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A Day in the Life of a Func onal Programmer Richard Dallaway

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Modern Development Func onal Programming with Types

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Why now?

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Why now? Modern compilers have powerful type system

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A Day in the Life

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Func onal Programming Is For Everyone Makes Change Easier

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The Morning Standup

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CRM RDBMS REST Server

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$ 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" } ]

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As a customer, I should have a subscrip on, so I can receive the company magazine.

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No subscrip on Ac ve subscrip on Lapsed subscrip on

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Algebraic Data Types Structural Recusion Unfamiliar names for simple powerful ideas

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sealed trait Subscription

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sealed trait Subscription case object Never extends Subscription case object Active extends Subscription case object Lapsed extends Subscription

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sealed trait Subscription case object Never extends Subscription case class Active(expires: Date) extends Subscription case object Lapsed extends Subscription

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sealed trait Subscription case object Never extends Subscription case class Active(expires: Date) extends Subscription case object Lapsed extends Subscription case class Customer( name : String, phone : String, sub : Subscription )

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sealed trait Subscription case object Never extends Subscription case class Active(expires: Date) extends Subscription case object Lapsed extends Subscription case class Customer( name : String, phone : String, sub : Subscription ) def transition(sub: Subscription, at: Date): Subscription = ???

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

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sealed trait Subscription case object Never extends Subscription case class Active(expires: Date) extends Subscription case object Lapsed extends Subscription case class Customer( 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 }

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“Marke ng just reminded me we give free subscrip ons to some customers” — Product Owner

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sealed trait Subscription case object Never extends Subscription case class Active(expires: Date) extends Subscription case object Lapsed extends Subscription

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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

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Move side effects out

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

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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 ...

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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 }

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ADTs Safe Versi le Separate data & behaviour

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Output JSON to the Client { "id": 1, "name": "Alice", "phone": "+1 555 1234", "sub": { "type": "expired" } }

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Type Class Big things from ny building blocks

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Turn Data into JSON text 7.toString

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

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Turn Data into JSON text 7.toString 7 "\"" + "Hello" + "\"" "Hello" { greeting: "Hello", count: 7 }

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Turn Data into JSON text 7.toString 7 "\"" + "Hello" + "\"" "Hello" { greeting: "Hello", count: 7 } Int => String String => String

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Turn Data into JSON text 7.toString 7 "\"" + "Hello" + "\"" "Hello" { greeting: "Hello", count: 7 } Int => String String => String T => String

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

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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 + "\"" }

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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

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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

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

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implicit def listFormat[T] = new JsonFormat[List[T]] { def format(values: List[T]): String = { ??? } }

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implicit def listFormat[T] = new JsonFormat[List[T]] { def format(values: List[T]): String = { val formattedValues = { ??? } "[" + formattedValues.mkString(",") + "]" } }

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implicit def listFormat[T] = new JsonFormat[List[T]] { def format(values: List[T]): String = { val formattedValues = for { v <- values } yield ??? "[" + formattedValues.mkString(",") + "]" } }

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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(",") + "]" } }

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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]

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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] ]

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Type Class Small Building Blocks Combined by the Compiler Used Everywhere

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

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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" })

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Used Everywhere ADTs Type Classes

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As a customer, I should have an control over my details, so I can update some or all of them.

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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() } } }

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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 } }

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Simplified Patch { "name" : "Robert" } or { "name" : "Robert", "phone : "+1 555 555" }

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Simplified Patch (Customer, Patch) => Customer

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

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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) = ???

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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)

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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] = ???

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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 } }

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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")

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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] )

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ADTs Safe, Versi le Type Classes Combining Small Building Blocks

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Scala Modern Powerful Compiler Func onal Programming JVM, Scala Na ve, Scala.js

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Help Make Change Easier

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Thank You h ps:/ /underscore.io/