,
<!--a href="http://en.wikipedia.org/wiki/Lisp_(programming_language)">Lisp</a>,-->
<a href="http://en.wikipedia.org/wiki/ML_(programming_language)">ML</a>,
<a href="http://caml.inria.fr/ocaml/index.en.html">OCaml</a>,
<a href="http://en.wikipedia.org/wiki/Scheme_(programming_language)">Scheme</a> and
<a href="http://www.scala-lang.org/">Scala</a>.
Clojure and Scala were written to
run on the Java Virtual Machine (JVM).
Other functional programming languages that have implementations
that run on the JVM include:
<a href="http://common-lisp.net/project/armedbear/">Armed Bear Common Lisp (ABCL)</a>,
<a href="http://ocamljava.x9c.fr/">OCaml-Java</a> and
<a href="http://www.gnu.org/software/kawa/">Kawa (Scheme)</a>.
</p>
<h2><a name="Overview">Clojure Overview</a></h2>
<p>
Clojure is a dynamically-typed, functional programming language
that runs on the JVM (Java 5 or greater)
and provides interoperability with Java.
A major goal of the language is to make it easier to implement
applications that access data from multiple threads (concurrency).
</p>
<p>
Clojure is pronounced the same as the word "closure".
The creator of the language, Rich Hickey, explains the name this way:
"I wanted to involve C (C#), L (Lisp) and J (Java).
Once I came up with Clojure, given the pun on closure,
the available domains and vast emptiness of the googlespace,
it was an easy decision."
</p>
<p>
Clojure is also available for the .NET platform.
ClojureCLR is an implementation of Clojure that runs on the
Microsoft Common Language Runtime instead of the JVM.
See <a href="https://github.com/clojure/clojure-clr">https://github.com/clojure/clojure-clr</a>.
</p>
<p>
In July 2011, ClojureScript was announced.
It compiles Clojure code to JavaScript. See
<a href="https://github.com/clojure/clojurescript">https://github.com/clojure/clojurescript</a>.
</p>
<p>
Clojure is an open source language released under the
<a href="http://www.eclipse.org/legal/epl-v10.html">Eclipse Public License v 1.0</a> (EPL).
This is a very liberal license.
See <a href="http://www.eclipse.org/legal/eplfaq.php">http://www.eclipse.org/legal/eplfaq.php</a> for more information.
</p>
<p>
Running on the JVM provides
portability, stability, performance and security.
It also provides access to a wealth of existing Java libraries
supporting functionality including
file I/O, multithreading, database access, GUIs, web applications,
and much more.
</p>
<p>
Each "operation" in Clojure is implemented as either
a function, macro or special form.
Nearly all functions and macros
are implemented in Clojure source code.
The differences between functions and macros are explained later.
Special forms are recognized by the Clojure compiler and
not implemented in Clojure source code.
There are a relatively small number of special forms
and new ones cannot be implemented.
They include
<a href="http://clojure.org/special_forms#try">catch</a>,
<a href="http://clojure.org/special_forms#toc1">def</a>,
<a href="http://clojure.org/special_forms#toc3">do</a>,
<a href="http://clojure.org/java_interop#dot">dot</a> ('.'),
<a href="http://clojure.org/special_forms#try">finally</a>,
<a href="http://clojure.org/special_forms#toc7">fn</a>,
<a href="http://clojure.org/special_forms#toc2">if</a>,
<a href="http://clojure.org/special_forms#toc4">let</a>,
<a href="http://clojure.org/special_forms#toc9">loop</a>,
<a href="http://clojure.org/special_forms#toc13">monitor-enter</a>,
<a href="http://clojure.org/special_forms#toc14">monitor-exit</a>,
<a href="http://clojure.org/java_interop#new">new</a>,
<a href="http://clojure.org/special_forms#toc5">quote</a>,
<a href="http://clojure.org/special_forms#toc12">recur</a>,
<a href="http://clojure.org/java_interop#set">set!</a>,
<a href="http://clojure.org/special_forms#try">throw</a>,
<a href="http://clojure.org/special_forms#try">try</a> and
<a href="http://clojure.org/special_forms#toc6">var</a>.
</p>
<p>
Clojure provides many functions that make it easy to operate on
"sequences" which are logical views of collections.
Many things can be treated as sequences.
