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In scala Option class is declared like
sealed abstract class _Option[+A]
case object _None extends _Option[Nothing] {}
final case class _Some[+A](x: A) extends _Option[A] {}
What is [+A]? Why not just [A]? Could it be [-A] and what it would mean?
Sorry if it is a duplicate but I couldn't find the answer on SO.
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=> is syntactic sugar for creating instances of functions. Recall that every function in scala is an instance of a class. For example, the type Int => String , is equivalent to the type Function1[Int,String] i.e. a function that takes an argument of type Int and returns a String .
It declares the class to be covariant in its generic parameter. For your example, it means that Option[T] is a subtype of Option[S] if T is a subtype of S . So, for example, Option[String] is a subtype of Option[Object] , allowing you to do: val x: Option[String] = Some("a") val y: Option[Object] = x.
It declares the class to be covariant in its generic parameter. For your example, it means that Option[T] is a subtype of Option[S] if T is a subtype of S. So, for example, Option[String] is a subtype of Option[Object], allowing you to do:
val x: Option[String] = Some("a")
val y: Option[Object] = x
Conversely, a class can be contravariant in its generic parameter if it is declared as -A.
Read above variances in Scala in the docs here.
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