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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Erlang

Elixer

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Elixir-Chap2

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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

# General property structure
A property has three parts for both Erlang and Elixir:
1. Type generator
2. Instance of type
3. Property expression

The functions generate the data for the test case we enter in the `TypeGenerator` position, called the **generators**. The framework will take these generators, execute them, and turn them into actual data, which will then be bound to the `InstanceOfType` variable. This variable is then made available within `PropertyExpression`, a piece of arbitrary code that must return true if the test fails or is false.  Since `PropertyExpression` is a single expression, we'll frequently see `begin ... end` blocks used in that area to wrap more complex sequences of expressions into a single one. 


# Property 

Let’s take a look at the `prop_test()` property. Standing out is the `?FORALL` Erlang macro, which has been imported through the module attribute from the property in the last lesson. The macro has this form:


```erlang
?FORALL(InstanceOfType, TypeGenerator,
        PropertyExpression).
```

We can translate the `?FORALL` macro:

```psuedocode
proper:forall(TypeGenerator, fun(InstanceOfType) -> PropertyExpression end).
```

Removing the need for the `begin ... end` expression might seem less exciting, but since all property-based testing frameworks implement the `?FORALL` macro interface, we’ll use that format for consistency and ease.

Understand the property structure to make writing properties easier.

Property Structure

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

Property Structure

General property structure