These include Java collections, Clojure-specific collections,
strings, streams, directory structures and XML trees.
New instances of Clojure collections can be created
from existing ones in an efficient manner because they are
<a href="http://en.wikipedia.org/wiki/Persistent_data_structure">persistent data structures</a>.
</p>
<p>
Clojure provides three ways of safely sharing mutable data,
all of which use mutable references to immutable data.
<a href="#Refs">Refs</a> provide synchronous access
to multiple pieces of shared data ("coordinated") by using
<a href="http://en.wikipedia.org/wiki/Software_transactional_memory">Software Transactional Memory</a> (STM).
<a href="#Atoms">Atoms</a> provide synchronous access
to a single piece of shared data.
<a href="#Agents">Agents</a> provide asynchronous access
to a single piece of shared data.
These are discussed in more detail in
the "<a href="#ReferenceTypes">Reference Types</a>" section.
</p>
<p>
Clojure is a
<a href="http://en.wikipedia.org/wiki/Lisp_(programming_language)">Lisp</a>
dialect. However, it makes some departures from older Lisps.
For example, older Lisps use the <code>car</code> function to get
the first item in a list. Clojure calls this <code>first</code>
as does Common Lisp.
For a list of other differences, see
<a href="http://clojure.org/lisps">http://clojure.org/lisps</a>.
</p>
<p>
Lisp has a syntax that many people love ... and many people hate,
mainly due to its use of parentheses and prefix notation.
If you tend toward the latter camp, consider these facts.
Many text editors and IDEs highlight matching parentheses, so it
isn't necessary to count them in order to ensure they are balanced.
Clojure function calls are less noisy than Java method calls.
A Java method call looks like this:
</p>
<div class="code">
<pre xml:space="preserve">
methodName(arg1, arg2, arg3);
</pre>
</div>
<p>
A Clojure function call looks like this:
</p>
<div class="code">
<pre xml:space="preserve">
(function-name arg1 arg2 arg3)
</pre>
</div>
<p>
The open paren moves to the front and
the commas and semicolon disappear.
This syntax is referred to as a "form".
There is simple beauty in the fact that
everything in Lisp has this form.
Note that the naming convention in Clojure is to use
all lowercase with hyphens separating words in multi-word names,
unlike the Java convention of using camelcase.
</p>
<p>
Defining functions is similarly less noisy in Clojure.
The Clojure <code>println</code> function
adds a space between the output from each of its arguments.
To avoid this, pass the same arguments
to the <code>str</code> function and
pass its result to <code>println</code> .
</p>
<div class="code">
<pre xml:space="preserve">
// Java
public void hello(String name) {
System.out.println("Hello, " + name);
}
; Clojure
(defn hello [name]
(println "Hello," name))
</pre>
</div>
<p>
Clojure makes heavy use of lazy evaluation.
This allows functions to be invoked
only when their result is needed.
"Lazy sequences" are collections of results
that are not computed until needed.
This supports the efficient creation of infinite collections.
</p>
<p>
Clojure code is processed in three phases:
read-time, compile-time and run-time.
At read-time the Reader reads source code and
converts it to a data structure, mostly a list of lists of lists.
At compile-time this data structure is converted to Java bytecode.
At run-time the bytecode is executed.
Functions are only invoked at run-time.
Macros are special constructs
whose invocation looks similar to that of functions,
but are expanded into new Clojure code at compile-time.
</p>
<p>
Is Clojure code hard to understand?
Imagine if every time you read Java source code and
encountered syntax elements like <code>if</code> statements,
<code>for</code> loops, and anonymous classes,
you had to pause and puzzle over what they mean.
There are certain things that must be obvious to
a person who wants to be a productive Java developer.
Likewise there are parts of Clojure syntax that must be obvious
for one to efficiently read and understand code.
Examples include being comfortable with the use of
<code>let</code>, <code>apply</code>, <code>map</code>,
<code>filter</code>, <code>reduce</code>
and anonymous functions ... all of which are described later.
</p>
<h2><a name="GettingStarted">Getting Started</a></h2>
<p>
Clojure code for your own library and application projects will
typically reside in its own directory (named after the project)
and will be managed by the
<a href="http://leiningen.org/index.html">Leiningen</a>
project management tool. Leiningen (or "lein" for short) will
take care of downloading Clojure for you and making it available
to your projects. To start using Clojure, you don't need to
install Clojure, nor deal with jar files or
the <code>java</code> command — just install and use
<code>lein</code> (instructions on the Leiningen homepage,
linked to above).
