Gain insights into property-based testing with PropEr in Elixir. Learn foundational principles, custom data generators, and advanced concepts for effective real-world application testing.

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Property-based testing relies on specifying some property of code, unlike unit tests, which specify the expected output in response to some inputs. In this course, we'll cover the concepts we need to feel confident about using even the most advanced features of PropEr with Elixir.

We'll start with the basic and foundational principles of property-based testing, see what the framework offers us to get started, make our way through thinking in properties, write our own custom data generators, and then learn more advanced concepts. We'll also see how you can use property-based testing in a realistic project.

By the time we finish this course, we should feel comfortable testing real-world applications using properties.

Property-Based Testing with PropEr in Elixir

## Introduction
Property-based testing requires us to approach testing differently from what we’re used to. As we’ve seen earlier, the core of properties involve coming up with rules about a program that should always remain true. However, we’ll need to find a way to turn these rules into executable code so that a specific framework (PropEr) can exercise them. We will also need to tell the framework about what kind of inputs it should generate to truly challenge the rules. This is called a __generator__. Once we combine the encoded rules and the generators, we have a property.

### Types of properties
There are two main types of property:

1. Stateless properties
2. Stateful properties

In traditional example-based testing, it’s usually a good idea to start with simple unit tests. In property-based testing, stateless properties are their equivalent. __Stateless properties__ are a great fit to validate isolated, stateless components and without major side effects. They are still usable for more complex stateful integration and system tests, but __stateful properties__ are more appropriate for those use cases.

The properties we’ve seen and discussed in the previous chapter were all stateless, albeit a bit abstract. In this chapter, we’ll make everything concrete and see how stateless properties are structured so that we can read and understand them. We’ll also see what data generators are offered out of the box by PropEr, along with some ways of composing them together. Finally, we’ll run some more properties. This will allow us to figure out how to read the results of failing test cases and learn how to fix them.


# Introduction
Property-based testing requires us to approach testing differently from what we’re used to. As we’ve seen earlier, the core of properties involve coming up with rules about a program that should always remain true. However, we’ll need to find a way to turn these rules into executable code so that a specific framework (PropEr) can exercise them. We will also need to tell the framework about what kind of inputs it should generate to truly challenge the rules. This is called a __generator__. Once we combine the encoded rules and the generators, we have a property.

## Types of properties
There are two main types of property:

1. Stateless properties
2. Stateful properties

In traditional example-based testing, it’s usually a good idea to start with simple unit tests. In property-based testing, stateless properties are their equivalent. __Stateless properties__ are a great fit to validate isolated, stateless components and without major side effects. They are still usable for more complex stateful integration and system tests, but __stateful properties__ are more appropriate for those use cases.

The properties we’ve seen and discussed in the previous chapter were all stateless, albeit a bit abstract. In this chapter, we’ll make everything concrete and see how stateless properties are structured so that we can read and understand them. We’ll also see what data generators are offered out of the box by PropEr, along with some ways of composing them together. Finally, we’ll run some more properties. This will allow us to figure out how to read the results of failing test cases and learn how to fix them.


Get started with properties and a better understanding of how they work to write them easily.

Introduction to Writing Properties

Foundations of Property-Based Testing

Writing Properties

Thinking in Properties

Custom Generators

Responsible Testing

Properties-Driven Development

Shrinking

Stateful Properties

Case Study: Bookstore

State Machine Properties

Conclusion

Introduction to Writing Properties

Introduction