Haskell Programming from First Principles: Pure functional programming without fear or frustation

Reader feedback....2

Contents....4

Authors' preface....21

Acknowledgements....26

Introduction....30

Why This Book....30

A few words to new programmers....35

Haskevangelism....36

What's in this book?....40

Best practices for examples and exercises....44

All You Need is Lambda....49

All You Need is Lambda....50

What is functional programming?....50

What is a function?....52

The structure of lambda terms....55

Beta reduction....58

Multiple arguments....63

Evaluation is simplification....68

Combinators....69

Divergence....71

Summary....72

Chapter Exercises....73

Answers....75

Definitions....79

Follow-up resources....81

Hello, Haskell!....82

Hello, Haskell....83

Interacting with Haskell code....84

Understanding expressions....88

Functions....91

Evaluation....95

Infix operators....98

Declaring values....105

Arithmetic functions in Haskell....115

Parenthesization....126

Let and where....133

Chapter Exercises....138

Definitions....142

Follow-up resources....144

Strings....145

Printing strings....146

A first look at types....146

Printing simple strings....148

Top-level versus local definitions....155

Types of concatenation functions....158

Concatenation and scoping....163

More list functions....167

Chapter Exercises....170

Definitions....176

Basic datatypes....179

Basic Datatypes....180

What are types?....181

Anatomy of a data declaration....181

Numeric types....185

Comparing values....195

Go on and Bool me....200

Tuples....208

Lists....213

Chapter Exercises....215

Definitions....220

Names and variables....223

Types....226

Types....227

What are types for?....228

How to read type signatures....230

Currying....240

Polymorphism....256

Type inference....265

Asserting types for declarations....270

Chapter Exercises....273

Definitions....287

Follow-up resources....294

Typeclasses....295

Typeclasses....296

What are typeclasses?....296

Back to Bool....298

Eq....300

Writing typeclass instances....305

Num....321

Type-defaulting typeclasses....326

Ord....332

Enum....342

Show....344

Read....351

Instances are dispatched by type....352

Gimme more operations....357

Chapter Exercises....362

Chapter Definitions....371

Typeclass inheritance, partial....374

Follow-up resources....374

More functional patterns....376

Make it func-y....377

Arguments and parameters....377

Anonymous functions....387

Pattern matching....392

Case expressions....408

Higher-order functions....413

Guards....425

Function composition....435

Pointfree style....440

Demonstrating composition....444

Chapter Exercises....448

Chapter Definitions....454

Follow-up resources....465

Recursion....467

Recursion....468

Factorial!....469

Bottom....479

Fibonacci numbers....483

Integral division from scratch....489

Chapter Exercises....496

Definitions....503

Lists....505

Lists....506

The list datatype....506

Pattern matching on lists....508

List's syntactic sugar....512

Using ranges to construct lists....513

Extracting portions of lists....517

List comprehensions....525

Spines and nonstrict evaluation....533

Transforming lists of values....548

Filtering lists of values....559

Zipping lists....561

Chapter Exercises....565

Definitions....574

Follow-up resources....577

Folding lists....578

Folds....579

Bringing you into the fold....579

Recursive patterns....582

Fold right....583

Fold left....596

How to write fold functions....609

Folding and evaluation....616

Summary....619

Scans....621

Chapter Exercises....626

Definitions....633

Follow-up resources....637

Algebraic datatypes....638

Algebraic datatypes....639

Data declarations review....640

Data and type constructors....642

Type constructors and kinds....645

Data constructors and values....647

What's a type and what's data?....653

Data constructor arities....659

What makes these datatypes algebraic?....662

newtype....668

Sum types....675

Product types....679

Normal form....684

Constructing and deconstructing values....690

Function type is exponential....715

Higher-kinded datatypes....722

Lists are polymorphic....725

Binary Tree....729

Chapter Exercises....738

Definitions....751

Signaling adversity....752

Signaling adversity....753

How I learned to stop worrying and love Nothing....753

Bleating either....757

Kinds, a thousand stars in your types....768

Chapter Exercises....780

Definitions....796

Building projects....798

Modules....799

Making packages with Stack....801

Working with a basic project....802

Making our project a library....807

Module exports....810

More on importing modules....813

Making our program interactive....821

do syntax and IO....827

Hangman game....832

Step One: Importing modules....835

Step Two: Generating a word list....841

Step Three: Making a puzzle....846

Adding a newtype....858

Chapter exercises....859

Follow-up resources....863

Testing....865

Testing....866

A quick tour of testing for the uninitiated....867

Conventional testing....869

Enter QuickCheck....881

Morse code....894

Arbitrary instances....911

Chapter Exercises....923

Definitions....933

Follow-up resources....934

Monoid, Semigroup....935

Monoids and semigroups....936

What we talk about when we talk about algebras....937

Monoid....938

How Monoid is defined in Haskell....940

Examples of using Monoid....941

Why Integer doesn't have a Monoid....943

Why bother?....949

Laws....951

Different instance, same representation....956

Reusing algebras by asking for algebras....959

Madness....971

Better living through QuickCheck....973

Semigroup....984

Strength can be weakness....989

Chapter exercises....992

Definitions....1003

Follow-up resources....1004

Functor....1005

Functor....1006

What's a functor?....1007

There's a whole lot of fmap goin' round....1010

Let's talk about f, baby....1013

Functor Laws....1027

The Good, the Bad, and the Ugly....1029

Commonly used functors....1035

Transforming the unapplied type argument....1053

QuickChecking Functor instances....1058

