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2. Data Types and Immutability
Overview
In this chapter, we start by discovering the concept of immutability and its relevance in modern programs. We then examine simple data types such as strings, numbers and booleans, highlighting subtle differences in different environments like Clojure and ClojureScript. After a first exercise, we move on to more elaborated data types with collections such as lists, vectors, maps and sets, learning along the way which to use in different situations. After touching on the collection and sequence abstractions, we learn new techniques for working with nested data structures, before finally moving on to the final activity: implementing our very own in-memory database.
By the end of this chapter, you will be able to work with the commonly used data types in Clojure.
Introduction
Computer hardware has evolved dramatically in the last few decades. On a typical computer, storage and memory capacity have both increased a millionfold compared to the early 1980s. Nonetheless, standard industry practices in software development and mainstream ways of programming are not that different. Programming languages such as C++, Java, Python, and Ruby still typically encourage you to change things in place, and to use variables and mutate the state of a program, that is, to do things as if we were programming on a computer with a minimal amount of memory. However, in our quest for efficiency, better languages, and better tools, we reach for higher-level languages. We want to get further away from machine code. We want to write less code and let the computers do the tedious work.
We don't want to think about the computer's memory anymore, such as where a piece of information is stored and whether it's safe and shareable, as much as...
Simple Data Types
A data type designates what kind of value a piece of data holds; it is a fundamental way of classifying data. Different types allow different kinds of operations: we can concatenate strings, multiply numbers, and perform logic algebra operations with Booleans. Because Clojure has a strong emphasis on practicality, we don't explicitly assign types to values in Clojure, but those values still have a type.
Clojure is a hosted language and has three notable, major implementations in Java, JavaScript, and .NET. Being a hosted language is a useful trait that allows Clojure programs to run in different environments and take advantage of the ecosystem of its host. Regarding data types, it means that each implementation has different underlying data types, but don't worry as those are just implementation details. As a Clojure programmer, it does not make much difference, and if you know how to do something in Clojure, you likely know how to do it in, say, ClojureScript...
Collections
Clojure is a functional programming language in which we focus on building the computations of our programs in terms of the evaluation of functions, rather than building custom data types and their associated behaviors. In the other dominant programming paradigm, object-oriented programming, programmers define the data types and the operations available on them. Objects are supposed to encapsulate data and communicate with each other by passing messages around. But there is an unfortunate tendency to create classes and new types of objects to customize the shape of the data, instead of using more generic data structures, which cascades into creating specific methods to access and modify the data. We have to come up with decent names, which is difficult, and then we pass instances of objects around in our programs. We create new classes all the time, but more code means more bugs. It is a recipe for disaster; it is an explosion of code, with code that is very specific...
Summary
In this chapter, we discovered the concept of immutability. We learned about Clojure's simple data types, as well as their implementation on different host platforms. We discovered the most common types of collections and sequences: maps, sets, vectors, and lists. We saw how to use them with generic collections and sequence operations. We learned how to read and update complex structures of nested collections. We also learned about the standard functions for using collection data structures, as well as more advanced usage with deeply nested data structures. In the next chapter, we will learn advanced techniques for working with functions.
