Scala Language Extractors Case Class Extractors
Example
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. Novalis created.
In particular, this can make matching over collections easy:
val ls = List(p1, p2, p3) // List of Person objects
ls.map(person => 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.
