This document discusses different approaches for implementing polymorphism in Scala without subclasses. It begins by introducing type classes as a way to add ad-hoc polymorphism. It then provides examples of implementing serialization of case classes using method overloading, subclasses, traits, and type classes. Type classes are highlighted as having advantages like being easily extensible and separating serialization logic from data models.

1. TYPE CLASSES
Ad-Hoc Polymorphismus ohne Unterklassen
Scala User Group Köln/Bonn
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2. POLYMORPHISMUS
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3. POLYMORPHISMUS
• Parametrisiert
• Generische Funktionen und Datentypen
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4. POLYMORPHISMUS
• Parametrisiert
• Generische Funktionen und Datentypen
• Ad-hoc
• Methoden- und Operator-Overloading
• Unterklassen (Subtype Polymorphismus)
• Type Classes
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5. CASE CLASSES SERIALISIEREN
package model {
case class Person(firstName: String, lastName: String)
case class Restaurant(name: String, address: String)
}
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6. CASE CLASSES SERIALISIEREN
package model {
case class Person(firstName: String, lastName: String)
case class Restaurant(name: String, address: String)
}
Neue
JSON CSV XML
Funktion
Person toJson toCsv toXml
Restaurant toJson toCsv toXml
Neuer Typ
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7. ÜBERLADEN
1. object JsonSerializer {
2. def toJson(person: Person): String = """{ name : "%s %s" }""".format(person.firstName, person.lastName)
3. def toJson(r: Restaurant) = """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
4. }
5.
6. object CsvSerializer {
7. def toCSV(person: Person): String = "%s,%s".format(person.firstName, person.lastName)
8. def toCSV(r: Restaurant): String = "%s,%s".format(r.name, r.address)
9. }
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8. ÜBERLADEN
1. object JsonSerializer {
2. def toJson(person: Person): String = """{ name : "%s %s" }""".format(person.firstName, person.lastName)
3. def toJson(r: Restaurant) = """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
4. }
5.
6. object CsvSerializer {
7. def toCSV(person: Person): String = "%s,%s".format(person.firstName, person.lastName)
8. def toCSV(r: Restaurant): String = "%s,%s".format(r.name, r.address)
9. }
+Einfach
- Kein Zusammenhang außer Methodenname
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9. UNTERKLASSEN
1. abstract class Serializer {
2. def toJson: String
3. def toCSV: String
4. }
5.
6. case class Person(firstName: String, lastName: String) extends Serializer {
7. override def toJson: String = """{ name : "%s %s" }""".format(firstName, lastName)
8. override def toCSV: String = "%s,%s".format(firstName, lastName)
9. }
10.
11. case class Restaurant(name: String, address: String) extends Serializer {
12. override def toJson(r: Restaurant) = """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
13. override def toCSV(r: Restaurant): String = "%s,%s".format(r.name, r.address)
14. }
1. import models._
2. Person("Max", "Mustermann").toJson
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10. UNTERKLASSEN
1. abstract class Serializer {
2. def toJson: String
3. def toCSV: String
4. }
5.
6. case class Person(firstName: String, lastName: String) extends Serializer {
7. override def toJson: String = """{ name : "%s %s" }""".format(firstName, lastName)
8. override def toCSV: String = "%s,%s".format(firstName, lastName)
9. }
10.
11. case class Restaurant(name: String, address: String) extends Serializer {
12. override def toJson(r: Restaurant) = """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
13. override def toCSV(r: Restaurant): String = "%s,%s".format(r.name, r.address)
14. }
1. import models._
2. Person("Max", "Mustermann").toJson
+ Einfach neue Klassen hinzuzufügen
- Modellklassen müssen von Serializer ableiten
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11. TRAITS
1. trait JsonSerializer {
2. def toJson: String
3. }
4.
5. trait CsvSerializer {
6. def toCSV: String
7. }
8.
9. case class Person(firstName: String, lastName: String) extends JsonSerializer with CsvSerializer {
10. override def toJson: String = """{ name : "%s %s" }""".format(firstName, lastName)
11. override def toCSV: String = "%s,%s".format(firstName, lastName)
12. }
13.
14. case class Restaurant(name: String, address: String) extends JsonSerializer with CsvSerializer {
15. override def toJson(r: Restaurant) = """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
16. override def toCSV(r: Restaurant): String = "%s,%s".format(r.name, r.address)
17. }
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12. TRAITS
1. trait JsonSerializer {
2. def toJson: String
3. }
4.
