Real World OCaml

509 Pages·2013·9 MB·English

Real World OCaml
Yaron Minsky, Anil Madhavapeddy, and Jason Hickey
Real World OCaml
by Yaron Minsky, Anil Madhavapeddy, and Jason Hickey
Copyright © 2014 Yaron Minsky, Anil Madhavapeddy, Jason Hickey. All rights reserved.
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November 2013: First Edition
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2013-10-31: First release
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ISBN: 978-1-449-32391-2
[LSI]
For Lisa, a believer in the power of words, who helps me find mine. —Yaron
For Mum and Dad, who took me to the library and unlocked my imagination. —Anil
For Nobu, who takes me on a new journey every day. —Jason

Table of Contents
Prologue. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xv
Part I. Language Concepts
1. A Guided Tour. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
OCaml as a Calculator 3
Functions and Type Inference 5
Type Inference 7
Inferring Generic Types 8
Tuples, Lists, Options, and Pattern Matching 10
Tuples 10
Lists 11
Options 16
Records and Variants 18
Imperative Programming 20
Arrays 20
Mutable Record Fields 21
Refs 22
For and While Loops 23
A Complete Program 25
Compiling and Running 26
Where to Go from Here 26
2. Variables and Functions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Variables 27
Pattern Matching and let 30
Functions 31
Anonymous Functions 31
Multiargument functions 33
v
Recursive Functions 34
Prefix and Infix Operators 35
Declaring Functions with Function 39
Labeled Arguments 40
Optional Arguments 43
3. Lists and Patterns. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
List Basics 49
Using Patterns to Extract Data from a List 50
Limitations (and Blessings) of Pattern Matching 52
Performance 52
Detecting Errors 54
Using the List Module Effectively 55
More Useful List Functions 58
Tail Recursion 61
Terser and Faster Patterns 63
4. Files, Modules, and Programs. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67
Single-File Programs 67
Multifile Programs and Modules 70
Signatures and Abstract Types 71
Concrete Types in Signatures 74
Nested Modules 75
Opening Modules 77
Including Modules 79
Common Errors with Modules 81
Type Mismatches 81
Missing Definitions 81
Type Definition Mismatches 81
Cyclic Dependencies 82
Designing with Modules 83
Expose Concrete Types Rarely 83
Design for the Call Site 84
Create Uniform Interfaces 84
Interfaces before implementations 85
5. Records. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Patterns and Exhaustiveness 88
Field Punning 91
Reusing Field Names 92
Functional Updates 96
Mutable Fields 97
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First-Class Fields 98
6. Variants. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Catch-All Cases and Refactoring 105
Combining Records and Variants 107
Variants and Recursive Data Structures 111
Polymorphic Variants 114
Example: Terminal Colors Redux 116
When to Use Polymorphic Variants 121
7. Error Handling. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
Error-Aware Return Types 123
Encoding Errors with Result 125
Error and Or_error 125
bind and Other Error Handling Idioms 127
Exceptions 128
Helper Functions for Throwing Exceptions 131
Exception Handlers 132
Cleaning Up in the Presence of Exceptions 132
Catching Specific Exceptions 133
Backtraces 135
From Exceptions to Error-Aware Types and Back Again 137
Choosing an Error-Handling Strategy 138
8. Imperative Programming. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 139
Example: Imperative Dictionaries 139
Primitive Mutable Data 143
Array-Like Data 143
Mutable Record and Object Fields and Ref Cells 145
Foreign Functions 146
for and while Loops 146
Example: Doubly Linked Lists 147
Modifying the List 149
Iteration Functions 150
Laziness and Other Benign Effects 151
Memoization and Dynamic Programming 153
Input and Output 159
Terminal I/O 160
Formatted Output with printf 161
File I/O 163
Order of Evaluation 165
Side Effects and Weak Polymorphism 167
Table of Contents | vii
The Value Restriction 168
Partial Application and the Value Restriction 170
Relaxing the Value Restriction 170
Summary 173
9. Functors. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 175
A Trivial Example 176
A Bigger Example: Computing with Intervals 177
Making the Functor Abstract 181
Sharing Constraints 182
Destructive Substitution 184
Using Multiple Interfaces 185
Extending Modules 189
10. First-Class Modules. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193
Working with First-Class Modules 193
Example: A Query-Handling Framework 199
Implementing a Query Handler 200
Dispatching to Multiple Query Handlers 202
Loading and Unloading Query Handlers 205
Living Without First-Class Modules 208
11. Objects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
OCaml Objects 212
Object Polymorphism 213
Immutable Objects 215
When to Use Objects 216
Subtyping 217
Width Subtyping 217
Depth Subtyping 218
Variance 219
Narrowing 222
Subtyping Versus Row Polymorphism 224
12. Classes. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 227
OCaml Classes 227
Class Parameters and Polymorphism 228
Object Types as Interfaces 230
Functional Iterators 232
Inheritance 233
Class Types 234
Open Recursion 235
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