</p>
<p>
Once you've installed lein, create a trivial project to start
playing around with:
</p>
<div class="code">
<pre xml:space="preserve">cd ~/temp
lein new my-proj
cd my-proj
lein repl # starts up the interactive REPL
</pre>
</div>
<p>
To create a new <i>application</i> project, do
"<code>lein new app my-app</code>"
</p>
<p>
For more about getting started, see
<a href="http://dev.clojure.org/display/doc/Getting+Started">http://dev.clojure.org/display/doc/Getting+Started</a>.
</p>
<h2><a name="Syntax">Clojure Syntax</a></h2>
<p>
Lisp dialects have a very simple, some would say beautiful, syntax.
Data and code have the same representation, lists of lists
that can be represented in memory quite naturally as a tree.
<code>(a b c)</code> is a call to a function named <code>a</code>
with arguments <code>b</code> and <code>c</code>.
To make this data instead of code, the list needs to be quoted.
<code>'(a b c)</code> or <code>(quote (a b c))</code>
is a list of the values
<code>a</code>, <code>b</code> and <code>c</code>.
That's it except for some special cases.
The number of special cases there are depends on the dialect.
</p>
<p>
The special cases are seen by some as syntactic sugar.
The more of them there are, the shorter certain kinds of code become
and the more readers of the code have to learn and remember.
It's a tricky balance.
Many of them have an equivalent function name
that can be used instead.
I'll leave it to you to decide if Clojure
has too much or too little syntactic sugar.
</p>
<p>
The table below briefly describes each of the special cases
encountered in Clojure code.
These will be described in more detail later.
Don't try to understand everything in the table now.
</p>
<table border="1">
<tr style="background:pink">
<th colspan="1" rowspan="1">Purpose</th>
<th colspan="1" rowspan="1">Sugar</th>
<th colspan="1" rowspan="1">Function</th>
</tr>
<tr>
<td colspan="1" rowspan="1">comment</td>
<td colspan="1" rowspan="1">
<code>; <i>text</i></code><br />
for line comments</td>
<td colspan="1" rowspan="1">
<code>(comment <i>text</i>)</code> macro<br />
for block comments
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
character literal (uses Java <code>char</code> type)</td>
<td colspan="1" rowspan="1">
<code>\<i>char</i></code>
<code>\tab</code><br />
<code>\newline</code>
<code>\space</code><br />
<code>\u<i>unicode-hex-value</i></code>
</td>
<td colspan="1" rowspan="1">
<code>(char <i>ascii-code</i>)</code><br />
<code>(char \u<i>unicode</i></code>)
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
string (uses Java <code>String</code> objects)</td>
<td colspan="1" rowspan="1">
<code>"<i>text</i>"</code>
</td>
<td colspan="1" rowspan="1">
<code>(str <i>char1</i> <i>char2</i> ...)</code><br />
concatenates characters and
many other kinds of values to create a string.
