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Types Working For You

Types Working For You

QCon London, 7 March 2016

Richard Dallaway

March 07, 2016
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  1. The only problem was we had no idea what the

    code was doing at first. We came across a strange symbol we hadn’t seen in our projects before The spaceship operator <|*|> Someone said out loud “what the hell is that?” http://jimplush.com/talk/
  2. The only problem was we had no idea what the

    code was doing at first. We came across a strange symbol we hadn’t seen in our projects before The spaceship operator <|*|> Someone said out loud “what the hell is that?” http://jimplush.com/talk/
  3. The only problem was we had no idea what the

    code was doing at first. We came across a strange symbol we hadn’t seen in our projects before The spaceship operator <|*|> Someone said out loud “what the hell is that?” http://jimplush.com/talk/
  4. The only problem was we had no idea what the

    code was doing at first. We came across a strange symbol we hadn’t seen in our projects before The spaceship operator <|*|> Someone said out loud “what the hell is that?” http://jimplush.com/talk/
  5. “It’s about having a maintainable code base where you can

    have 
 people cross projects easily and get new hires up to speed rapidly”
  6. 1. Expressions, types, & values 2. Objects and classes 3.

    Algebraic data types 4. Structural recursion 5. Sequencing computation 6. Type classes
  7. 1. Expressions, types, & values 2. Objects and classes 3.

    Algebraic data types 4. Structural recursion 5. Sequencing computation 6. Type classes
  8. A logged in user has: • an ID; and •

    facts we know about them
  9. Two Patterns and (product types) or (sum types) Sum and

    product together make algebraic data types
  10. A logged in user has: • an ID; and •

    facts we know about them
 An anonymous has: • an ID
  11. sealed trait Visitor case class Anonymous(id: Id)
 extends Visitor case

    class User(id: Id, facts: Set[Fact])
 extends Visitor
  12. def serveAd(v: Visitor): Advert = v match { case User(_,

    info) => relevantAd(info) case Anonymous(id) => adRotation(id) }
  13. def serveAd(v: Visitor): Advert = v match { case User(_,

    info) => relevantAd(info) case Anonymous(id) => adRotation(id) }
  14. def serveAd(v: Visitor): Advert = v match { case User(_,

    info) => relevantAd(info) case Anonymous(id) => adRotation(id) } Structure
  15. ADT & Structural Recursion Straightforward part of Scala. Clear, productive,

    occurs frequently. Be opinionated in what you use. Structure helps us.
  16. Combining lists Concatenating strings Union of sets Combining things in

    a loop Chaining logical operations Adding numbers Building up a JavaScript expression Showing errors in a UI ...
  17. For any T Empty Combine A zero for T A

    way to combine two Ts and give me back a T
  18. The boss asks… What’s the total visits to the web

    site? def report(vs: List[Int]): Int = ???
  19. For any T Empty Combine A zero for T A

    way to combine two Ts and give me back a T
  20. val addition = new Monoid[Int] { def empty = 0

    def combine(x: Int, y: Int) = x+y }
  21. def fold(vs: List[Int]): Int = vs match { case Nil

    => 0 case v :: rest => v + fold(rest) } fold(List(1,2,3))
 // 6
  22. def fold(vs: List[Int]): Int = vs match { case Nil

    => 0 case v :: rest => v + fold(rest) } fold(List(1,2,3))
 // 6
  23. def fold(vs: List[Int], m: Monoid[Int]): Int = vs match {

    case Nil => 0 case v :: rest => v + fold(rest) } fold(List(1,2,3), addition)
 // 6
  24. def fold(vs: List[Int], m: Monoid[Int]): Int = vs match {

    case Nil => m.empty case v :: rest => m.combine(v, fold(rest,m)) } fold(List(1,2,3), addition)
 // 6
  25. def fold[T](vs: List[T], m: Monoid[T]): T = vs match {

    case Nil => m.empty case v :: rest => m.combine(v, fold(rest,m)) } fold(List(1,2,3), addition)
 // 6
  26. def fold[T](vs: List[T], m: Monoid[T]): T = vs match {

    case Nil => ??? case v :: rest => ??? } fold(List(1,2,3), addition)
 // 6
  27. def fold[T](vs: List[T], m: Monoid[T]): T = vs match {

    case Nil => m.empty case v :: rest => ??? } fold(List(1,2,3), addition)
 // 6
  28. The boss asks… What’s the total visits to the web

    site? def report(vs: List[Int]): Int = fold(vs, addition)
  29. HyperLogLog Empty Combine new HLL() HLL.plus Armon Dadgar (Papers We

