Types Working For You Richard Dallaway, @d6y underscore.io Modern - - PowerPoint PPT Presentation

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Types Working For You Richard Dallaway, @d6y underscore.io Modern - - PowerPoint PPT Presentation

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Types Working For You Richard Dallaway, @d6y underscore.io Modern type system with lots of power Two Themes Straightforward Scala Types Working for Us Progression Part 1 Straightforward Scala Part 2 Functional Programming Part 3

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Types Working For You

Richard Dallaway, @d6y

underscore.io

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Modern type system with lots of power

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Two Themes

Straightforward Scala Types Working for Us

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Progression

Part 1 Straightforward Scala Part 2 Functional Programming Part 3 Typelevel Programming

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Straightforward Scala

— Part 1 —

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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

  • ur projects before

The spaceship operator <|*|> Someone said out loud “what the hell is that?”

http://jimplush.com/talk/

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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

  • ur projects before

The spaceship operator <|*|> Someone said out loud “what the hell is that?”

http://jimplush.com/talk/

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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

  • ur projects before

The spaceship operator <|*|> Someone said out loud “what the hell is that?”

http://jimplush.com/talk/

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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

  • ur projects before

The spaceship operator <|*|> Someone said out loud “what the hell is that?”

http://jimplush.com/talk/

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“It’s about having a maintainable code base where you can have 
 people cross projects easily and get new hires up to speed rapidly”

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Power!

Protect the team from it and Get the benefit of it

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What can we do?

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

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Algebraic data types

data into code

Structural recursion transformation

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Model data with logical

  • rs and logical ands
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A website visitor is:

  • anonymous; or
  • logged in
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A logged in user has:

  • an ID; and
  • facts we know about

them

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Two Patterns and (product types)

  • r (sum types)

Sum and product together make algebraic data types

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Structure of the code follows the structure of the data

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A website visitor is:

  • anonymous; or
  • logged in
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sealed trait Visitor case class Anonymous()
 extends Visitor case class User()
 extends Visitor

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A logged in user has:

  • an ID; and
  • facts we know about

them
 An anonymous has:

  • an ID
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sealed trait Visitor case class Anonymous()
 extends Visitor case class User()
 extends Visitor

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sealed trait Visitor case class Anonymous(id: Id)
 extends Visitor case class User(id: Id, facts: Set[Fact])
 extends Visitor

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Structural recursion

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def serveAd(v: Visitor): Advert = ???

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Structure of the code follows the structure of the data

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def serveAd(v: Visitor): Advert = ???

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

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

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

Structure

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ADT & Structural Recursion

Straightforward part of Scala. Clear, productive, occurs frequently. Be opinionated in what you use. Structure helps us.

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Help from FP Ideas

— Part 2 —

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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 ...

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A combine function and an empty value

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Addition

Empty Combine +

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Set

Empty Combine Set.empty union

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For any T

Empty Combine A zero for T A way to combine two Ts and give me back a T

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A combine function and an empty value

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Monoid A combine function and an empty value …and laws

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The boss asks…

What’s the total visits to the web site?

def report(vs: List[Int]): Int = ???

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For any T

Empty Combine A zero for T A way to combine two Ts and give me back a T

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For any T

trait Monoid[T] { def empty: T def combine(x: T, y: T): T }

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val addition = new Monoid[Int] { def empty = 0 def combine(x: Int, y: Int) = x+y }

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fold

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

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fold(1,2,3) 1 + fold(2,3) 2 + fold(3) 3 + fold() 0 + 3 + 2 + 1 = 6

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fold(1,2,3) 1 + fold(2,3) 2 + fold(3) 3 + fold() 0 + 3 + 2 + 1 = 6

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fold(1,2,3) 1 + fold(2,3) 2 + fold(3) 3 + fold() 0 + 3 + 2 + 1 = 6

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fold(1,2,3) 1 + fold(2,3) 2 + fold(3) 3 + fold() 0 + 3 + 2 + 1 = 6

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fold(1,2,3) 1 + fold(2,3) 2 + fold(3) 3 + fold() 0 + 3 + 2 + 1 = 6

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fold(1,2,3) 1 + fold(2,3) 2 + fold(3) 3 + fold() 0 + 3 + 2 + 1 = 6

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

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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

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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

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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

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Split on cases, inspect values you have

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

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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

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But back to Monoids…

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The boss asks…

What’s the total visits to the web site?

def report(vs: List[Int]): Int = fold(vs, addition)

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Benefits Composition Flexibility Problem Solving

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The boss asks…

How many distinct visitors?

def report(vs: List[Visitor]): Int = ???

