4 Rules of Simple Design in Functional Programming

Let's see how Category Theory helps us in writing good functional code!

A practical example of how to design a small application using functional programming principles and respecting Kent Beck's 4 rules.
Functional Programming goals are the same than Object-Oriented Programming
You don’t need a special language or library
You need to study and practice a different paradigm

Transcript

1. The 4 Rules
   of Simple Design
   and Category Theory
   The 4 Rules
   of Simple Design
   and Category Theory
   Uberto
   Barbini
   @ramtop
   

   View Slide

2. “What is the relation between
   Category Theory
   and Code Quality?
   They seem two completely
   different things to me”
   Marcus Biel @MarcusBiel
   

   View Slide

3. Object-Oriented
   Functional
   TL;DR
   

   View Slide

4. Code Quality and Categories:
   Category Theory helps us
   To improve our functional design
   

   View Slide

5. Code Quality and Categories
   Why Functional Programming?
   How Categories can help?
   

   View Slide

6. The Alternative
   

   View Slide

7. Kent Beck’s
   Four Rules of Simple Design:
   Passes the tests
   (It works)
   Reveals intention
   (Easy to read and understand)
   No duplication
   (DRY: Don’t Repeat Yourself)
   Fewest elements
   (remove anything that doesn't
   serve the three previous rules)
   

   View Slide

8. Functional Programming is a
   programming paradigm that treats
   computation as the evaluation of
   mathematical functions and avoids
   changing-state and mutable data.
   Wikipedia
   

   View Slide

9. Alan Kay John McCarthy
   Smalltalk Lisp
   Conversations Transformations
   

   View Slide

10. A Story of Two Books
    

    View Slide

11. Category Tinted Glasses
    

    View Slide

12. The proof of the pudding is
    in the eating
    

    View Slide

13. The proof of the Functional
    Programmer is in the coding
    

    View Slide

14. What we need is:
    Higher Order Functions
    Immutable Data
    Lazyness
    

    View Slide

15. But at the end…
    it’s all about morphisms
    “If I haven’t convinced you yet that category
    theory is all about morphisms then I haven’t
    done my job properly.”
    Bartosz Milewski
    https://bartoszmilewski.com/2015/11/17/its-all-about-morphisms/
    

    View Slide

16. Morphism examples in code
    Date → String
    User → Date
    String → Int
    (User → Date) → (User → String)
    

    View Slide

17. Morphism examples in code
    Date → String
    dateFormat()
    User → Date
    getBirthday()
    String → Int
    length()
    (User → Date) → (User → String)
    

    View Slide

18. What is a Category
    

    View Slide

19. London Tube Category
    

    View Slide

20. Event Source Category
    for pizza delivery service
    NewOrder
    Ready
    Cancelled
    Returned
    Dispatched
    Closed
    https://skillsmatter.com/skillscasts/11486-functional-cqrs
    AddItem
    Cancel
    Dispatch
    Return
    Close
    

    View Slide

21. Birthday Greetings Kata
    Problem: write a program that loads a set of employee records from
    a flat file and then sends a greetings email to all employees whose
    birthday is today. The flat file is a sequence of records, separated by
    newlines; this are the first few lines:
    Doe, John, 1982/10/08, john.doe@foobar.com
    Ann, Mary, 1975/09/11, mary.ann@foobar.com
    http://matteo.vaccari.name/blog/archives/154
    

    View Slide

22. Hexagonal OO Design
    Problem: write a program that loads a set of employee records from
    a flat file and then sends a greetings email to all employees whose
    birthday is today. The flat file is a sequence of records, separated by
    newlines; this are the first few lines:
    

    View Slide

23. Categories Functional Design
    Our weapons:
    Immutable Types
    Pure Functions
    Laziness
    No Exceptions
    

    View Slide

24. Define Arrows From the Outside
    Filename → Emails sent
    

    View Slide

25. Define Arrows From the Outside
    Filename → Emails sent
    Filename → Text → EmailData → Emails sent
    

    View Slide

26. Define Arrows From the Outside
    Filename → Emails sent
    Filename → Text → EmailData → Emails sent
    … → Text → CSVrows → Employee → EmailData → …
    

