Brighton Java: Day in the life...

Transcript

1. A Day in the Life of a Func onal Programmer

   Richard Dallaway (@d6y)

2. Agenda Three examples

3. Func onal Programming Is For Everyone Makes Change Easier

4. The Morning Standup

5. CRM RDBMS REST Server

6. $ http GET :8080/customers HTTP/1.1 200 OK Content-Type: application/json [

   { "id": 1, "name": "Alice", "phone": "+1 555 1234" }, { "id": 2, "name": "Bob", "phone": "+1 555 5678" } ]

7. As a customer, I should have a subscrip on, so

   I can receive the magazine.

8. No subscrip on Ac ve subscrip on Lapsed subscrip on

9. Algebraic Data Types Structural Recursion Unfamiliar names for simple powerful

   ideas

10. sealed trait Subscription

11. sealed trait Subscription case object Never extends Subscription case object

    Active extends Subscription case object Lapsed extends Subscription

12. sealed trait Subscription case object Never extends Subscription case class

    Active(expires: Date) extends Subscription case object Lapsed extends Subscription

13. sealed trait Subscription case object Never extends Subscription case class

    Active(expires: Date) extends Subscription case object Lapsed extends Subscription case class Customer( id : Long, name : String, phone : String, sub : Subscription )

14. sealed trait Subscription case object Never extends Subscription case class

    Active(expires: Date) extends Subscription case object Lapsed extends Subscription case class Customer( id : Long, name : String, phone : String, sub : Subscription ) def transition(sub: Subscription, at: Date): Subscription = ???

15. sealed trait Subscription case object Never extends Subscription case class

    Active(expires: Date) extends Subscription case object Lapsed extends Subscription case class Customer( id : Long, name : String, phone : String, sub : Subscription ) def transition(sub: Subscription, at: Date): Subscription = sub match { case Lapsed | Never => sub case Active(exp) => ??? }

16. sealed trait Subscription case object Never extends Subscription case class

    Active(expires: Date) extends Subscription case object Lapsed extends Subscription case class Customer( id : Long, name : String, phone : String, sub : Subscription ) def transition(sub: Subscription, at: Date): Subscription = sub match { case Lapsed | Never => sub case Active(exp) => if (at > exp) Lapsed else sub }

17. “Marke ng just reminded me we give free subscrip ons

    to some customers” — Product Owner

18. sealed trait Subscription case object Never extends Subscription case class

    Active(expires: Date) extends Subscription case object Lapsed extends Subscription

19. sealed trait Subscription case object Never extends Subscription case class

    Active(expires: Date) extends Subscription case object Lapsed extends Subscription case object Freebie extends Subscription def transition(sub: Subscription, at: Date): Subscription = sub match { case Lapsed | Never => sub case Active(exp) => if (at > exp) Lapsed else sub } ERROR: non‐exhaus ve match

20. Move side effects out

21. def transition(c: Customer, at: Date): Action = ???

22. def transition(c: Customer, at: Date): Action = ??? sealed trait

    Action case object NoAction extends Action case class EncourageRenewal(c: Customer) extends Action case class Expire(c: Customer) extends Action ...

23. def transition(c: Customer, at: Date): Action = ??? sealed trait

    Action case object NoAction extends Action case class EncourageRenewal(c: Customer) extends Action case class Expire(c: Customer) extends Action ... def perform(a: Action): Unit = a match { case NoAction => case Expire(c) => sendSMS(c.phone) ...etc }

24. ADTs Safe Versa le Separate data & behaviour

25. Output JSON to the Client { "id": 1, "name": "Alice",

    "phone": "+1 555 1234", "sub": { "type": "expired" } }

26. Type Class Big things from ny building blocks

27. Turn Data into JSON text 7.toString 7

28. Turn Data into JSON text 7.toString 7 "\"" + "Hello"

    + "\"" "Hello"

