Property based testing 101

Yet another testing talk?

This talk is different

People already said a lot about testing

Why do we test?

Property Based Testing success stories

• Riak
• Level DB
• Volvo

How many tests are enough?

Unit Testing vs Property Based Testing

• What does unit, integration, system etc tests have in common?
• Example based vs Property based

What is property?

• length(list) == length(reverse(list))
• assert reverse(reverse(list)) equals list
• assert reverse(list)[k] equals list[length(list)-k]

What else?

• Generator
• Shrinking

Generator

list <- generate_list

• [1]
• [-1, 3, 5]
• [-50, -10, 3, 14, 25, 100]
• etc

Shrinking

Let's assume reverse function that will remove element 42 if found

• Failed: [-10, -8, ..., 42, ... 87, 100]
• Shrinking...
• Minimal failing test case: [42]

Sample test

• list <- generate_list
• k <- generate_int(0, length(list)-1)
• assert length(reverse(list)) equals length(list)
• assert reverse(reverse(list)) equals list
• assert reverse(list)[k] equals list[length(list)-k]

Why should I care?

Bad stuff?

• Time consuming
• Learning curve

Thank you & questions

Lukasz Skotarek

@dreat_ on Twitter

dreat on Elixir/3cityIt/HS Slack

mail: contact@dreat.info