the typeclass pattern an alternative to inheritance Northeast Scala Symposium March 9, 2012 Seth Tisue seth@tisue.net @SethTisue http://tisue.net

The plan 1. Concepts 2. Example 3. Pros & cons

Why typeclasses in Scala?

Why typeclasses?

Why? POLYMORPHISM DECOUPLING

Polymorphism The same operation working on different types of values. “Parametric” vs. “ad hoc” polymorphism

Origins pre-Haskell 1989: Philip Wadler & Stephen Blott, “How to make ad-hoc polymorphism less ad hoc” [link] (POPL ’89) 1990: Haskell

Typeclasses and functional programming FP = immutability functions as values no side effects typeclasses = decoupled ad hoc polymorphism

Haskell: Type class (language feature) Scala: Typeclass (pattern)

Polymorphism in OO languages – Overloading – Inheritance – Pattern matching + Traits / interfaces + Type parameters, generic types

Examples scala.math.Numeric scala.math.Ordering typesafe equality [link] everything else in Scalaz (monoids, monads, applicative functors...) extended example: serialization

Serialization example case class Person( name: String, age: Int) case class Restaurant( name: String, brunch: Boolean)

Serialization: Overloading def serialize(p: Person) = "Person(" + p.name + "," + p.age + ")" def serialize(r: Restaurant) = "Restaurant(" + r.name + "," + r.brunch + ")" Uh oh...

Serialization: Inheritance abstract class Serializable { def serialize: String } class Person(...) extends Serializable { override def serialize = ... } class Restaurant(...) extends Serializable { override def serialize = ... } Uh oh...

Serialization: trait (interface) trait Serializable { def serialize: String } class Person(...) extends Serializable with ... { override def serialize = ... } class Restaurant(...) extends Serializable with ... { override def serialize = ... } Uh oh?

Serialization: pattern matching aka “typecasing” def serialize(x: Any) = x match { case p: Person => "Person(" + p.name + ... case r: Restaurant => "Restaurant(" + r.name + ... ... } } def serializeToJSON(x: Any) = x match { ...

Serialization: Typeclass (almost) trait Serializable[T] { def ser(t: T): String } def serialize[T](t: T, s: Serializable[T]) = s.ser(t) object PersonIsSerializable extends Serializable[Person] { def ser(p: Person) = "Person(" + p.name + ... } object RestaurantIsSerializable ...

So far so good scala> serialize(Person("Seth", 40), PersonIsSerializable) res7: String = Person(Seth,40) scala> serialize(Restaurant(...), RestaurantIsSerializable) res8: String = Restaurant(...) but it’s not automatic yet

Serialization: Typeclass trait Serializable[T] { def ser(t: T): String } def serialize[T](t: T) (implicit s: Serializable[T]) = s.ser(t) implicit object PersonIsSerializable extends Serializable[Person] { def ser(p: Person) = "Person(" + p.name + ... } implicit object RestaurantIsSerializable ...

It works! scala> serialize(Person("Seth", 40)) res7: String = Person(Seth,40) and it's type-safe: scala> serialize("Seth") [...] error: could not find implicit value for parameter s: Serializable[java.lang.String] serialize("Seth")

Syntax: “context bound” let's define something that works across the whole typeclass: implicit def ListIsSerializable[T] (implicit ev: Serializable[T]) = ... can be shortened to: implicit def ListIsSerializable[T : Serializable] = new Serializable[List[T]] { def ser(xs: List[T]) = xs.map(serialize(_)) .mkString("List(", ",", ")") } scala> serialize(List(Person("Seth", 40), Person("Kaarin", 43))) res1: String = List(Person(Seth,40),Person(Kaarin,43))

Syntax: OO style so far, function style: scala> serialize(Person("Seth", 40)) res7: String = Person(Seth,40) but suppose I'd rather write: scala> Person("Seth", 40).serialize res8: String = Person(Seth,40)

Syntax: OO style trait Serializable[T] { def ser(t: T): String } implicit def addSerialize[T](t: T) (implicit s: Serializable[T]) = new { def serialize = s.ser(t) } scala> Person("Seth", 40).serialize res8: String = Person(Seth,40) implicit parameters and implicit conversion

What have we got? Type safety Extensibility (add serializable types) Extensibility (add new kinds of serialization) Decoupling

Advantage: Scala “Unlike with [Haskell’s] type classes, the scope of an implicit parameter can be controlled, and competing implicit parameters can coexist in different parts of one program.” Odersky et al, “An Overview of the Scala Programming Language” (2004)

Example: disambiguating scala> implicit val ByName: Ordering[Person] = Ordering.by(_.name) ByName: Ordering[Person] = ... scala> people.sorted res3: List[Person] = List(Person(Kaarin,43), Person(Seth,40)) scala> implicit val ByAge: Ordering[Person] = Ordering.by(_.age) ByAge: Ordering[Person] = ... scala> people.sorted error: ambiguous implicit values: both value ByName of type => Ordering[Person] and value ByAge of type => Ordering[Person] match expected type scala.math.Ordering[Person] scala> people.sorted(ByName) res5: List[Person] = List(Person(Kaarin,43), Person(Seth,40)) scala> people.sorted(ByAge) res6: List[Person] = List(Person(Seth,40), Person(Kaarin,43))

Typeclasses are static Object oriented dispatch happens at runtime Pattern matching happens at runtime Typeclasses work at compile time only

This is not dynamic dispatch! This works: scala> serialize(List(Person("Seth", 40), Person("Kaarin", 43))) res1: String = List(Person(Seth,40),Person(Kaarin,43)) But this doesn't: scala> serialize(List(Person("Seth", 40), Restaurant("Haveli", true))) error: could not find implicit value for parameter s: Serializable[List[Product with Serializable]]

Interaction with inheritance [link 1] (Stack Overflow) [link 2] (Stack Overflow)

Other downsides Pattern boilerplate Not obvious “this is a typeclass” Implicit parameters complicate error messages, complicate use of higher order functions, etc. Less discoverable in Scaladoc, IDE, etc Performance overhead

Language proposals performance: Josh Suereth (& Jorge Ortiz, David Hall): SIP-13: Implicit classes [link] Martin Odersky, Jeff Olson, Paul Phillips, Josh Suereth: SIP-15: Value classes [link] syntax: “Note: [SIP-13] does not include any syntax sugar fixes for defining typeclasses. I have an alternative, much more controversial, SIP that outlines an annotation-based approach to generating typeclass boilerplate. I can send it to those who are interested, because it’s designed to be achieved via annotations + a compiler plugin currently.” — Josh Suereth, scala-lang group [link]

Further reading Scala in Depth, section 7.3 Josh Suereth (book, Manning, 2012) [info/preorder] “Typeclasses in Scala” Erik Osheim (PHASE talk, 2011) [slides]

thank you for listening ask something Seth Tisue seth@tisue.net @SethTisue http://tisue.net