29. Turn Data into JSON text 7.toString 7 "\"" + "Hello"

    + "\"" "Hello" { greeting: "Hello", count: 7 }

30. Turn Data into JSON text 7.toString 7 "\"" + "Hello"

    + "\"" "Hello" { greeting: "Hello", count: 7 } Int => String String => String

31. Turn Data into JSON text 7.toString 7 "\"" + "Hello"

    + "\"" "Hello" { greeting: "Hello", count: 7 } Int => String String => String T => String

32. trait JsonFormat[T] { def format(value: T): String }

33. trait JsonFormat[T] { def format(value: T): String } val intFormat

    = new JsonFormat[Int] { def format(value: Int): String = value.toString } val strFormat = new JsonFormat[String] { def format(value: String): String = "\"" + value + "\"" }

34. trait JsonFormat[T] { def format(value: T): String } val intFormat

    = new JsonFormat[Int] { def format(value: Int): String = value.toString } val strFormat = new JsonFormat[String] { def format(value: String): String = "\"" + value + "\"" } def outputJson[T](value: T)(jf: JsonFormat[T]) = jf.format(value) outputJson(7)(intFormat) // 7

35. trait JsonFormat[T] { def format(value: T): String } implicit val

    intFormat = new JsonFormat[Int] { def format(value: Int): String = value.toString } implicit val strFormat = new JsonFormat[String] { def format(value: String): String = "\"" + value + "\"" } def outputJson[T](value: T)(implicit jf: JsonFormat[T]) = jf.format(value) outputJson(7) // 7

36. [ "Hello", "there" ] [ 1, 2, 3, 4 ]

    [ [1,2], [3,4] ]

37. implicit def listFormat[T] = new JsonFormat[List[T]] { def format(values: List[T]):

    String = { ??? } }

38. implicit def listFormat[T] = new JsonFormat[List[T]] { def format(values: List[T]):

    String = { val formattedValues = { ??? } "[" + formattedValues.mkString(",") + "]" } }

39. implicit def listFormat[T] = new JsonFormat[List[T]] { def format(values: List[T]):

    String = { val formattedValues = for { v <- values } yield ??? "[" + formattedValues.mkString(",") + "]" } }

40. implicit def listFormat[T](implicit jf: JsonFormat[T]) = new JsonFormat[List[T]] { def

    format(values: List[T]): String = { val formattedValues = for { v <- values } yield jf.format(v) "[" + formattedValues.mkString(",") + "]" } }

41. implicit def listFormat[T](implicit jf: JsonFormat[T]) = new JsonFormat[List[T]] { def

    format(values: List[T]): String = { val formattedValues = for { v <- values } yield jf.format(v) "[" + formattedValues.mkString(",") + "]" } } def outputJson[T](value: T)(implicit jf: JsonFormat[T]) = jf.format(value) outputJson( List(1,2,3,4) ) [1,2,3,4]

42. implicit def listFormat[T](implicit jf: JsonFormat[T]) = new JsonFormat[List[T]] { def

    format(values: List[T]): String = { val formattedValues = for { v <- values } yield jf.format(v) "[" + formattedValues.mkString(",") + "]" } } def outputJson[T](value: T)(implicit jf: JsonFormat[T]) = jf.format(value) outputJson( List(1,2,3,4) ) [1,2,3,4] outputJson( List( List(1,2), List(3,4) ) ) [ [1,2], [3,4] ]

43. Type Class Small Building Blocks Combined by the Compiler Used

    Everywhere

44. case class Customer( name : String, phone : String, id

    : Long ) sql" select name, phone, id from customer ".query[Customer]

45. case class Customer( name : String, phone : String, id

    : Long ) sql" select name, phone, id from customer ".query[Customer] implicit val SubscriptionMeta: Meta[Subscription] = Meta[String].nxmap(ch => ch match { case "A" => Active case "L" => Lapsed case "N" => Never }, sub => sub match { case Active => "A" case Lapsed => "L" case Never => "N" })

46. Used Everywhere ADTs Type Classes

47. As a customer, I want to update the details you

    hold for me.