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Set

Empty Combine Set.empty union

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The boss says…

Argh! 
 The servers are OutOfMemory

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HyperLogLog

Empty Combine new HLL() HLL.plus

Armon Dadgar (Papers We Love, 2015)
 “Bloom Filters and HyperLogLog”

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The boss asks…

Who are the really keen 
 visitors to the site?

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Count-Min Sketch

Empty Combine new CMS() CMS.plus

Laura Bledaite (Scala eXchange 2015) 
 “Count-Min Sketch in Real Data Applications”

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We can safely run 
 a parallel version 


  • f fold
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Laws

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a + 0 = a (a + b) + c = a + (b + c)

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Identity & Associativity

a combine empty = a (a combine b) combine c 
 = a combine (b combine c)

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a combine b combine combine c d e f

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Errors: 10 Warnings: 0

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Its a monoid I know this

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…so we fold

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Summary

Types and laws give us flexibility & help lead us to solutions. They help us every day.

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A Taste of Typelevel

— Part 3 —

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Date Metric Mon Low Tue High

csv( List(“Date”, “Metric”), List( List(“Mon”, “Low”), List(“Tue”, “High”) ) )

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Date Mon Low Tue High

csv( List(“Date”), List( List(“Mon”, “Low”), List(“Tue”, “High”) ) )

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How can we prevent that error happening again?

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def csv( hdrs: List[String], rows: List[List[String]] ): String = ???

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def csv[N <: Nat]( hdrs: List[String], rows: List[List[String]] ): String = ???

import shapeless._ import syntax.sized._

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def csv[N <: Nat]( hdrs: Sized[List[String], N], rows: List[Sized[List[String], N]] ): String = ???

import shapeless._ import syntax.sized._

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csv( Sized(“Date”), List( Sized(“Mon”, “Low”), Sized(“Tue”, “High”) ) )

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csv( Sized(“Date”), List( Sized(“Mon”, “Low”), Sized(“Tue”, “High”) ) ) Sized[List, 1] Sized[List, 2]

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How?

Sized(“Date”) constructs Sized[Nat] Nat implements numbers as types

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sealed trait Nat trait Succ[P <: Nat] extends Nat trait Zero extends Nat

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Zero 0 Succ[Zero] 1 Succ[Succ[Zero]] 2 Succ[Succ[Succ[Zero]]] 3

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sealed trait Nat trait Succ[P <: Nat] extends Nat trait Zero extends Nat

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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]]]

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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]]]

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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]]]

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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.

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Merging Fields

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

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PATCH /user/123 {
 “name” : “Batman”
 }

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case class User(
 id : Long,
 name : String,
 email : Option[String]) case class Update(
 name : Option[String],
 email : Option[Option[String]])

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val user = User(
 123L, 
 “Bruce Wayne”,
 Some(“bruce@example.org”)) val update = Update( 
 Some(“Batman”), 
 None)

How do we get to…

User(
 123L, 
 “Batman”, 
 Some(“bruce@example.org”))

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Bulletin

https://github.com/davegurnell/bulletin

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How?

User

String Option[String] … Option[String] Option[
 Option[String] ] …

Update

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How?

User

String Option[String] … Option[String] Option[
 Option[String] ] …

Update

Head

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How?

User

String Option[String] … Option[String] Option[
 Option[String] ] …

Update

Head The Rest…

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How?

Type constraints Implicit methods HLists Labelled generic Macros …

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val user = User(
 123L, 
 "Bruce Wayne”,
 Some(“bruce@example.org”)) val update = Update( 
 Some(“Batman”), 
 None) import bulletin._ val updated = user.merge(update) // User(
 // 123L, 
 // “Batman”, 
 // Some(“bruce@example.org”))

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val user = User(
 123L, 
 "Bruce Wayne”,
 Some(“bruce@example.org”)) val update = Update( 
 Some(“Batman”), 
 None) import bulletin._ val updated = user.merge(update) // User(
 // 123L, 
 // “Batman”, 
 // Some(“bruce@example.org”))

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Summary

The compiler can help (maybe more than you thought). Reduce boilerplate code.

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Using Power Tools

Can go one of two ways…

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Using Power Tools

Can go one of two ways… What the hell is that? It’s a monoid!
 I know this

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Simple Types Power Share

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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.’

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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

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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

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Thanks!

Richard Dallaway, @d6y

underscore.io

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Thanks!

Richard Dallaway, @d6y

underscore.io

Amanda Laucher Wesley Reisz Noel Welsh Dave Gurnell Miles Sabin Jono Ferguson Julio Capote Alessandro Zoffoli