5. trait CsvSerializer {
6. def toCSV: String
7. }
8.
9. case class Person(firstName: String, lastName: String) extends JsonSerializer with CsvSerializer {
10. override def toJson: String = """{ name : "%s %s" }""".format(firstName, lastName)
11. override def toCSV: String = "%s,%s".format(firstName, lastName)
12. }
13.
14. case class Restaurant(name: String, address: String) extends JsonSerializer with CsvSerializer {
15. override def toJson(r: Restaurant) = """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
16. override def toCSV(r: Restaurant): String = "%s,%s".format(r.name, r.address)
17. }
- Serialisieren hat nichts mit der Logik zu tun,
Code steht nicht immer zur Verfügung
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13. TYPE CASING
1. object JsonSerializer {
2. def toJson(x: AnyRef): String = x match {
3. case person: Person => """{ name : "%s %s" }""".format(person.firstName, person.lastName)
4. case r: Restaurant => """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
5. }
6.
7. object CsvSerializer {
8. def toCSV(x: AnyRef): String = x match {
9. case person: Person => "%s,%s".format(person.firstName, person.lastName)
10. case r: Restaurant => "%s,%s".format(r.name, r.address)
11. }
12. }
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14. TYPE CASING
1. object JsonSerializer {
2. def toJson(x: AnyRef): String = x match {
3. case person: Person => """{ name : "%s %s" }""".format(person.firstName, person.lastName)
4. case r: Restaurant => """{ name : "%s"; address: "%s" }""".format(r.name, r.address)
5. }
6.
7. object CsvSerializer {
8. def toCSV(x: AnyRef): String = x match {
9. case person: Person => "%s,%s".format(person.firstName, person.lastName)
10. case r: Restaurant => "%s,%s".format(r.name, r.address)
11. }
12. }
+ Einfach neue Funktionen hinzuzufügen
- Viele Eingriffe bei neuem Typ
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15. TYPE CLASSES (1)
1. package serializer
2.
3. trait CsvSerializable[T] {
4. def toCsv(t: T): String
5. }
6.
7. object CsvSerializable {
8. import models._
9.
10. def toCsv[T](t: T)(implicit serializer: CsvSerializable[T]): String =
11. serializer.toCsv(t)
12.
13. implicit object UserSerializable extends CsvSerializable[Person] {
14. def toCsv(person: User) String =
15. "%s,%s".format(person.firstName, person.lastName)
16. }
17. }
1. import serializer._
2. val user = User(name = "Max Musterbro")
3. val csvHelper = implicitly[CsvSerializable[User]]
4. csvHelper.toCsv(user)
5.
6. // Oder
7. import serializer.CsvSerializable._
8. toCsv(User(name = "Max Musterbro"))
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16. TYPE CLASSES (2)
1. trait JsonSerializable[T] {
2. def toJson(t: T): String
3. }
4.
5. object JsonSerializable {
6. import models._
7.
8. implicit object UserSerializable extends JsonSerializable[Person] {
9. def toJson(person: Person): String =
10. """{ name : "%s", status: "%s" }""".format(person.firstName, person.lastName)
11. }
12.
13. class UserSerializer(p: Person) {
14. def toJson: String = UserSerializable.toJson(p)
15. }
16.
17. implicit def UserToSerializer(p: Person) = new UserSerializer(p)
1. import serializer.JsonSerializable._
2. User(name = "Max Musterbro").toJson
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17. TYPE CLASSES (3)
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18. TYPE CLASSES (3)
• Definition
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19. TYPE CLASSES (3)
• Definition
• Trait mit Parameter
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20. TYPE CLASSES (3)
• Definition
• Trait mit Parameter
• Companion Object mit Default Implementationen
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21. TYPE CLASSES (3)
• Definition
• Trait mit Parameter
• Companion Object mit Default Implementationen
• Beispiele in Scala: scala.math.Ordering und scala.math.Numeric
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22. TYPE CLASSES (3)
• Definition
• Trait mit Parameter
• Companion Object mit Default Implementationen
• Beispiele in Scala: scala.math.Ordering und scala.math.Numeric
•+ Gut erweiterbar
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23. TYPE CLASSES (3)
• Definition
• Trait mit Parameter
• Companion Object mit Default Implementationen
• Beispiele in Scala: scala.math.Ordering und scala.math.Numeric
•+ Gut erweiterbar
• - Viel Code, implicits manchmal undurchsichtig
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24. > CODE
• https://twitter.com/scalacgn
• http://xing.to/scala
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