Ways to create a type

There are four ways to define a type in Scala:

• By using a literal to define a singleton type
• By using the type keyword to define an alias for an abstract or concrete type 
• By defining a class, object, or trait to create a concrete type
• By defining a method that creates a method type

Singleton types were introduced in Scala 2.13, and we'll look at them in detail later in this chapter. For now, let's try to define and refer to some concrete types in Scala REPL:

scala> class SomeClass
defined class SomeClass
scala> object SomeObject
defined object SomeObject
scala> :type SomeObject
SomeObject.type
scala> :type new SomeClass
SomeClass
scala> trait SomeTrait {
     | def usage(a: SomeClass, b: SomeObject.type): SomeTrait
     | }
defined trait SomeTrait

The type can be referred before it is fully defined, as shown by the example of SomeTrait.

When annotating types, traits and classes can be used directly, but the type of an object needs to be referenced by using its type operator. The a.type form in Scala describes a singleton type. Depending upon whether p conforms to scala.AnyRef, it denotes either a set of values, [a, null], or just an a. As we usually don't use null in Scala programs, we can say that a.type denotes a type containing a single element, a. Normally, it is not used in regular code because it is easier to reference an object directly than to pass it as a parameter, but this style has found its use in some advanced libraries to implement parts of the internal DSL. 

In Scala 2.13, there is a new marker trait Singleton that can be applied as an upper bound on a type parameter, which indicates that the singleton type should be inferred for this parameter:

scala> def singleIn[T <: Singleton](t: T): T = t
singleIn: [T <: Singleton](t: T)T

scala> final val t = singleIn(42)
t: 42 = 42

A method type does not denote a value, nor does it appear directly in the program. It is an internal representation of a method definition. It is represented as a sequence of parameter names with respective types and a return type of the method. The method type is important into know about because, if a method name is used as a value, its type is implicitly converted to the corresponding function type. As we defined in the usage method, the compiler internally created a method type called (a: SomeClass, b: SomeObject.type)SomeTrait.