Programming with Values


This lesson is divided into 5 sections:

  1. Short warm-up discussion (~20 minutes)
  2. Lecture
  3. Student exercise
  4. Share out
  5. Review potential refactor


Available here



  • What does ‘single responsibility’ mean in a programming context?
  • How does programming with an eye towards the single responsibility principle help us as programmers?
  • In past projects, what have you decided to test? What have you decided not to test?
  • Why?


Data Pains - limiting our interface to our objects

  • Common pain point when dealing with external interfaces: we make the external data dump the only interface for sending information into the object
  • The action of bringing outside data in is contained within the object, so our only way to provide it is to put the data in the right place (often a file) and let the object slurp it in
  • Programming with files/HTTP sources/Network data drives us to program with Locations rather than values
  • Rather than telling you the value of something I’ll tell you it’s on the whiteboard in the other classroom. Then you have to go look it up.
  • What does this mean for our tests?
  • What would an alternative look like?
  • Can we preserve the simple data interface (i.e. provide data in the form of ruby objects) as well as the “slurp it up from a file” interface?
  • Best of both worlds: Testability and flexibility when you want to provide standard basic objects; Ease of use / “Do it all” when you need to pull in a bunch of external data
  • Additional win: If the importing task becomes more complex, we can easily extract it into a separate entity
  • The interface between our objects is simple data, rather than external infrastructure

Exercise - TDD Pizza Parlor

Keep these principles in mind as you complete this short Pizza Parlor Exercise


  • What did you test?
  • What didn’t you test?
  • Can you test more if you refactored your code?

Review Potential Refactors

  • Can we isolate the file I/O?
  • Can we extract it to a class?

Further Reading / Watching