Exercises: Instances of Func....1062

Ignoring possibilities....1063

A somewhat surprising functor....1072

More structure, more functors....1076

IO Functor....1078

What if we want to do something different?....1082

Functors are unique to a datatype....1087

Chapter exercises....1089

Definitions....1094

Follow-up resources....1098

Applicative....1100

Applicative....1101

Defining Applicative....1102

Functor vs. Applicative....1105

Applicative functors are monoidal functors....1107

Applicative in use....1115

Applicative laws....1154

You knew this was coming....1163

ZipList Monoid....1168

Chapter Exercises....1183

Definitions....1186

Follow-up resources....1186

Monad....1188

Monad....1189

Sorry — a monad is not a burrito....1189

Do syntax and monads....1202

Examples of Monad use....1211

Monad laws....1236

Application and composition....1247

Chapter Exercises....1254

Definition....1257

Follow-up resources....1259

Applying structure....1260

Applied structure....1261

Monoid....1261

Functor....1269

Applicative....1274

Monad....1281

An end-to-end example: URL shortener....1285

That's a wrap!....1305

Follow-up resources....1306

Foldable....1307

Foldable....1308

The Foldable class....1309

Revenge of the monoids....1310

Demonstrating Foldable instances....1316

Some basic derived operations....1321

Chapter Exercises....1328

Follow-up resources....1329

Traversable....1330

Traversable....1331

The Traversable typeclass definition....1332

sequenceA....1333

traverse....1335

So, what's Traversable for?....1339

Morse code revisited....1340

Axing tedious code....1344

Do all the things....1348

Traversable instances....1352

Traversable Laws....1355

Quality Control....1356

Chapter Exercises....1357

Follow-up resources....1362

Reader....1363

Reader....1364

A new beginning....1365

This is Reader....1375

Breaking down the Functor of functions....1376

But uh, Reader?....1382

Functions have an Applicative too....1385

The Monad of functions....1393

Reader Monad by itself is boring....1399

You can change what comes below, but not above....1402

You tend to see ReaderT, not Reader....1403

Chapter Exercises....1403

Definition....1410

Follow-up resources....1411

State....1412

State....1413

What is state?....1413

Random numbers....1415

The State newtype....1419

Throw down....1422

Write State for yourself....1431

Get a coding job with one weird trick....1433

Chapter exercises....1440

Follow-up resources....1442

Parser combinators....1443

Parser combinators....1444

A few more words of introduction....1446

Understanding the parsing process....1447

Parsing fractions....1464

Haskell's parsing ecosystem....1473

Alternative....1477

Parsing configuration files....1492

Character and token parsers....1508

Polymorphic parsers....1513

Marshalling from an AST to a datatype....1522

Chapter Exercises....1539

Definitions....1548

Follow-up resources....1549

Composing types....1552

Composing types....1553

Common functions as types....1554

Two little functors sittin' in a tree, L-I-F-T-I-N-G....1559

Twinplicative....1562

Twonad?....1564

Exercises: Compose Instances....1566

Monad transformers....1568

IdentityT....1571

Finding a pattern....1590

Monad transformers....1594

Monad transformers....1595

MaybeT....1595

EitherT....1603

ReaderT....1605

StateT....1609

Types you probably don't want to use....1614

Recovering an ordinary type from a transformer....1616

Lexically inner is structurally outer....1618

MonadTrans....1622

MonadIO aka zoom-zoom....1645

Monad transformers in use....1649

Monads do not commute....1665

Transform if you want to....1665

Chapter Exercises....1666

Defintion....1675

Follow-up resources....1676

Nonstrictness....1677

Laziness....1678

Observational Bottom Theory....1679

Outside in, inside out....1681

What does the other way look like?....1685

Can we make Haskell strict?....1686

Call by name, call by need....1705

Nonstrict evaluation changes what we can do....1706

Thunk Life....1708

Sharing is caring....1712

Refutable and irrefutable patterns....1734

Bang patterns....1737

Strict and StrictData....1741

Adding strictness....1743

Chapter Exercises....1750

Follow-up resources....1753

Basic libraries....1755

Basic libraries and data structures....1756

Benchmarking with Criterion....1757

Profiling your programs....1775

Constant applicative forms....1780

Map....1785

Set....1789

Sequence....1792

Vector....1796

String types....1810

Chapter Exercises....1823

Follow-up resources....1827

IO....1829

IO....1830

Where IO explanations go astray....1831

The reason we need this type....1834

Sharing....1836

IO doesn't disable sharing for everything....1843

Purity is losing meaning....1845

IO's Functor, Applicative, and Monad....1848

Well, then, how do we MVar?....1854

Chapter Exercises....1857

Follow-up resources....1858

When things go wrong....1860

Exceptions....1861

The Exception class and methods....1862

This machine kills programs....1873

Want either? Try!....1880

The unbearable imprecision of trying....1885

Why throwIO?....1888

Making our own exception types....1892

Surprising interaction with bottom....1899

Asynchronous Exceptions....1901

Follow-up Reading....1905

Final project....1907

Final project....1908

fingerd....1908

Exploring finger....1910

Slightly modernized fingerd....1920

Chapter Exercises....1935

Welcome to a new way to learn Haskell. Perhaps you are coming to this book frustrated by previous attempts to learn Haskell. Perhaps you have only the faintest notion of what Haskell is. Perhaps you are coming here because you are not convinced that anything will ever be better than Common Lisp/Scala/Ruby/whatever language you love, and you want to argue with us. Perhaps you were just looking for the 18 billionth (n.b.: this number may be inaccurate) monad tutorial, certain that this time around you will understand monads once and for all. Whatever your situation, welcome and read on! It is our goal here to make Haskell as clear, painless, and practical as we can, no matter what prior experiences you’re bringing to the table.