Book Image

Scala Functional Programming Patterns

By : Atul S. Khot
Book Image

Scala Functional Programming Patterns

By: Atul S. Khot

Overview of this book

Scala is used to construct elegant class hierarchies for maximum code reuse and extensibility and to implement their behavior using higher-order functions. Its functional programming (FP) features are a boon to help you design “easy to reason about” systems to control the growing software complexities. Knowing how and where to apply the many Scala techniques is challenging. Looking at Scala best practices in the context of what you already know helps you grasp these concepts quickly, and helps you see where and why to use them. This book begins with the rationale behind patterns to help you understand where and why each pattern is applied. You will discover what tail recursion brings to your table and will get an understanding of how to create solutions without mutations. We then explain the concept of memorization and infinite sequences for on-demand computation. Further, the book takes you through Scala’s stackable traits and dependency injection, a popular technique to produce loosely-coupled software systems. You will also explore how to currying favors to your code and how to simplify it by de-construction via pattern matching. We also show you how to do pipeline transformations using higher order functions such as the pipes and filters pattern. Then we guide you through the increasing importance of concurrent programming and the pitfalls of traditional code concurrency. Lastly, the book takes a paradigm shift to show you the different techniques that functional programming brings to your plate. This book is an invaluable source to help you understand and perform functional programming and solve common programming problems using Scala’s programming patterns.

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Scala Functional Programming Patterns
Atul S. Khot
Dec, 2015 | 298 pages
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Table of Contents (19 chapters)
Scala Functional Programming Patterns
Scala Functional Programming Patterns
Credits
Credits
About the Author
About the Author
Aknowledgement
Aknowledgement
About the Reviewers
About the Reviewers
www.PacktPub.com
www.PacktPub.com
Preface
Preface
Free Chapter
1
Grokking the Functional Way
Grokking the Functional Way
Abstractions
Concise expression
Functions
Immutable
Referential transparency
The problem – grouping continuous integers
Scala idioms
Patterns and those aha! moments
Summary
2
Singletons, Factories, and Builders
Singletons, Factories, and Builders
Singletons – being one and only one
Null Objects – singletons in another garb
Scala singletons
Builders
Summary
3
Recursion and Chasing your Own Tail
Recursion and Chasing your Own Tail
Recursive structures
Pattern matching
Tail recursion to the rescue
Getting the nth element of a list
An expression parser
Persistent data structures
Two forms of recursion
Summary
4
Lazy Sequences – Being Lazy, Being Good
Lazy Sequences – Being Lazy, Being Good
Illusion and reality – the proxy pattern
Hibernate's lazy loading
Lazy val – calling by need
Infinite sequences – Scala streams
Recursive streams
Memoization and the flyweight pattern
Call by name
Streams are collections
Sieve of Eratosthenes
A view to a collection
Summary
5
Taming Multiple Inheritance with Traits
Taming Multiple Inheritance with Traits
The iterator design pattern
Interfaces as types
The dreaded diamond
Traits – Scala's rich interfaces
Mix-ins – rich interfaces
Frills and thrills – the decorator pattern
Scala's easy and breezy decorations – stackable modifications
Dependencies injection pattern
A taste of the cake pattern
Sealed traits
Defeating the dreaded diamond
Summary
6
Currying Favors with Your Code
Currying Favors with Your Code
Functions as first-class values
Roping in a scope
Local functions – hiding and biding their time
The underscore – Scala's Swiss army knife
A taste of the curry
The loan pattern
Serving the curry
Frills and thrills – decorators again
Wrapping it up
Summary
7
Of Visitors and Chains of Responsibilities
Of Visitors and Chains of Responsibilities
A tale of two hierarchies
The Visitor pattern
Partial functions
The chain of responsibility
Summary
8
Traversals – Mapping/Filtering/Folding/Reducing
Traversals – Mapping/Filtering/Folding/Reducing
Iterating the Scala way
A validation problem
Foreach–sugary sweetener
Monads
Reduce
Summary
9
Higher Order Functions
Higher Order Functions
The strategy design pattern
A strategy in Scala land
Functors
Monads
FlatMap
Monoids
An inverted index
Pipes and filters
Pipes and filters – the Scala version
It is a Monoid
Lazy collections
Summary
10
Actors and Message Passing
Actors and Message Passing
The recursive grep
The producer/consumer pattern
Threads – masters and slaves
Events
Immutability is golden
Akka actors take the stage
Summary
11
It's a Paradigm Shift
It's a Paradigm Shift
Verbosity
Sorting it out!
Functional error handling
Threads and futures
Parser combinators
Summary
Index
Index

An expression parser

We will look at an instructive example of how recursion and immutability go hand in hand. We will look at an infix expression parser.

Note

An infix notation is where the operator comes in between two operands, for example, 3+4+5-6 is the infix notation.

We will look at the iterative Java version and then at the recursive Scala version. We support only operators + and * and also provide bracketed sub-expressions.

Evaluating (1+2)*3*(2+4) expression should give us the output as 54 and evaluating (1+2)*3+4 expression should give us the output as 13. The grammar for our expression parser looks as shown in the following code. Note how each sub-expression is an expression composed of other sub-expressions, terms, and factors. In short, the grammar is recursively defined. Here is the grammar:

Expr: Term | Term + Expr
Term: Factor | Factor * Term
Factor: [0-9][0-9]+ | '(' Expr ')'

Here is a diagrammatic representation of the flow:

Figure 3.5: The expression tree

Look at the bracketed...

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