    Love, 2015)
 “Bloom Filters and HyperLogLog”
  30. Count-Min Sketch Empty Combine new CMS() CMS.plus Laura Bledaite (Scala

    eXchange 2015) 
 “Count-Min Sketch in Real Data Applications”
  31. a + 0 = a (a + b) + c

    = a + (b + c)
  32. Identity & Associativity a combine empty = a (a combine

    b) combine c 
 = a combine (b combine c)
  33. Summary Types and laws give us flexibility & help lead

    us to solutions. They help us every day.
  34. Date Metric Mon Low Tue High csv( List(“Date”, “Metric”), List(

    List(“Mon”, “Low”), List(“Tue”, “High”) ) )
  35. def csv[N <: Nat]( hdrs: List[String], rows: List[List[String]] ): String

    = ??? import shapeless._ import syntax.sized._
  36. def csv[N <: Nat]( hdrs: Sized[List[String], N], rows: List[Sized[List[String], N]]

    ): String = ??? import shapeless._ import syntax.sized._
  37. sealed trait Nat trait Succ[P <: Nat] extends Nat trait

    Zero extends Nat type One = Succ[Zero] type Two = Succ[One] implicitly[Succ[Zero] =:= One] implicitly[Succ[One] =:= Succ[Succ[Zero]]]
  38. sealed trait Nat trait Succ[P <: Nat] extends Nat trait

    Zero extends Nat type One = Succ[Zero] type Two = Succ[One] implicitly[Succ[Zero] =:= One] implicitly[Succ[One] =:= Succ[Succ[Zero]]]
  39. sealed trait Nat trait Succ[P <: Nat] extends Nat trait

    Zero extends Nat type One = Succ[Zero] type Two = Succ[One] implicitly[Succ[Zero] =:= One] implicitly[Succ[One] =:= Succ[Succ[Zero]]]
  40. sealed trait Nat trait Succ[P <: Nat] extends Nat trait

    Zero extends Nat type One = Succ[Zero] type Two = Succ[One] implicitly[Succ[Zero] =:= Two] error: Cannot prove that Succ[Zero] =:= Two.
  41. case class User(
 id : Long,
 name : String,
 email

    : Option[String]) val user = User(
 123L, 
 “Bruce Wayne”,
 Some(“[email protected]”))
  42. case class User(
 id : Long,
 name : String,
 email

    : Option[String]) case class Update(
 name : Option[String],
 email : Option[Option[String]])
  43. val user = User(
 123L, 
 “Bruce Wayne”,
 Some(“[email protected]”)) val

    update = Update( 
 Some(“Batman”), 
 None) How do we get to… User(
 123L, 
 “Batman”, 
 Some(“[email protected]”))
  44. val user = User(
 123L, 
 "Bruce Wayne”,
 Some(“[email protected]”)) val

    update = Update( 
 Some(“Batman”), 
 None) import bulletin._ val updated = user.merge(update) // User(
 // 123L, 
 // “Batman”, 
 // Some(“[email protected]”))
  45. val user = User(
 123L, 
 "Bruce Wayne”,
 Some(“[email protected]”)) val

    update = Update( 
 Some(“Batman”), 
 None) import bulletin._ val updated = user.merge(update) // User(
 // 123L, 
 // “Batman”, 
 // Some(“[email protected]”))
  46. Using Power Tools Can go one of two ways… What

    the hell is that? It’s a monoid!
 I know this
  47. 2008 ‘The name Scala stands for “scalable language.”
 
 The

    language is so named because it was designed to grow with the demands of its users.’
  48. What have we seen? Some straightforward parts of Scala
 —Clear,

    maintainable, helpful Encoding ideas in types
 —flexibility, leads us to solutions
 
 Let the compiler do it 
 —when it make sense for your demands
  49. Summary Scala scaling with your needs
 —be opinionated in what

    you use, more when needed Types working for us, not stopping us
 —functional programming, share what you learn
  50. Thanks! Richard Dallaway, @d6y underscore.io Amanda Laucher Wesley Reisz Noel

    Welsh Dave Gurnell Miles Sabin Jono Ferguson Julio Capote Alessandro Zoffoli