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

# What is responsible testing?

Understanding when and where properties work best is important for responsible testing, and that is what we are going to learn in this chapter.

Throughout the chapter, we'll take on a practical project based on the [birthday greeting "kata" by _Matteo Vaccari_](http://matteo.vaccari.name/blog/archives/154). This exercise asks us to organize unit tests for a little application in a manner such that as few tests as possible will need to be modified or trashed when implementation details or requirements change.

The exercise will contain parts having to do with data storage, text parsing, data manipulation, and templating. We will only focus on unit tests, and we’ll leave integrations and systems with side effects for later chapters. In this chapter, we'll go through how to choose between traditional example-based testing and property-based testing. 

# The specifications of the example

We’re going to write a program that will first load a set of employee records from a flat file and then send a greeting email to each employee whose birthday is today. In our example, the flat file we receive contains employee records and looks a bit like a comma-separated-values, CSV, file. We’re given this sample:

```CSV
last_name, first_name, date_of_birth, email
Doe, John, 1982/10/08, john.doe@foobar.com
Ann, Mary, 1975/09/11, mary.ann@foobar.com
```

The email sent on an employee’s birthday should contain text like:

```text
Subject: Happy birthday!
Happy birthday, dear John!
```

On its own, this is straightforward. The challenge comes from the additional constraints that we’re given:

- The tests written should be unit tests, meaning none of the tests should talk to a database, touch the filesystem, interact with the network, or toy with the environment. Tests that do any of these are qualified as integration tests and are out of our scope.

- We won’t use CSV format forever. Eventually, a database or web service should be used to fetch the employee records, similarly to send the email. The tests should be written to require as few modifications as possible whenever these implementation details change.

Get introduced to the concept of responsible testing and what we will go through in this chapter.

Introduction to Responsible Testing

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 Responsible Testing

What is responsible testing?