专业 Clojure 开发指南：实战与深度解析

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Clojure

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xix

INTRODUCTION

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CONVENTIONS

To help you get the most from the text and keep track of what’s happening, we’ve used a number of

conventions throughout the book.

NOTE Notes indicates notes, tips, hints, tricks, and/or asides to the current

discussion.

As for styles in the text:

➤

We highlight new terms and important words when we introduce them.

➤

We show code within the text like so: persistence.properties.

➤

We show all code snippets in the book using this style:

FileSystem fs = FileSystem.get(URI.create(uri), conf);

InputStream in = null;

try {

➤

URLs in text appear like this: http://<Slave Hostname>:50075.

SOURCE CODE

As you work through the examples in this book, you may choose either to type in all the code man-

ually, or to use the source code  les that accompany the book. All of the source code used in this

book is available for download at

www.wiley.com. Speci cally for this book, the code download is

on the Download Code tab at:

www.wiley.com/go/professionalclojure

You can also search for the book at www.wrox.com by ISBN (the ISBN for this book is 9781119267171

to  nd the code. And a complete list of code downloads for all current Wrox books is available at

www.wiley.com/dynamic/books/download.aspx.

NOTE Because many books have similar titles, you may  nd it easiest to search

by ISBN; this book’s ISBN is 978-1-119-26727-0.

Once you download the code, just decompress it with your favorite compression tool. Alternately,

you can go to the main Wrox code download page at

www.wrox.com/dynamic/books/download

.aspx

to see the code available for this book and all other Wrox books.

INTRODUCTION

 ast.indd 05/03/2016 Page xx

ERRATA

We make every effort to ensure that there are no errors in the text or in the code. However, no one

is perfect, and mistakes do occur. If you  nd an error in one of our books, like a spelling mistake

or faulty piece of code, we would be very grateful for your feedback. By sending in errata, you may

save another reader hours of frustration, and at the same time, you will be helping us provide even

higher quality information.

To  nd the errata page for this book, go to

www.wiley.com/go/

and click the Errata link. On

this page you can view all errata that has been submitted for this book and posted by Wrox

editors.

If you don’t spot “your” error on the Book Errata page, go to

www.wrox.com/contact/techsupport

.shtml

and complete the form there to send us the error you have found. We’ll check the informa-

tion and, if appropriate, post a message to the book’s errata page and  x the problem in subsequent

editions of the book.

P2P.WROX.COM

For author and peer discussion, join the P2P forums at http://p2p.wrox.com. The forums are a

web-based system for you to post messages relating to Wrox books and related technologies and

interact with other readers and technology users. The forums offer a subscription feature to e-mail

you topics of interest of your choosing when new posts are made to the forums. Wrox authors, edi-

tors, other industry experts, and your fellow readers are present on these forums.

http://p2p.wrox.com, you will  nd a number of different forums that will help you, not only as

you read this book, but also as you develop your own applications. To join the forums, just follow

these steps:

1. Go to http://p2p.wrox.com and click the Register link.

2. Read the terms of use and click Agree.

3. Complete the required information to join, as well as any optional information you wish to

provide, and click Submit.

4. You will receive an e-mail with information describing how to verify your account and com-

plete the joining process.

NOTE You can read messages in the forums without joining P2P, but in order to

post your own messages, you must join.

❘

CHAPTER 1 HAVE A BEGINNER’S MIND

c01.indd 04/23/2016 Page 2

Forget everything you know, or think you know about programming, and instead approach it as if

you were learning your very  rst programming language. Otherwise, you’ll just be learning a new

syntax, and your Clojure code will look more like Java/C/Ruby and less like Clojure is designed to

look. Learning Clojure/Lisp will even affect the way you write in other languages, especially with

Java 8 and Scala becoming more popular.

FUNCTIONAL THINKING

C, C++, C#, Java, Python, Ruby, and even to some extent Perl, all have very similar syntax. They

make use of the same programming constructs and have an emphasis on an imperative style of pro-

gramming. This is a style of programming well suited to the von Neumann architecture of comput-

ing that they were designed to execute in. This is probably most apparent in the C language, where

you are responsible for allocating and de-allocating memory for variables, and dealing directly with

pointers to memory locations. Other imperative languages attempt to hide this complexity with

varying degrees of success.

In computer science, imperative programming is a programming paradigm that

uses statements that change a program’s state.

This C-style of programming has dominated the programming scene for a very long time, because

it  ts well within the dominant hardware architectural paradigm. Programs are able to execute

very ef ciently, and also make ef cient use of memory, which up until recently had been a very real

constraint. This ef ciency comes at the cost of having more complex semantics and syntax, and it is

increasingly more dif cult to reason about the execution, because it is so dependent upon the state

of the memory at the time of execution. This makes doing concurrency incredibly dif cult and error

prone. In these days of cheap memory and an ever growing number of multiple core architectures, it

is starting to show its age.

Functional programming, however, is based on mathematical concepts, rather than any given com-

puting architecture. Clojure, in the spirit of Lisp, calls itself a general-purpose language; however, it

does provide a number of functional features and supports the functional style of programming very

well. Clojure as a language not only offers simpler semantics than its imperative predecessors, but it

also has arguably a much simpler syntax. If you are not familiar with Lisp, reading and understand-

ing Clojure code is going to take some practice. Because of its heavy focus on immutability, it makes

concurrency simple and much less error prone than having to manually manage locks on memory

and having to worry about multiple threads reading values simultaneously. Not only does Clojure

provide all of these functional features, but it also performs object-oriented programming better

than its Java counterpart.

Value Oriented

Clojure promotes a style of programming commonly called “value-oriented programming.”

Clojure’s creator, Rich Hickey, isn’t the  rst person to use that phrase to describe functional

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