Lies My OO Teacher Told Me
- 1. Lies My OO Teacher Told Me Eric Normand
- 2. Registration for Classes • Go to a department • Read the list of classes given • Put your name on the list for each class • Repeat
- 3. Background • These are lies I have personally experienced in my education. • Most of my education was in Object Oriented Programming. • It may seem like I’m attacking Object Oriented Programming. • But I’m not. I have a lot of respect for OOP. • But these are things I’ve learned through functional programming. • And they are universally applicable.
- 4. For the video and transcript of this presentation, click here: https://lispcast.com/lies-my-oo-teacher-told-me/
- 5. Object Oriented Modeling Process• Take a natural-language description of the problem • Underline all of the nouns. • Make a class for each noun. • Look for possessive words like “have”, those become references. • Underline all of the verbs. • Make a method for each verb.
- 6. Student Registration • A university allows students to register for courses. Each student may register for multiple courses. And each course can have multiple students.
- 7. Student Registration • A university allows students to register for courses. Each student may register for multiple courses. And each course can have multiple students.
- 10. LIES!
- 11. Lie #1: Noun and Verb? • Student • Study • Studious • Register • Registration • Registry
- 12. Takeaways • We have to look deeper than part of speech. • We have many more options for modeling.
- 13. Command-Query Separation • Queries: • Methods that are like questions you ask the object. • No modifications. • Commands: • Methods that modify the object or call a command on another object.
- 14. Questions - Queries • What is your name? • What is your Student ID? • What classes are you registered for?
- 15. Actions - Commands • Change your name to “xxx”. • Change your Student ID to “xxx”. • Register for X course.
- 16. LIES!
- 17. Lie #2: Those are interesting questions • These are questions, but do they modify? • What would it take to register you for this course? • I bet your schedule would be better if you added this course? Is it? • What was your schedule last year? • I know you haven’t decided, but could you give me an example schedule?
- 18. Takeaways • Easy questions like “What is your name?” are uninteresting. • Interesting questions are more like commands in an alternate reality. • Hypotheticals • Counterfactuals
- 19. Model relationships with references • Student.courses - list of pointers to Courses • Course.students - list of pointers to Students • University.courses - list of pointers to Courses • University.students - list of pointers to Students
- 20. LIES!
- 21. Lie #3: Tracking relationships with pointers • Pointing to mutable object. • It’s changing as you look at it. • Pointers are ephemeral and out of your control. • Pointers are different each time you run. • There could be two Student objects with the same Student ID. • Same real person, different objects.
- 22. Takeaways • Model relationships with a value • Something immutable • With equality semantics • That represents identity
- 23. Let’s model the registration process • Construct a University • Construct a Student • Construct the Course they want • Call the register method on Student with Course • … • Profit!
- 24. LIES!
- 25. Lie #4: We want to model the actors • Why is the register method on Student? • Because “A student registers for a course” • We need to record the course in Student.courses • Why is the register method on Course? • Because we need to record the student in Courses.students. • Will these lists get out of sync? • Why are we modeling students and courses, anyway? • OO programming was made for Simulations.
- 26. Takeaways • What should we simulate? • Exercise: • How would you run registration if all you had were paper and pen?
- 27. LIES!
- 28. Object Oriented Programming • You can do all of this with OOP. • Many good programmers already do. • Lie #1: Noun/verb • Many OO refactorings convert between class and method • Lie #2: Queries are interesting • Simulation is great for counterfactuals. • Lie #3: Relationships with pointers • Value objects. • Lie #4: Modeling all of the actors • Easily done in OOP. • OOP is great for exploring different models.
- 29. Eric Normand Follow Eric on: Eric Normand @EricNormand [email protected]