Functional Reactive Programming

362 Pages·2016·10.109 MB·English

by  Blackheath, Jones

Stephen Blackheath
Anthony Jones
F Heinrich Apfelmus
OREWORD BY
M A N N I N G
Functional Reactive Programming
ii
Functional Reactive
Programming
STEPHEN BLACKHEATH
ANTHONY JONES
MANNING
SHELTER ISLAND
iv
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Printed in the United States of America
1 2 3 4 5 6 7 8 9 10 – EBM – 21 20 19 18 17 16
brief contents
1 ■ Stop listening! 1
2 ■ Core FRP 26
3 ■ Some everyday widget stuff 60
4 ■ Writing a real application 65
5 ■ New concepts 94
6 ■ FRP on the web 111
7 ■ Switch 131
8 ■ Operational primitives 169
9 ■ Continuous time 186
10 ■ Battle of the paradigms 201
11 ■ Programming in the real world 215
12 ■ Helpers and patterns 232
13 ■ Refactoring 262
14 ■ Adding FRP to existing projects 273
15 ■ Future directions 283

v
vi BRIEF CONTENTS
contents
foreword xv
preface xvii
acknowledgments xviii
about this book xix
about the cover xxii
1 Stop listening! 1
1.1 Project, meet complexity wall 2
1.2 What is functional reactive programming? 3
A stricter definition 3 ■ Introducing Sodium 4
1.3 Where does FRP fit in? The lay of the land 4
1.4 Interactive applications: what are events? 5
1.5 State machines are hard to reason about 6
1.6 Interactive applications without the bugs 7
1.7 Listeners are a mainstay of event handling, but … 7
1.8 Banishing the six plagues of listeners 8
1.9 Why not just fix listeners? 9
1.10 “Have you tried restarting it?” or why state is problematic 9
1.11 The benefit of FRP: dealing with complexity 10
1.12 How does FRP work? 11
Life cycle of an FRP program 13
1.13 Paradigm shift 14
Paradigm 14 ■ Paradigm shift 15
1.14 Thinking in terms of dependency 15
vii
viii CONTENTS
1.15 Thinking declaratively: what the program is, not what it
does 17
1.16 Conceptual vs. operational understanding of FRP 19
Opening your mind to FRP 20 ■ What’s really going on when
the code runs? 21
1.17 Applying functional programming to event-based code 22
1.18 Summary 25
2 Core FRP 26
2.1 The Stream type: a stream of events 27
2.2 The map primitive: transforming a value 30
Transforming a stream 31
2.3 The components of an FRP system 32
Combining primitives 32 ■ Separating I/O from logic 33
2.4 Referential transparency required 33
2.5 The Cell type: a value that changes over time 34
Why Stream and Cell? 36 ■ The constant primitive: a cell with
a constant value 37 ■ Mapping cells 37
2.6 The merge primitive: merging streams 38
Simultaneous events 39 ■ Collection variants of merge 42
How does merge do its job? 42
2.7 The hold primitive: keeping state in a cell 43
2.8 The snapshot primitive: capturing the value of a cell 45
2.9 Looping hold and snapshot to create an accumulator 47
Forward references 47 ■ Constructing FRP in an explicit
transaction 48 ■ Accumulator code 49
Does snapshot see the new value or the old value? 50
2.10 The filter primitive: propagating an event only sometimes 52
2.11 The lift primitive: combining cells 53
2.12 The never primitive: a stream that never fires 55
2.13 Referential transparency dos and don’ts 55
2.14 FRP cheat sheet 57
2.15 Summary 58
3 Some everyday widget stuff 60
3.1 Spinner as a standalone SWidget 60
3.2 Form validation 62
3.3 Summary 64
CONTENTS ix
4 Writing a real application 65
4.1 The petrol pump example 66
4.2 Running the petrol pump example 68
4.3 Code, meet outside world 68
4.4 The life cycle of a petrol pump fill 73
Code for LifeCycle 75
4.5 Is this really better? 77
4.6 Counting liters delivered 78
4.7 Showing dollars’ of fuel delivered 79
4.8 Communicating with the point-of-sale system 82
4.9 Modularity illustrated: a keypad module 85
4.10 Notes about modularity 87
The form of a module 87 ■ Tuples vs. classes 88
Explicit wiring 88 ■ When inputs and outputs proliferate 88
Some bugs are solved, some aren’t 88 ■ Testability 89
4.11 Adding a preset dollar amount 89
4.12 What have you achieved? 92
4.13 Summary 93
5 New concepts 94
5.1 In search of the mythical von Neumann machine 94
Why so slow? The cache 96 ■ The madness of bus
optimization 98 ■ How does this relate to FRP? 101
5.2 Compositionality 101
When complexity gets out of control 101 ■ Reductionism and
engineering 102 ■ Compositionality is no longer optional 104
5.3 Lack of compositionality illustrated 104
Why the OO version lacks compositionality 105
5.4 Compositionality: eliminating whole classes of bugs 106
5.5 Don’t pull out the rug: use immutable values 107
Immutable data structures 107
5.6 Clarity of intent 108
5.7 The consequences of cheap abstraction 109
5.8 Summary 109
6 FRP on the web 111
6.1 RxJS 112