# A journey to Property-Based Testing

The purpose of this workshop is to understand by the practice an approach to testing that allows us to identify and check our business invariants. We will discover :

* How it differs from traditional sampling approach (example-based testing)
* Its advantages and limits

### Source code

Source code is available [here](https://github.com/ythirion/property-based-testing-kata) in :

* Scala : Scalatest / Scalacheck
* C# 10  : XUnit / FsCheck / Language-Ext / FluentAssertions 

### Connection - Calculator

Connect participants to the topic by making them write Unit Tests on the Calculator class :

![](https://1936518372-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MAffO8xa1ZWmgZvfeK2%2Fuploads%2F0GdzzbAMuaflkIqd5yEC%2Fjourney-to-pbt.png?alt=media\&token=8164f438-6156-4046-a77e-b80c8e2f79b7)

Debriefing :

![](https://1936518372-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MAffO8xa1ZWmgZvfeK2%2Fuploads%2Fr2TqNTUgi5qyrX2varfP%2Fdebriefe1.png?alt=media\&token=bc6f5ead-bb30-458b-b99e-02d023d8189c)

### Concepts - PBT

* Go through slides 4 to 15 to explain the different concepts : 
  * Example-Based Testing vs PBT
  * What is PBT ?
  * Addition properties example
  * PBT by hands
  * Scalacheck / FsCheck / QuickCheck
  * Associated concepts
* Demonstrate how to write properties on the Calculator, you can use the [step-by-step guide](https://github.com/ythirion/property-based-testing-kata/blob/main/C%23/PBTKata.Tests/Math/Solution/step-by-step.md) to prepare yourself

![](https://1936518372-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MAffO8xa1ZWmgZvfeK2%2Fuploads%2Fw4CJ79PlXUSLeIR9PyBl%2Fimage.png?alt=media\&token=f853f2a3-9e67-4bee-83d1-ea48ed6c3066)

### Concrete Practice

#### Postal Parcel

Open \`PostalParcel\` : 

* Identify properties – 5’
* What are the invariants ?
* Write them in \`PostalParcelPropertiesFlatSpec\` with ScalaCheck / FsCheck - 15’
* Collective debriefing – 5’
  * [Scala](https://github.com/ythirion/property-based-testing-kata/blob/main/scala/src/test/scala/post/solution/step-by-step.md)
  * [C#](https://github.com/ythirion/property-based-testing-kata/blob/main/C%23/PBTKata.Tests/Post/Solution/step-by-step.md)

#### Bank withdrawal

* Open \`AccountService\` :•Identify properties – 5’ 
* What should we do to check those properties ?

![](https://1936518372-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MAffO8xa1ZWmgZvfeK2%2Fuploads%2Fn3d3sv31QFOqajMl4Bkz%2Fbank.png?alt=media\&token=f044e3fc-dd36-4812-aac7-bb4ac9329fa8)

* Demonstrate how you could write properties on those knowing that Command and Account are tightly coupled as inputs : 
  * Using properties + input filtering (when)
  * Using properties and Builder
  * How we would write tests on Bank withdrawal in real life (with Parameterized tests

#### Rental Calculator - PBT as a refactoring tool

* Open \`RentalCalculator\`
* Imagine this code is running in production and business is happy with it
*   Imagine you need to adapt it, but you need to be sure that you don’t introduce any regression
* How PBT can help us ?

![](https://1936518372-files.gitbook.io/~/files/v0/b/gitbook-x-prod.appspot.com/o/spaces%2F-MAffO8xa1ZWmgZvfeK2%2Fuploads%2FoV4Y062ZTV1olSSP9rEA%2FrentalCalc.png?alt=media\&token=24ddb2af-51f2-47e8-8647-0abefc28e712)

* Let them code - 15'
* Collective debriefing - 5'
  * [Scala](https://github.com/ythirion/property-based-testing-kata/blob/main/scala/src/test/scala/rental/solution/step-by-step.md)
  * [C#](https://github.com/ythirion/property-based-testing-kata/blob/main/C%23/PBTKata.Tests/Rentals/Solution/step-by-step.md)

### Conclusion

* When to apply PBT in real life
* Associated anti-patterns
* Reflect : Think about your last development, how this approach could have helped you ?

### Resources

* [ScalaCheck documentation](https://github.com/typelevel/scalacheck/blob/main/doc/UserGuide.md)
* Scala exercises on PBT : <https://www.scala-exercises.org/scalacheck/generators>
* ["Functions for nothing, and your tests for free" Property-based testing and F# - George Pollard](https://youtu.be/8oALNLdyOyM)
* Property based testing - step by step : [https://www.leadingagile.com/2018/04/step-by-step-toward-property-based-testing/](https://romainberthon.blog/2019/01/08/a-la-decouverte-du-property-based-testing/)

{% embed url="<https://speakerdeck.com/thirion/a-journey-to-property-based-testing>" %}
Slide deck to support workshop
{% endembed %}


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