Clean Code Cookbook

Chapter 18. Globals

Write shy code—modules that don’t reveal anything unnecessary to other modules and that don’t rely on other modules’ implementations.

David Thomas and Andrew Hunt, The Pragmatic Programmer: Your Journey to Mastery

18.0 Introduction

Most modern languages support global functions, classes, and attributes. There’s a hidden cost when you use any of these artifacts. Even when you are creating an object using new(), you will introduce a tight coupling to a global class unless you apply some of the following recipes.

18.1 Reifying Global Functions

Problem

You have global functions that you can call from anywhere. 

Solution

Global functions bring a lot of coupling. Narrow their scope.

Discussion

Discouraged by object-oriented programming, many mixed languages support global functions. They create coupling and hurt readability since they are difficult to trace. As coupling grows, maintainability and testability become more problematic. You can start by wrapping the function in a context object. For example, you can find external resource access, database access, singletons (see Recipe 17.2, “Replacing Singletons”), global classes, time, and operating system resources.

This example calls a method from a global database:

class Employee {
    function taxesPayedUntilToday() {
        return database()->select(
            "SELECT TAXES FROM EMPLOYEE".
            " WHERE ID = " . $this->id() .
            " AND DATE < " . currentDate());
    }
}

By making persistence contextual, you can decouple the database ...