    View Slide

27. Define Arrows From the Outside
    Filename → Emails sent
    Filename → Text → EmailData → Emails sent
    … → Text → CSVrows → Employee → EmailData → …
    … → CSVrows → Employee → isTodayBirthday → …
    

    View Slide

28. Filename
    TextFile
    EmailData
    Emails sent
    CSVrow
    Employee
    TodayBirthday
    GreetingTemplate
    Today
    

    View Slide

29. data class Employee(val firstName: String,
    val lastName: String,
    val dateOfBirth: LocalDate,
    val email: EmailAddress)
    data class Email (val recipient: EmailAddress,
    val subject: String,
    val text: String)
    inline class EmailAddress(val raw: String)
    inline class CsvRow(val raw: String)
    Objects → Types
    

    View Slide

30. typealias EmployeeToEmail = (Employee) -> Email
    class EmailTemplate(val msgTemplate: String): EmployeeToEmail {
    override fun invoke(e: Employee): Email =
    Email(e.email, "Greetings",
    msgTemplate.replace("%", e.firstName))
    }
    fun rowToEmployee(csv: CsvRow): Employee =
    csv.raw.split(",").let{
    Employee(
    firstName = it[1].trim(),
    lastName = it[0].trim(),
    email = EmailAddress(it[3].trim()),
    dateOfBirth = LocalDate.parse(it[2].trim(), LOCAL_DATE))
    }
    Morphisms → Pure Functions
    

    View Slide

31. Filename
    TextFile
    EmailData
    Emails sent
    CSVrow
    Employee
    TodayBirthday
    GreetingTemplate
    Today
    What about the rest?
    IO, collection, time, errors...
    

    View Slide

32. Filename
    TextFile
    EmailData
    Emails sent
    CSVrow
    Employee
    TodayBirthday
    GreetingTemplate
    Today
    Categories
    What about the rest?
    IO, collection, time, errors...
    

    View Slide

33. But, but…
    how can we switch category?
    

    View Slide

34. View Slide

35. Simplest functor in our code:
    string.length()
    

    View Slide

36. Generics are type builders
    List is not a type but you can build types
    List
    List
    List
    Etc.
    

    View Slide

37. Generics are type builders
    List is not a type but you can build types
    List
    List
    List
    Etc.
    Employee → List
    is a Functor to the Category of List
    List is said to have a Functor instance
    

    View Slide

38. Employee → List
    is a Functor to the Category of List
    List is said to have a Functor instance
    map is how you translate functions (morphisms)
    val employees: List = …
    val names: List =
    employees.map { emp → emp.name }
    List.map(f: (T) -> R): List
    

    View Slide

39. Functors are like Fork lifts, they lift a
    function from
    Employee → String
    to
    List → List
    val employees: List = …
    val names: List =
    employees.map { emp → emp.name }
    List.map(f: (T) -> R): List
    

    View Slide

40. Functor for Birthday Filter
    

    View Slide

41. Functor for Errors
    

    View Slide

42. Functor to Read from IO
    

    View Slide

43. The Result
    

    View Slide

44. CHECK!
    Passes the tests
    Reveals intention
    No duplication
    Fewest elements
    

    View Slide

45. Takeouts
    Functional Programming goals are the same than
    Object-Oriented Programming
    You don’t need a special language or library
    You need to study and practice a different paradigm
    github.com/uberto/birthdaykata
    

    View Slide

46. Uberto Barbini
    Blog (jvm performance, kotlin, functional programming):
    medium.com/@ramtop
    Twitter:
    @ramtop
    Programmer OOP
    TDD
    Kotlin
    Agile
    Functional
    Programming
    Finance
    Industry
    

    View Slide

47. View Slide

48. View Slide

49. View Slide

50. View Slide

51. View Slide

52. View Slide