Book Image

Crystal Programming

By : George Dietrich, Guilherme Bernal
Book Image

Crystal Programming

By: George Dietrich, Guilherme Bernal

Overview of this book

Crystal is a programming language with a concise and user-friendly syntax, along with a seamless system and a performant core, reaching C-like speed. This book will help you gain a deep understanding of the fundamental concepts of Crystal and show you how to apply them to create various types of applications. This book comes packed with step-by-step explanations of essential concepts and practical examples. You'll learn how to use Crystal’s features to create complex and organized projects relying on OOP and its most common design patterns. As you progress, you'll gain a solid understanding of both the basic and advanced features of Crystal. This will enable you to build any application, including command-line interface (CLI) programs and web applications using IOs, concurrency and C bindings, HTTP servers, and the JSON API. By the end of this programming book, you’ll be equipped with the skills you need to use Crystal programming for building and understanding any application you come across.
Table of Contents (26 chapters)
1
Part 1: Getting Started
5
Part 2: Learning by Doing – CLI
10
Part 3: Learn by Doing – Web Application
13
Part 4: Metaprogramming
18
Part 5: Supporting Tools

Reading annotations

In Crystal, you normally invoke a method on an object in order to access some data stored within. Annotations are no different. The Annotation type exposes three methods that can be used to access the data defined on the annotation in different ways. However, before you can access the data on the annotation, you need to get a reference to an Annotation instance. This can be accomplished by passing the Annotation type to the #annotation method defined on the types that support annotations, including TypeNode, Def, and MetaVar. For example, we can use this method to print the annotation applied to a specific class or method, if present:

annotation MyAnnotation; end
@[MyAnnotation]
class MyClass
  def foo
    {{pp @type.annotation MyAnnotation}}
    {{pp @def.annotation MyAnnotation}}
  end
end
MyClass.new.foo

The #annotation method will return NilLiteral if no annotation of the provided type is applied...