Erlang编程精要

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"Erlang Programming" Erlang是一种面向并发的、函数式的编程语言，最初由爱立信开发，用于构建高度可靠和可扩展的电信系统。《Erlang编程指南》由Francesco Cesarini和Simon Thompson撰写，是学习Erlang语言的重要参考资料。书中深入探讨了Erlang的核心特性，包括它的并发模型、分布式计算、故障恢复机制以及模式匹配等概念。 1. 并发性：Erlang的设计核心之一就是并发处理。它采用轻量级进程（processes）的概念，这些进程之间的通信通过消息传递完成，而不是共享内存。这种模型降低了竞态条件和死锁的风险，使得编写高并发程序变得更加简单。 2. 函数式编程：Erlang是函数式的，这意味着它强调纯函数，没有副作用，变量不可变。这使得代码更容易理解和测试，也有利于并行执行。 3. 模式匹配：Erlang中的模式匹配是其语法的一大特色，允许在函数定义时指定输入值的模式，提高了代码的可读性和简洁性。 4. 错误处理与容错：Erlang的哲学是“让失败快速发生”，它鼓励通过设计来容忍失败，而不是避免失败。当进程崩溃时，可以自动重启，这种机制使得系统具有高度的容错性。 5. 分布式计算：Erlang支持跨多台机器的分布式计算，节点之间可以轻松通信，创建大规模的分布式系统。 6. OTP（Open Telecom Platform）：OTP是Erlang的标准库，包含了一系列的库、设计模式和工具，用于构建高效、可靠、可维护的系统。 7. 热更新：Erlang允许在运行时更新代码，而无需停止整个系统，这对于保持服务连续性至关重要。 8. 并发编程的工具：如监控器、链接和陷阱退出等，这些都是Erlang提供的用于管理和调试并发程序的工具。 9. 数据类型：Erlang提供了原子（atoms）、列表、元组、二进制、整数和浮点数等基本数据类型，以及动态类型的特性。 10. 编译与解释：Erlang代码既可以被编译成字节码执行，也可以解释执行，提供了开发和调试的灵活性。这本书不仅涵盖了Erlang的基础语法和特性，还讲解了如何利用Erlang构建实际的并发系统，适合对并发编程和函数式编程感兴趣的读者。通过阅读《Erlang编程指南》，读者可以学习到如何有效地利用Erlang解决复杂的问题，特别是在需要构建高可用性、分布式系统时。

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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