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
keyword; an interned string;
keywords with the same name refer to the same object;
often used for map keys
</td>
<td colspan="1" rowspan="1">
<code>:<i>name</i></code>
</td>
<td colspan="1" rowspan="1">
<code>(keyword "<i>name</i>")</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
keyword resolved in the current namespace
</td>
<td colspan="1" rowspan="1">
<code>::<i>name</i></code>
</td>
<td colspan="1" rowspan="1">
none
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">regular expression</td>
<td colspan="1" rowspan="1">
<code>#"<i>pattern</i>"</code><br />
quoting rules differ from function form
</td>
<td colspan="1" rowspan="1">
<code>(re-pattern <i>pattern</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
treated as whitespace;
sometimes used in collections to aid readability
</td>
<td colspan="1" rowspan="1">
<code>,</code> (a comma)</td>
<td colspan="1" rowspan="1">N/A</td>
</tr>
<tr>
<td colspan="1" rowspan="1">list - a linked list</td>
<td colspan="1" rowspan="1">
<code>'(<i>items</i>)</code><br />
doesn't evaluate items
</td>
<td colspan="1" rowspan="1">
<code>(list <i>items</i>)</code><br />
evaluates items
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">vector - similar to an array</td>
<td colspan="1" rowspan="1">
<code>[<i>items</i>]</code>
</td>
<td colspan="1" rowspan="1">
<code>(vector <i>items</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">set</td>
<td colspan="1" rowspan="1">
<code>#{<i>items</i>}</code><br />
creates a hash set
</td>
<td colspan="1" rowspan="1">
<code>(hash-set <i>items</i>)</code><br />
<code>(sorted-set <i>items</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">map</td>
<td colspan="1" rowspan="1">
<code>{<i>key-value-pairs</i>}</code><br />
creates a hash map
</td>
<td colspan="1" rowspan="1">
<code>(hash-map <i>key-value-pairs</i>)</code><br />
<code>(sorted-map <i>key-value-pairs</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
add metadata to a symbol or collection
</td>
<td colspan="1" rowspan="1">
<code>^{<i>key-value-pairs</i>} <i>object</i></code><br />
processed at read-time
</td>
<td colspan="1" rowspan="1">
<code>(with-meta <i>object</i> <i>metadata-map</i>)</code><br />
processed at run-time
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
get metadata map from a symbol or collection
</td>
<td colspan="1" rowspan="1">
</td>
<td colspan="1" rowspan="1">
<code>(meta <i>object</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
gather a variable number of arguments<br />
in a function parameter list</td>
<td colspan="1" rowspan="1">
<code>& <i>name</i></code>
</td>
<td colspan="1" rowspan="1">N/A</td>
</tr>
<tr>
<td colspan="1" rowspan="1">conventional name given to<br />
function parameters that aren't used</td>
<td colspan="1" rowspan="1">
<code>_</code> (an underscore)</td>
<td colspan="1" rowspan="1">N/A</td>
</tr>
<tr>
<td colspan="1" rowspan="1">construct a Java object;<br />
note the period after the class name</td>
<td colspan="1" rowspan="1">
<code>(<i>class-name</i>. <i>args</i>)</code>
</td>
<td colspan="1" rowspan="1">
<code>(new <i>class-name</i> <i>args</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">call a Java method</td>
<td colspan="1" rowspan="1">
<code>(. <i>class-or-instance</i> <i>method-name</i> <i>args</i>)</code> or <br />
<code>(.<i>method-name</i> <i>class-or-instance</i> <i>args</i>)</code>
</td>
<td colspan="1" rowspan="1">none</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
call several Java methods, threading the result
from each into the next as its first argument;<br />
each method can have additional arguments
specified inside the parens;<br />
note the double period</td>
<td colspan="1" rowspan="1">
<code>(.. <i>class-or-object</i>
(<i>method1 args</i>) (<i>method2 args</i>) ...)</code>
</td>
<td colspan="1" rowspan="1">none</td>
</tr>
<tr>
<td colspan="1" rowspan="1">create an anonymous function</td>
<td colspan="1" rowspan="1">
<code>#(<i>single-expression</i>)</code><br />
use <code>%</code> (same as <code>%1</code>), <code>%1</code>,
<code>%2</code> and so on for arguments
</td>
<td colspan="1" rowspan="1">
<code>(fn [<i>arg-names</i>] <i>expressions</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">dereference a Ref, Atom or Agent</td>
<td colspan="1" rowspan="1">
<code>@<i>ref</i></code>
</td>
<td colspan="1" rowspan="1">
<code>(deref <i>ref</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">
get <code>Var</code> object instead of<br />
the value of a symbol (var-quote)</td>
<td colspan="1" rowspan="1">
<code>#'<i>name</i></code>
</td>
<td colspan="1" rowspan="1">
<code>(var <i>name</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">syntax quote (used in macros)</td>
<td colspan="1" rowspan="1">
<code>`</code>
</td>
<td colspan="1" rowspan="1">none</td>
</tr>
<tr>
<td colspan="1" rowspan="1">unquote (used in macros)</td>
<td colspan="1" rowspan="1">
<code>~<i>value</i></code>
</td>
<td colspan="1" rowspan="1">
<code>(unquote <i>value</i>)</code>
</td>
</tr>
<tr>
<td colspan="1" rowspan="1">unquote splicing (used in macros)</td>
<td colspan="1" rowspan="1">
<code>~@<i>value</i></code>
</td>
<td colspan="1" rowspan="1">none</td>
</tr>
<tr>
<td colspan="1" rowspan="1">auto-gensym
(used in macros to generate a unique symbol name)</td>
<td colspan="1" rowspan="1">
<code><i>prefix</i>#</code>
</td>
<td colspan="1" rowspan="1">
<code>(gensym <i>prefix</i>?)</code>
</td>
</tr>
</table>
<p>
Lisp dialects use prefix notation rather than
the typical infix notation used by most programming languages
for binary operators such as <code>+</code> and <code>*</code>.
