Container types

We've seen Scala's class hierarchy, hence we are aware of many collection types such as List, Set, and Map. What's different about these types along with types such as Option and Either, is that they all expect you to provide a type and then instantiate. We call List as a container type because it works that way. We use a list to contain elements of a certain data type. Similarly, we can think of an Option as a binary containerized type, as Option can be some value or None. The Either type goes the same way. In Scala, when we create such container types, we tend to use a type parameter to declare and provide a concrete type, such as String,Int, Boolean, and so on when we instantiate. Take a look how Option is declared in Scala (more on Option and Either types in the next chapter):

sealed abstract class Option[+A] extends Product  
   with Serializable 

It takes a type parameter A. It's possible to provide more than one type parameter if your type expects more than one type...