Erlang编程指南：函数式语言的高并发解决方案

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"Erlang编程指南是一本由Francesco Cesarini和Simon Thompson编写的书籍，主要介绍Erlang编程语言。Erlang是一种专门针对高并发、高容错环境设计的小众函数式编程语言。该书由O'Reilly Media出版，并提供了英文版在线阅读的选项。" 在Erlang编程指南中，读者将深入学习到Erlang的核心概念和特性。Erlang作为一种函数式编程语言，其主要特点包括： 1. **并行处理**：Erlang设计时考虑了分布式计算，支持轻量级进程（lightweight processes）的大量并发执行，这些进程间的通信高效且可靠，使得Erlang成为构建大规模并发系统的选择。 2. **错误恢复和容错性**：Erlang强调系统的健壮性和容错性，通过模式如“let it crash”来鼓励快速失败和自我修复，从而提高了整体系统的稳定性和可靠性。 3. **函数式编程**：Erlang的函数式编程风格鼓励不可变数据和纯函数，这有助于减少副作用，提高代码可读性和测试性。 4. **模式匹配**：Erlang中的模式匹配是其语法的一大特色，允许在函数调用中进行解构和比较，简化了复杂数据结构的处理。 5. **热代码升级**：Erlang系统支持在运行时进行代码替换和升级，无需停止服务，这对于维护大型分布式系统至关重要。 6. **BEAM虚拟机**：Erlang运行在BEAM（Erlang Virtual Machine）上，它优化了并发处理和内存管理，为Erlang的性能提供了坚实的基础。 7. **OTP（Open Telecom Platform）**：Erlang OTP提供了一套标准库和设计模式，包括进程间通信、分布式服务、监控和日志等功能，用于构建高效、可靠的分布式应用。 8. **简洁的文本协议和网络通信**：Erlang天生适合处理网络协议和通信，其内置的简单协议处理能力和强大的TCP/IP支持使其在构建网络服务方面表现出色。通过阅读本书，开发者可以学习如何编写Erlang代码，理解其并发模型，掌握如何利用Erlang进行错误处理和调试，以及如何构建可扩展和容错的应用。此外，书中还可能涵盖Erlang与其他技术的集成，如Web开发框架如 Cowboy 或 Mochiweb，以及数据库接口等。《Erlang Programming》不仅是一本技术指南，也是深入理解和运用Erlang语言的宝贵资源，对于那些希望在高并发、分布式领域工作或对函数式编程感兴趣的开发者来说，这本书无疑是一份重要的学习资料。

Preface

What made us start writing this book in the first place is the enthusiasm we share for

Erlang.

We wanted to help get the word out, giving back a little of what the community

has given to us. Although we both got into Erlang for very different reasons, the end

result was the same: lots of fun hours doing lots of fun stuff at a fraction of the effort

it would have taken with other languages. And best of all, it is not a tool we use for

hobby projects, but one we use on a daily basis in our real jobs!

Francesco: Why Erlang?

The year was 1994. While studying computer science at Uppsala University, one of the

courses I took was on parallel programming. The lecturer held up the first edition of

Concurrent Programming in Erlang (Prentice Hall) and said, “Read it.” He then held

up a handout and added, “These are the exercises, do them,” after which Erlang barely

got a mention; it was quickly overshadowed with the theory of threads, shared memory,

semaphores, and deadlocks.

As the main exercise for this course, we had to implement a simulated world inhabited

by carrots, rabbits, and wolves. Rabbits would roam this world eating carrots that grew

in random patches. When they had eaten enough carrots, the rabbits would get fat and

split in two. Wolves ran around eating up the rabbits; if they managed to catch and eat

enough rabbits, they would also get fat and split. Rabbits and wolves within a certain

distance of each other would broadcast information on food and predators. If a rabbit

found a carrot patch, other rabbits would quickly join him. If a wolf found a rabbit,

the pack would start chasing it.

The final result was amusingly fun to watch. The odd rabbit would run straight into a

group of wolves, while others would run in other directions, sometimes stopping to

grab a carrot en route. Every carrot patch, rabbit, and wolf was represented as an Erlang

process communicating through message passing.

The exercise took me about 40 hours to solve. Although I enjoyed using Erlang and

was positively surprised at the simplicity of its concurrency model and lack of OS

threads for every process, I did not think that much of it right there and then. After all,

it was one of the dozen or so languages I had to learn for my degree. Having used ML

in my functional programming courses and ADA in my real-time programming courses,

for me Erlang was just another language in the crowd. That changed a few months later

when I started studying object-oriented programming.

In the object-oriented (OO) programming course, we were given the same simulated

world lab but had to solve it with Eiffel, an OO language our new lecturer insisted was

ideal for simulations. Although I had already solved the same problem and was able to

reuse a good part of the algorithms, it took me and a fellow student 120 man-hours to

solve.

