4 Rules of Simple Design in Functional Programming

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

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

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Object-Oriented Functional TL;DR

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

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

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The Alternative

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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)

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

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Alan Kay John McCarthy Smalltalk Lisp Conversations Transformations

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A Story of Two Books

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Category Tinted Glasses

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The proof of the pudding is in the eating

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The proof of the Functional Programmer is in the coding

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What we need is: Higher Order Functions Immutable Data Lazyness

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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/

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

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

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What is a Category

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London Tube Category

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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

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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, [email protected] Ann, Mary, 1975/09/11, [email protected] http://matteo.vaccari.name/blog/archives/154

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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:

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

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Define Arrows From the Outside Filename → Emails sent

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

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

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

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Filename TextFile EmailData Emails sent CSVrow Employee TodayBirthday GreetingTemplate Today

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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

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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

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

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

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But, but… how can we switch category?

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No content

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Simplest functor in our code: string.length()

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

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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

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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

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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

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Functor for Birthday Filter

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Functor for Errors

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Functor to Read from IO

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The Result

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CHECK! Passes the tests Reveals intention No duplication Fewest elements

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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

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