Gain insights into property-based testing with PropEr in Erlang. Delve into foundational principles, common frameworks, custom data generators, and applying testing in realistic projects for robust code validation.

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Property-based testing relies on specifying some property of code, rather than unit tests which specify the expected output in response to some inputs. In this course, you’ll cover the concepts you need to get started, up to the point where you feel confident enough to use the most advanced features of PropEr with Erlang.

You’ll start smoothly with the basic and foundational principles of property-based testing. From there, you will cover some of the common frameworks, how to think in properties, how to write your own custom data generators, and more. You'll also see how you can use Property-Based Testing in a realistic project.

By the time you finish this course, you will be comfortable testing real-world applications using properties.

Property-Based Testing with PropEr in Erlang

Let's look at the `prop_test()` property being tested by the framework to figure out what the macro ends up doing at run time. Let's take a look in the terminal widget below. 

You can use the following terminal to run the command `rebar3 proper` to see the output. 

> **Note:** The setup has been done for you, so just run the commands to learn the concepts.

In this output, PropEr finds our test suite (the `prop_foundations` module), which at the moment only contains the `prop_test()` property. Once PropEr finds the test suite, it will execute the individual properties.  Every period represents an instance of PropEr taking our property and testing it against an input. In this case, we have one property that has been run a hundred times, and succeeded at every attempt. For each run, PropEr did the following:

1. Expanded the generator (which was defined with the `term()` function provided by PropEr) to a given piece of data.
2. Passed the expanded data to the property, binding it to the `InstanceofType` variable.
3. Validated that the actual `PropertyExpression` returned true.

That `term()` generator isn’t a standard in Erlang. That’s where property-based testing frameworks do a bit of magic. The included file directive at the top of the file is causing all of it:

`-include_lib("proper/include/proper.hrl").`

The included `proper.hrl` file contains macro code and inlines and imports multiple functions provided by PropEr. The `term()` function is actually shorthand for the `proper_types:term()` function call. Try playing with it in the shell by using the command in the terminal:
```
rebar3 as test shell
```

After it builds the output and the shell prelude, the shell will open and we can input to it. Try using these commands:

1. `proper_types:term().`
2. `proper_gen:pick(proper_types:number()).`
3. `proper_gen:pick(proper_types:term()).`


Understand the flow of the execution for property-based testing.

Execution Model

Foundations of Property-Based Testing

Writing Properties

Thinking in Properties

Custom Generators

Responsible Testing

Properties-Driven Development

Shrinking

Targeted Properties

Stateful Properties

Case Study: Bookstore

State Machine Properties

Conclusion

Execution Model