For example, in Java one might write <code>a + b + c</code>,
whereas in a Lisp dialect this becomes <code>(+ a b c)</code>.
One benefit of this notation is that any number of arguments
can be specified without repeating the operator.
Binary operators from other languages are Lisp functions
that aren't restricted to two operands.
</p>
<p>
One reason Lisp code contains more parentheses
than code in other languages is that it also uses them
where languages like Java use curly braces.
For example, the statements in a Java method are inside curly braces,
whereas the expressions in a Lisp function are inside
the function definition which is surrounded by parentheses.
</p>
<p>
Compare the following snippets of Java and Clojure code
that each define a simple function and invoke it.
The output from both is "edray" and "orangeay".
</p>
<div class="code">
<pre xml:space="preserve">
// This is Java code.
public class PigLatin {
public static String pigLatin(String word) {
char firstLetter = word.charAt(0);
if ("aeiou".indexOf(firstLetter) != -1) return word + "ay";
return word.substring(1) + firstLetter + "ay";
}
public static void main(String args[]) {
System.out.println(pigLatin("red"));
System.out.println(pigLatin("orange"));
}
}
</pre>
</div>
<p/>
<div class="code">
<pre xml:space="preserve">
; This is Clojure code.
; When a set is used as a function, it returns the argument if it is
; in the set and nil otherwise. When used in a boolean context,
; that indicates whether the argument is in the set.
(def vowel? (set "aeiou"))
(defn pig-latin [word] ; defines a function
; word is expected to be a string
; which can be treated like a sequence of characters.
(let [first-letter (first word)] ; assigns a local binding
(if (vowel? first-letter)
(str word "ay") ; then part of if
(str (subs word 1) first-letter "ay")))) ; else part of if
(println (pig-latin "red"))
(println (pig-latin "orange"))
</pre>
</div>
<p>
Clojure supports all the common data types such as booleans
(with literal values of <code>true</code> and <code>false</code>),
integers, decimals,
characters (see "character literal" in the table above) and strings.
It also supports ratios which retain a numerator and denominator
so numeric precision is not lost when they are used in calculations.
</p>
<p>
Symbols are used to name things.
These names are scoped in a namespace,
either one that is specified or the default namespace.
Symbols evaluate to their value.
To access the <code>Symbol</code> object itself, it must be quoted.
</p>
<p>
Keywords begin with a colon and are used as unique identifiers.
Examples include keys in maps and enumerated values
(such as <code>:red</code>, <code>:green</code>
and <code>:blue</code>).
</p>
<p>
It is possible in Clojure, as it is in any programming language,
to write code that is difficult to understand.
Following a few guidelines can make a big difference.
Write short, well-focused functions to make them
easier to read, test and reuse.
Use the "extract method" refactoring pattern often.
Deeply nested function calls can be hard to read.
Limit this nesting where possible,
often by using <code>let</code> to break
complicated expressions into several less complicated expressions.
Passing anonymous functions to named functions is common.
However, avoid passing anonymous functions
to other anonymous functions
because such code is difficult to read.
</p>
<h2><a name="REPL">REPL</a></h2>
<p>
REPL stands for read-eval-print loop.
This is a standard tool in Lisp dialects that
allows a user to enter expressions, have them read and evaluated,
and have their result printed.
It is a very useful tool for testing and
gaining an understanding of code.
</p>
<p>
To start a REPL, enter "<code>lein repl</code>" at a command prompt.
This will display a prompt of "<code>user=></code>".
The part before "<code>=></code>"
indicates the current default namespace.
Forms entered after this prompt are evaluated
and their result is output.
Here's a sample REPL session that shows both input and output.
</p>
<div class="code">
<pre xml:space="preserve">
user=> (def n 2)
#'user/n
user=> (* n 3)
6
</pre>
</div>
<p>
<code>def</code> is a special form that