Book Image

The Clojure Workshop

By : Joseph Fahey, Thomas Haratyk, Scott McCaughie, Yehonathan Sharvit, Konrad Szydlo
Book Image

The Clojure Workshop

By: Joseph Fahey, Thomas Haratyk, Scott McCaughie, Yehonathan Sharvit, Konrad Szydlo

Overview of this book

The Clojure Workshop is a step-by-step guide to Clojure and ClojureScript, designed to quickly get you up and running as a confident, knowledgeable developer. Because of the functional nature of the language, Clojure programming is quite different to what many developers will have experienced. As hosted languages, Clojure and ClojureScript can also be daunting for newcomers because of complexities in the tooling and the challenge of interacting with the host platforms. To help you overcome these barriers, this book adopts a practical approach. Every chapter is centered around building something. As you progress through the book, you will progressively develop the 'muscle memory' that will make you a productive Clojure programmer, and help you see the world through the concepts of functional programming. You will also gain familiarity with common idioms and patterns, as well as exposure to some of the most widely used libraries. Unlike many Clojure books, this Workshop will include significant coverage of both Clojure and ClojureScript. This makes it useful no matter your goal or preferred platform, and provides a fresh perspective on the hosted nature of the language. By the end of this book, you'll have the knowledge, skills and confidence to creatively tackle your own ambitious projects with Clojure and ClojureScript.

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Table of Contents (17 chapters)
Preface
Preface
About the Book
1
1. Hello REPL!
1. Hello REPL!
Introduction
REPL Basics
Evaluation of Clojure Code
Basic Special Forms
Truthiness, nil, and equality
Summary
Free Chapter
2
2. Data Types and Immutability
2. Data Types and Immutability
Introduction
Simple Data Types
Collections
Summary
3
3. Functions in Depth
3. Functions in Depth
Introduction
Destructuring
Advanced Call Signatures
Higher-Order Programming
Multimethods
Summary
4
4. Mapping and Filtering
4. Mapping and Filtering
Introduction
map and filter
Using Lazy Sequences
Importing a Dataset from a CSV File
Exercise 4.13: Querying the Data with filter
Summary
5
5. Many to One: Reducing
5. Many to One: Reducing
Introduction
Initializing reduce
Reducing without reduce
Summarizing Tennis Scores
Introduction to Elo
Summary
6
6. Recursion and Looping
6. Recursion and Looping
Introduction
Clojure's Most Procedural Loop: doseq
Solving Complex Problems with Recursion
Summary
7
7. Recursion II: Lazy Sequences
7. Recursion II: Lazy Sequences
Introduction
A Simple Lazy Sequence
Lazy Trees
Summary
8
8. Namespaces, Libraries and Leiningen
8. Namespaces, Libraries and Leiningen
Introduction
Namespaces
Importing Clojure Functions with require and use
Leiningen—A Build Tool in Clojure
Working with External Libraries
Leiningen Profiles
Summary
9
9. Host Platform Interoperability with Java and JavaScript
9. Host Platform Interoperability with Java and JavaScript
Introduction
Using Java in Clojure
Working with Java I/O
Using JavaScript in ClojureScript
Reactive Web Programming Using Rum
Exceptions and Errors in Clojure
Errors in JavaScript
Summary
10
10. Testing
10. Testing
Introduction
Why Testing Is Important
Using the Expectations Testing Library
Property-Based Testing
Testing in ClojureScript
Summary
11
11. Macros
11. Macros
Introduction
What is a Macro?
Syntax Quoting
Macros in ClojureScript
Macro Hygiene
Summary
12
12. Concurrency
12. Concurrency
Introduction
Concurrency in General
Futures
Coordination
Summary
13
13. Database Interaction and the Application Layer
13. Database Interaction and the Application Layer
Introduction
Connecting to a Database
Introduction to Connection Pools
Introduction to the Application Layer
Summary
14
14. HTTP with Ring
14. HTTP with Ring
Introduction
HTTP, Web Servers, and REST
Static Files
Summary
15
15. The Frontend: A ClojureScript UI
15. The Frontend: A ClojureScript UI
Introduction
Hiccup instead of HTML
The Virtual DOM and Component Lifecycle
Summary
Appendix
Appendix
1. Hello REPL!
2. Data Types and Immutability
3. Functions in Depth
4. Mapping and Filtering
5. Many to One: Reducing
6. Recursion and Looping
7. Recursion II: Lazy Sequences
8. Namespaces, Libraries and Leiningen
9. Host Platform Interoperability with Java and JavaScript
10. Testing
11. Macros
12. Concurrency
13. Database Interaction and the Application Layer
14. HTTP with Ring
15. The Frontend: A ClojureScript UI

Introduction

This chapter is about using Clojure's reduce function and about reducing in general. By that, we mean starting with a sequence and boiling it down to a single thing. ("Reducing" is also cooking term, after all.) map and filter were about taking the sequence you have and turning it into the sequence you want: sequence in, sequence out. But that's not always what we want. Even simple operations on a sequence, such as calculating an average, a sum, or a maximum, cannot be directly calculated this way. That's where reduce, as well as a wider family of functions and patterns, comes in: sequence in, something else out. It's "something else" because the result might be a number, a string, a map, or even another sequence.

In the previous chapter, we saw that functions such as map and filter only look at one element at a time: how should we transform this item? Should we discard this item, or keep it? This is a powerful approach because...

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