This was the eye-opener that led me to believe the declarative and concurrent features

in Erlang had to be the direction in which software development was heading. At the

time, I was not sure whether the language that would lead the way in this paradigm

shift was going to be Erlang, but I was certain that whatever language it was, it would

be heavily influenced by Erlang and its ancestors. I picked up the phone and called Joe

Armstrong, one of the inventors of Erlang. A week later, I visited the Ericsson Computer

Science Lab for an interview, and I have never looked back.

Simon: Why Erlang?

I have worked in functional programming since the early 1980s, and have known about

Erlang ever since it was first defined about 20 years ago. What I find most attractive

about Erlang is that it’s a language that was designed from the start to solve real and

difficult problems, and to do it in an elegant and powerful way. That’s why we’ve seen

Erlang used in more and more systems in recent years.

It’s also a small language, which makes writing tools for it much more practical than

for a language such as Java, C++, or even Haskell. This, and the quality of the libraries

we’ve been able to build on in our work, has helped the functional programming group

at Kent to be very productive in implementing the Wrangler refactoring tool for Erlang.

Who Should Read This Book?

We have written this book to introduce you to programming in Erlang. We don’t expect

that you have programmed in Erlang before, nor do we assume that you are familiar

with functional programming in other languages.

We do expect you to have programmed in Java, C, Ruby, or another mainstream lan-

guage, and we’ve made sure that we point out to you where Erlang differs from what

you’re used to.

xvi | Preface

How to Read This Book

We wrote this book in two parts, the first to be read sequentially and the second can

be read concurrently (or sequentially in whatever order you like), as the chapters are

independent of each other.

The first 11 chapters of the book cover the core parts of Erlang:

• Chapter 1 gives a high-level introduction to the language, covering its key features

for building high-availability, robust concurrent systems. In doing this, we also

describe how Erlang came to be the way it is, and point out some of its high-profile

success stories, which explain why you may want to adopt Erlang in one of your

projects.

• The basics of sequential programming in Erlang are the subject of Chapters 2 and

3. In these chapters, we cover the central role of recursion in writing Erlang pro-

grams, as well as how single assignment in Erlang is quite different from the way

variables are handled in other languages, such as C and Java.

• While covering sequential programming, we also introduce the basic data types of

Erlang—numbers, atoms, strings, lists, and tuples—comparing them with similar

types in other languages. Other types are covered later: records in Chapter 7, and

function types and binaries in Chapter 9. Large-scale storage in ETS tables is the

topic of Chapter 10.

• Erlang’s distinctiveness comes to the fore in Chapters 4–6, which together cover

the concurrent aspects of Erlang, embodied in message passing communication

between concurrently executing processes running in separate memory spaces.

• It is possible to “hot-swap” code in a system, supporting software upgrades in run-

ning systems: this is the topic of Chapter 8.

• To conclude this part of the book, we cover distributed programming in Chap-

ter 11. This allows different Erlang runtime systems (or nodes), which might be

running on the same or different machines, to work together and interact as a

distributed system.

In the remaining chapters, we cover a variety of different topics largely independent of

each other. These include the following:

• The Open Telecom Platform (OTP) gives a set of libraries and design principles

supporting the construction of robust, scalable systems in Erlang; this is the subject

of Chapter 12.

• The Erlang distribution contains some standard computing applications: we cover

the Mnesia database in Chapter 13 and the wxErlang GUI programming library in

Chapter 14.

Preface | xvii

• Erlang distribution gives one mechanism for linking Erlang systems to each other.

Chapter 15 shows how Erlang supports programming across the Internet using

sockets, and Chapter 16 covers the various ways in which Erlang can interwork

with systems written in C, Java, and Ruby, as well as many other languages.

• The standard Erlang distribution comes with a number of very useful tools, and

we cover some of these next. Chapter 17 explains in depth how all aspects of Erlang

systems can be traced without degrading their performance, and Chapter 18 covers

tools for checking the correctness of programs, and for constructing documenta-

tion for Erlang systems. Unit testing, and how it is supported by EUnit, is the

subject of Chapter 19.

• The last chapter, Chapter 20, looks at how to write programs that are elegant,

readable, and efficient, and pulls together into one place much of the accumulated

experience of the Erlang community.

The Appendix covers how to get started with Erlang, how to use the Erlang shell,

popular tools for Erlang, and how to find out more about Erlang.

Each chapter is accompanied by a set of exercises, and you can download all the code

in this book from its website:

http://www.erlangprogramming.org

The website also has references to further reading as well as links to the major sites

supporting the Erlang community.

wrote this book to be compatible with Erlang Release 13 (R13-B). Most of the

features we describe will work with earlier releases; known incompatibilities with more

recent earlier releases are detailed on our website.

Conventions Used in This Book

The following typographical conventions are used in this book:

Italic

Indicates new terms, URLs, filenames, file extensions, and occasionally, emphasis

and keyword phrases.

Constant width

Indicates computer coding in a broad sense. This includes commands, options,

variables, attributes, keys, requests, functions, methods, types, classes, modules,

properties, parameters, values, objects, events, event handlers, XML and XHTML

tags, macros, and keywords.

Constant width bold

Indicates commands or other text that the user should type literally.

xviii | Preface

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