BDD (Behavior-driven development) is a software development process focusing on active collaboration, which illustrates and automates the requirements using key examples of the problem domain. In BDD the formalized examples use a natural language-based DSL driven by the Given/When/Then keywords. At the same time, property-based testing (PBT) uses abstract (mathematical) formulas to declare expectations for the output values given some constraints on the input. The PBT tools try to disproof that the application fulfills these requirements by taking samples from the valid input value space. The experience shows that for understanding and properly implementing the requirements, the team has to understand the requirements as a set of abstract rules, where collecting key examples can help a lot. BDD is strong at managing and automating these key examples and PBT is strong at defining and automating the rules, so the question should arise: can these two methods somehow support each-other? Can we increase the value of BDD by formalizing and testing the rules? Can we increase the value of PBT by a better communication of the rules and constraints we have? This session shows an experiment of combining BDD and PBT. It provides examples of how the specification could look like in this combined world and what it could be used for. And there is a working, open-source prototype too!

1. Budapest, 26-28 October 2016
PROPERTY BASED BDD EXAMPLES
Presented by Gaspar Nagy
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gaspar@specsolutuions.eu
@gasparnagy, http://gasparnagy.com

2. BDD PBT
• Behavior Driven
Development
• ~Specification by
Example (SbE)
• ~Acceptance Test Driven
Development (ATDD)
• ~Keyword Driven Testing
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• Property Based Testing
• Property Testing
• ~Random Testing
• ~Model-based Testing

3. Scenario: Correct easy answer scores 10
Given I register a team
When I submit a correct easy answer
Then my score should be 10
Collaboration
Automation with
Cucumber/SpecFlow
What is BDD?

4. This is an example!
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Scenario: Correct easy answer scores 10
Given I register a team
When I submit a correct easy answer
Then my score should be 10

5. How many examples you need?
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Scenario: Add two numbers
Given I have entered 1 into the calculator
And I have entered 2 into the calculator
When I press add
Then the result should be 3 on the screen
Scenario: Add two numbers
Given I have entered 5 into the calculator
And I have entered -7 into the calculator
When I press add
Then the result should be -2 on the screen
Scenario: Add two numbers
Given I have entered 2 into the calculator
And I have entered 0 into the calculator
When I press add
Then the result should be 2 on the screen

6. BDD is for…
understanding & validating
business requirements
through illustrative examples
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7. PBT is for…
verifying
implementation
through checking statements about the
output for many different possible inputs
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source: http://blog.jessitron.com/2013/04/property-based-testing-what-is-it.html
the “properties”

8. For example for addition…
• Commutative property: a + b = b + a
• Associative property: (a + b) + c = a + (b + c)
• Identity property: a + 0 = a
• Distributive property: a * (b + c) = a*b + a*c
We would like to verify these for ~ALL input
combinations!
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9. There is a tool for doing this!
• QuickCheck (Haskell) is the canonical framework, but
there are many different ports of it to other
programming languages
• QuickCheck for Java
• PhpQuickCheck for PHP
• ScalaCheck for Scala
• FsCheck for .NET (F#, C#)
• … (see more at https://en.wikipedia.org/wiki/QuickCheck)
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10. FsCheck Sample
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11. When the implementation is wrong…
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12. BDD PBT
• 1 + 2 = 3
• 5 + -7 = -2
• 2 + 0 = 2
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• Commutative
• Associative
• Identity
• Distributive

13. Examples in SpecFlow
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14. Identity property BDD style…
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15. Defining constraints and expectations…
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16. More real life examples…
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17. Summary
• BDD turns examples into automated tests
• PBT automates rules with many different input
• The power of this two can be combined to achieve an
executable specification
• The BDD and the PBT tools can work together for this
• See http://github.com/gasparnagy/SpecFlow.FsCheck
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18. QUESTIONS?
gaspar@specsolutuions.eu
@gasparnagy
http://gasparnagy.com
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Special thanks to
• Ciaran McNulty
• Konstantin Kudryashov