Design Patterns and Scala

Question

I'm writing this question to maintain a register of design patterns associated with Scala, standards patterns or only from this language.

Associated questions:

Scala Catalog of functional Design Patterns

Thanks to all who contribute

Answer 1

Let's start with the "Singleton pattern":

object SomeSingleton //That's it

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I would additionally propose the "Using-functions-of-higher-order pattern". Instead of e. g. iterating through a collection by yourself you supply functions to the methods the classes provide.

In Scala you basically say what you intend to do:

//declare some example class
case class Person(name: String, age: Int) 

//create some example persons
val persons = List(Person("Joe", 42), Person("Jane", 30), Person("Alice", 14), Person("Bob", 12))

//"Are there any persons in this List, which are older than 18?"
persons.exists(_.age > 18)
// => Boolean = true

//"Is every person's name longer than 4 characters?"
persons.forall(_.name.length > 4)
// => Boolean = false

//"I need a List of only the adult persons!"
persons.filter(_.age >= 18)
// => List[Person] = List(Person(Joe,42), Person(Jane,30))

//"Actually I need both, a list with the adults and a list of the minors!"
persons.partition(_.age >= 18)
// => (List[Person], List[Person]) = (List(Person(Joe,42), Person(Jane,30)),List(Person(Alice,14), Person(Bob,12)))

//"A List with the names, please!"
persons.map(_.name)
// => List[String] = List(Joe, Jane, Alice, Bob)    

//"I would like to know how old all persons are all together!"
persons.foldLeft(0)(_ + _.age)
// => Int = 98

Doing this in Java would have meant touching the elements of a collection yourself and mix your application logic with flow control code.

More information about the Collection classes.

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This nice EPFL paper about Deprecating the Observer Pattern might be of interest, too.

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Typeclasses are one approach to structure common features of classes where inheritance doesn't really fit.

Answer 2

A list like this has been collated already. See http://scala.sygneca.com/patterns/start