48. class CustomerAPI(db: Database) { val service = HttpService { case

    GET -> Root / "customers" => Ok(db.list) case GET -> Root / "customers" / IntVar(id) => db.find(id).flatMap { case Some(customer) => Ok(customer) case None => NotFound() } } }

49. class CustomerAPI(db: Database) { val service = HttpService { case

    GET -> Root / "customers" => Ok(db.list) case GET -> Root / "customers" / IntVar(id) => db.find(id).flatMap { case Some(customer) => Ok(customer) case None => NotFound() } case PATCH -> Root / "customers" / IntVar(id) => decode the patch lookup the customer in the database found a record? somehow apply the patch to the record update the database otherwise, 404 the request } }

50. Simplified Patch { "name" : "Robert" } or { "name"

    : "Robert", "phone : "+1 555 555" }

51. Simplified Patch (Customer, Patch) => Customer

52. case class Customer( id : Long, name : String, phone

    : String ) case class CustomerPatch( name : Option[String], phone : Option[String] )

53. case class Customer( id : Long, name : String, phone

    : String ) case class CustomerPatch( name : Option[String], phone : Option[String] ) trait Merge[Into, From] { def merge(i: Into, f: From): Into } def merge[Into, From](i: Into, f: From) = ???

54. case class Customer( id : Long, name : String, phone

    : String ) case class CustomerPatch( name : Option[String], phone : Option[String] ) trait Merge[Into, From] { def merge(i: Into, f: From): Into } def merge[Into, From](i: Into, f: From) (implicit m: Merge[Into, From]) = m.merge(i, f)

55. case class Customer( id : Long, name : String, phone

    : String ) case class CustomerPatch( name : Option[String], phone : Option[String] ) trait Merge[Into, From] { def merge(i: Into, f: From): Into } def merge[Into, From](i: Into, f: From) (implicit m: Merge[Into, From]) = m.merge(i, f) implicit def customerMerge: Merge[Customer, CustomerPatch] = ???

56. case class Customer( id : Long, name : String, phone

    : String ) case class CustomerPatch( name : Option[String], phone : Option[String] ) trait Merge[Into, From] { def merge(i: Into, f: From): Into } def merge[Into, From](i: Into, f: From) (implicit m: Merge[Into, From]) = m.merge(i, f) implicit def customerMerge: Merge[Customer, CustomerPatch] = ??? implicit def optMerge = new Merge[String, Option[String]] { def merge(initial: String, update: Option[String]) = update match { case Some(value) => value case None => initial } }

57. case class Customer( id : Long, name : String, phone

    : String ) case class CustomerPatch( name : Option[String], phone : Option[String] ) trait Merge[Into, From] { def merge(i: Into, f: From): Into } def merge[Into, From](i: Into, f: From) (implicit m: Merge[Into, From]) = m.merge(i, f) implicit def customerMerge: Merge[Customer, CustomerPatch] = ??? merge( Customer(1, "Bob", "555-123"), CustomerPatch(Some("Robert"), None) ) Customer(1, "Robert", "555-123")

58. Pu ng it all together... case req @ PATCH ->

    Root / "customers" / IntVar(id) => req.decode[CustomerPatch] { patch => db.find(id).flatMap { case None => NotFound() case Some(customer) => val updated = merge(customer, patch) Ok(db.update(updated)) } } val merge = bulletin.AutoMerge[Customer, CustomerPatch] case class CustomerPatch( name : Option[String], phone : Option[String] )

59. ADTs Safe, Versa le Type Classes Combining Small Building Blocks

60. Scala Modern Powerful Compiler Func onal Programming JVM, Scala.js, Scala

    Na ve
61. None

62. Func onal Programming Is For Everyone Makes Change Easier

63. Thank You h ps:/ /underscore.io/ h ps:/ /underscore.io/books/essen al‐scala/ h

    ps:/ /www.meetup.com/Func onal‐Brighton/