Introduction to type classes in Scala Explain how JSON Reads/Writes/Formats work in Play! Framework

1. type classes in Scala
And how Json Reads/Writes/Formats
work in play framework?
Yann Simon - @simon_yann

2. toString, hashCode on model
case class Person(name: String, age: Int)
val persons =
List(
Person("bob", 34),
Person("alice", 34))
persons.toString()
res0: String = List(Person(bob,34), Person(alice,34))
persons.hashCode()
res1: Int = -408691241

3. persons.toString
toString()List
Person
String Int
toString()
toString() toString()

4. persons.hashCode
hashCodeList
Person
String Int
hashCode
hashCode hashCode

5. What about persons.toJson?
persons.toXML? .toCSV?
persons.otherHashCode?
persons.otherToString?

6. Why persons.toString?
persons.hashCode?
• part of the standard JVM library
package java.lang;
public class Object {
public native int hashCode();
public boolean equals(Object obj) { … }
public String toString() { … }
public final native void notify();
public final native void notifyAll();
public final native void wait(long timeout) throws InterruptedException;
public final void wait(long timeout, int nanos) throws InterruptedException { … }
public final void wait() throws InterruptedException { … }
}

7. Why persons.toString?
persons.hashCode?
• needed by Map, Set, serialization…
• mix technical concerns with domain model
• no liberty for further improvement
• ex: #2011-003 multiple implementations denial-of-
service via hash algorithm collision
A variety of programming languages suffer from a denial-of-service
(DoS) condition against storage functions of key/value pairs in hash
data structures, the condition can be leveraged by exploiting
predictable collisions in the underlying hashing algorithms.

8. a good toJson?
• Separated from domain model
• Flexible
• User can override library choices

9. persons to json
List
Person
String Int
listToJson
personToJson
stringToJson intToJson
trait toJson[A]

10. ToJson[A] strategy is a
parameter of toJson[A]
def toJson[A](a: A)(serializer: ToJson[A]): String =
serializer.serialize(a)
toJson("hello")(stringToJson)
toJson(3)(intToJson)
toJson(Person("bob", 34))(personToJson)
toJson(persons)(listToJson(personToJson))

11. find the right ToJson[A]
based on type
def toJson[A](a: A)(implicit serializer: ToJson[A]): String =
serializer.serialize(a)
toJson("hello")
toJson(3)
toJson(Person("bob", 34))
toJson(persons)

12. find the right ToJson[A]
based on type
toJson("hello")
// is different from
toJson(3)
$ scalac -Xprint:4 Test.scala
[…]
Test.this.toJson[String]("hello")(Test.this.stringToJson);
Test.this.toJson[Int](3)(Test.this.intToJson);

13. Type classes
• Define a contract (ex in ToJson[A]: (a: A) -> String)
• Define strategy for a tree of classes (like the domain model)
• Use type for find the suitable implementation
• Each implicit can be overridden / set explicitly => the user has the
power to override defaults
• Type classes resolved at compilation time
• Be sure that the strategy is defined for each class
• No runtime introspection necessary

14. In Play! framework
package play.api.libs.json
object Json {
def toJson[T](o: T)(implicit tjs: Writes[T]): JsValue = tjs.writes(o)
def fromJson[T](json: JsValue)(implicit fjs: Reads[T]): JsResult[T] =
fjs.reads(json)
}
/**
* Default Serializers.
*/
trait DefaultWrites {
implicit object IntWrites extends Writes[Int] {
def writes(o: Int) = JsNumber(o)
}
implicit object StringWrites extends Writes[String] {
def writes(o: String) = JsString(o)
}
implicit def traversableWrites[A: Writes] = new Writes[Traversable[A]] {
def writes(as: Traversable[A]) = JsArray(as.map(toJson(_)).toSeq)
}
}

15. In Play! framework
package play.api.libs.json
trait Format[A] extends Writes[A] with Reads[A]
/**
* Default Json formatters.
*/
trait DefaultFormat {
implicit def GenericFormat[T](implicit fjs: Reads[T], tjs: Writes[T]):
Format[T] = {
new Format[T] {
def reads(json: JsValue) = fjs.reads(json)
def writes(o: T) = tjs.writes(o)
}
}
}

16. implicitly syntax
def toJson[A](a: A)(implicit toJson: ToJson[A]): String =
toJson.serialize(a)
// can be written as:
def toJson[A : ToJson](a: A): String =
implicitly[ToJson[A]].serialize(a)
toJson("hello")
toJson(3)
toJson(Person("bob", 34))
toJson(persons)

17. persons.toJson
implicit class JsonOps[A](a: A)(implicit ser: ToJson[A]) {
def toJson: String = ser.serialize(a)
}
"hello".toJson
3.toJson
Person("bob", 34).toJson
persons.toJson

18. Questions?
• code on https://github.com/yanns/scala-type-classes-
demo
• more info:
• https://non.github.io/cats/typeclasses.html
• http://danielwestheide.com/blog/2013/02/06/the-
neophytes-guide-to-scala-part-12-type-classes.html