Learn you some Erlang for great good! : a beginner's guide

628 Pages·2013·15.48 MB·English

by  Fred Hébert

Learn You Some erLang
for great good!

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Erlang is the language of choice for • Passing messages, raising errors, and
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Erlang maestro Fred Hébert starts slow swyrnatpapxe adn ydo fuurn hcetiaodn aalr oduensidg nth cea sni minptilme i- example programms, Learn You Some Erlang Gonreat GoEordla! ng for
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of writing distributed, concurrent sia and ETS e
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taasx ,y oitus ddiavtea i nsttoru Ecrtluarnegs’,s i tfsu ntycptieo nsayls tfeanm- who used to teach Erlang. He is currently Ee
potatoes of the language: concurrency, into the sometimes-crazy, always-thrilling

distributed computing, hot code loading, (toars yla wcwok rotlhdrl,ed yr eooofu f’El!l)r ,ll aeanangrdn. baabsoiuc tf:unctional wPaocrkkeindg wointh a l irgehaftlh-teiamrete bdi dildluinstgr aptliaotnfos ramn d ra
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as you dive into Erlang’s functional fan- d i s wtriitbhwu tthheoed uocsoTemPd p fturoat itmnegea,wc hhoo rEtk rclaondge. lHoaed isin cgu, rrentlytw hoer lbde osft Ewralay ntog .le earn Erlang. f
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Erlanga snudc hw aas h noat mtoepdi cE arlmanogn gu steord aoyf ’tsh e year About the tAuthor
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Common Test YoTuHE SFIoNEmST eIN EGErElKa EnNTgER TfAoINrM GENrT™eat Fred Hébert is a self-taught programmer r
as you dwivwew .ninosttaorc hE.crolmang’s functional fan- who used to teaoch Erlang. He is currently e
• Building and releasing your applications Good!, is widely regarded as
with the oTP framework tasy wtohreld b, eyostu ’wlla lye atorn l eaabronu Et:rlang. working on a reaol-time bidding plaSHtEfolvrEm In : a
This book u“seIs RLeIpEK oFvLeAr—T”a durable binding that won’t snap shut. PrICE: $49.95a (n$5d2 .w95a CsD nn)a mePdr doEgrrlaamnmgI nugs learn goufa tghEeS/ Eyrelaanr g t
• Testing your applications with Eunit and 2012. His online !tutorial, Learn H ébert
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Common Test
You Some Erlang for Great
o Fred Hébert
• Building and releasing your applications Good!, is widely regarded as
THE FINEST IN GEEK ENTERTAINMENT™
o
www.nostarch.com with the oTP framework the best way to learn Erlang.
d Foreword by Joe Armstrong
SHElvE In:
!
“I LIE FLAT” PrICE: $49.95 ($52.95 CDn) ProgrammIng languagES/Erlang
This book uses RepKover—a durable binding that won’t snap shut.
Hébert
THE FINEST IN GEEK ENTERTAINMENT™
www.nostarch.com Fred Hébert
SHElvE In:
“I LIE FLAT” PrICE: $49.95 ($52.95 CDn) ProgrammIng languagES/Erlang Foreword by Joe Armstrong
This book uses RepKover—a durable binding that won’t snap shut.
Hébert
San Francisco
Fred Hébert
Foreword by Joe Armstrong
Learn You Some erLang for great good! Copyright © 2013 by Fred Hébert.
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Brief Contents
About the Author . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xvii
Foreword by Joe Armstrong . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xix
Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .xxi
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxiii
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Chapter 1: Starting Out . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Chapter 2: Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Chapter 3: Syntax in Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Chapter 4: Types (or Lack Thereof) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .55
Chapter 5: Hello Recursion! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61
Chapter 6: Higher-Order Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .77
Chapter 7: Errors and Exceptions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .87
Chapter 8: Functionally Solving Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Chapter 9: A Short Visit to Common Data Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . 121
Chapter 10: The Hitchhiker’s Guide to Concurrency . . . . . . . . . . . . . . . . . . . . . . . . . . . .135
Chapter 11: More on Multiprocessing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
Chapter 12: Errors and Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
Chapter 13: Designing a Concurrent Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .175
Chapter 14: An Introduction to OTP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .199
Chapter 15: Rage Against the Finite-State Machines . . . . . . . . . . . . . . . . . . . . . . . . . . . .219
Chapter 16: Event Handlers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247
Chapter 17: Who Supervises the Supervisors? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .263
Chapter 18: Building an Application . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 281
Chapter 19: Building Applications the OTP Way . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .303
Chapter 20: The Count of Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .315
Chapter 21: Release Is the Word . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .335
Chapter 22: Leveling Up in the Process Quest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .353
Chapter 23: Buckets of Sockets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 375
Chapter 24: EUnited Nations Council . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .397
Chapter 25: Bears, ETS, Beets: In-Memory NoSQL for Free! . . . . . . . . . . . . . . . . . . . . . . 419
Chapter 26: Distribunomicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .441
Chapter 27: Distributed OTP Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .473
Chapter 28: Common Test for Uncommon Tests . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .485
Chapter 29: Mnesia and the Art of Remembering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 511
Chapter 30: Type Specifications and Dialyzer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .543
Afterword . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 573
Appendix: On Erlang’s Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .577
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 581
vi Brief Contents
Contents in De tail
about the author xvii
foreword by Joe armstrong xix
Preface xxi
To the Foreigner . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxi
To the Erlang Regular . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxii
To the Person Who Has Read This Online . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xxii
acknowLedgmentS xxiii
IntroductIon 1
So What’s Erlang? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Don’t Drink Too Much Kool-Aid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
What You Need to Dive In . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Where to Get Help . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1
StartIng out 7
Using the Erlang Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Entering Shell Commands . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Exiting the Shell . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Some Erlang Basics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Numbers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Invariable Variables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Atoms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Boolean Algebra and Comparison Operators . . . . . . . . . . . . . . . . . . . . . . . . 14
Tuples . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
List Comprehensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Working with Binary Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Bit Syntax . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
Bitwise Binary Operations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26
Binary Strings . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Binary Comprehensions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
2
moduLeS 31
What Are Modules? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31
Creating Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
Compiling Code . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Compiler Options . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Defining Macros . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
More About Modules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Metadata . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40
Circular Dependencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
3
SYntax In functIonS 43
Pattern Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43
Fancier Patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 45
Variables in a Bind . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Guards, Guards! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
What the If ?! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
In case . . . of . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52
Which Should We Use? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 54
4
tYPeS (or Lack thereof) 55
Dynamite-Strong Typing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Type Conversions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
To Guard a Data Type . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58
For Type Junkies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60
5
heLLo recurSIon! 61
How Recursion Works . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 62
Length of a List . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
Length of a Tail Recursion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
More Recursive Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
A Duplicate Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
A Reverse Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66
A Sublist Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
A Zip Function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Quick, Sort! . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70
More Than Lists . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Thinking Recursively . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
6
hIgher-order functIonS 77
Let’s Get Functional . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Anonymous Functions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
More Anonymous Function Power . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Function Scope and Closures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
Maps, Filters, Folds, and More . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Fold Everything . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
More Abstractions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
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