Brighton Java: Day in the life...

Presentation to Brighton Java. https://www.meetup.com/Brighton-Java/events/239733337/

Recording: https://www.youtube.com/watch?v=S5BvWPIj-TY

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
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/