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Scala Extractors


Tuple Extractors

x and y are extracted from the tuple:

val (x, y) = (1337, 42)
// x: Int = 1337
// y: Int = 42

To ignore a value use _:

val (_, y: Int) = (1337, 42)
// y: Int = 42

To unpack an extractor:

val myTuple = (1337, 42)
myTuple._1 // res0: Int = 1337
myTuple._2 // res1: Int = 42

Note that tuples have a maximum length of 22, and thus ._1 through ._22 will work (assuming the tuple is at least that size).

Tuple extractors may be used to provide symbolic arguments for literal functions:

val persons = List("A." -> "Lovelace", "G." -> "Hopper")
val names = List("Lovelace, A.", "Hopper, G.")
assert {
 names ==
 (persons map { name =>
 s"${name._2}, ${name._1}"
assert {
 names ==
 (persons map { case (given, surname) =>
 s"$surname, $given"

Case Class Extractors

A case class is a class with a lot of standard boilerplate code automatically included. One benefit of this is that Scala makes it easy to use extractors with case classes.

case class Person(name: String, age: Int) // Define the case class
val p = Person("Paola", 42) // Instantiate a value with the case class type
val Person(n, a) = p // Extract values n and a
// n: String = Paola
// a: Int = 42

At this juncture, both n and a are vals in the program and can be accessed as such: they are said to have been 'extracted' from p. Continuing:

val p2 = Person("Angela", 1337)
val List(Person(n1, a1), Person(_, a2)) = List(p, p2)
// n1: String = Paola
// a1: Int = 42
// a2: Int = 1337

Here we see two important things:

• Extraction can happen at 'deep' levels: properties of nested objects can be extracted.

•Not all elements need to be extracted. The wildcard _ character indicates that that particular value can be anything, and is ignored. No val is created.

In particular, this can make matching over collections easy:

val ls = List(p1, p2, p3) // List of Person objects => person match {
 case Person(n, a) => println("%s is %d years old".format(n, a))

Here, we have code that uses the extractor to explicitly check that person is a Person object and immediately pull out the variables that we care about: n and a.


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