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

# The layout
State machine properties are very similar to regular stateful properties, conceptually speaking. They share the same three major components:

- A model, which represents what the system should do at a high level.
- A generator for commands, which represent the execution flow of the program.
- An actual system, which is validated against our model.

Since FSM properties are really a specialization of stateful properties, the differences are subtle. 

# The model

The model for state machine properties keeps the same spirit as other models. The model must be a straightforward version of our system to ensure that the real system acts according to our expectations. FSM properties are made of three parts:

1. A data structure that represents the expected state of the system.
2. The name of a state representing the name of the modeled finite state machine (this can be either an atom or a tuple).

3. A function that transforms the model’s data based on commands that could be applied to the system, named `next_state_data`. 

Although the model is similar to stateful properties, it contains names that are important to keep in mind.

> **Note**: When dealing with stateful components in Elixir, we often refer to their “state” as the ensemble of data that is living within the component. In fact, “state” and “data” are often used interchangeably.
When dealing with finite state machines, we need to be accurate. "State" refers to the name of the state that the FSM is in. For example, a lock is in either locked or unlocked state. "Data" refers to the actual information and records being held by the FSM. The combination to open the lock is 3-13-37 and may be held in its data.

# The commands

As with stateful properties, FSM properties have command generators. They work a bit differently since they’re based on the state. These command generators contain the following:

1. A list of functions sharing the name of the states, each of which return a list of symbolic calls with generators defining their arguments, along with the name of the next transition.
2. A series of preconditions defining whether a given symbolic call would make sense to apply according to the current model state and data.
3. An optional callback that defines the probability for a given state transition to happen. If no weight is defined, all transitions are equally likely.

# The validation

When it comes to validating our system against the model, we’ll once again borrow a lot from stateful properties. The mechanism here relies fully on postconditions, with the one distinction that the state names involved in a transition are considered in the system.


With that in mind, we can now look at the execution mode of FSM properties. 

Take a look at the layout and details of finite-state properties. 

Laying Out State Machine Properties

Take a look at the layout and details of finite-state 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

Laying Out State Machine Properties

The layout