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#scala 
trait Buffer {
  type T
  val element: T
}

Abstract types are types whose identity is not precisely known. In the example above, we only know that each object of class Buffer has a type member T, but the definition of class Buffer does not reveal to what concrete type the member type T corresponds. Like value definitions, we can override type definitions in subclasses. This allows us to reveal more information about an abstract type by tightening the type bound (which describes possible concrete instantiations of the abstract type).

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Abstract Types - Scala Documentation
s; types along with values are members of objects. Furthermore, both forms of members can be concrete and abstract. Here is an example which defines both a deferred value definition and an abstract type definition as members of class Buffer. <span>trait Buffer { type T val element: T } Abstract types are types whose identity is not precisely known. In the example above, we only know that each object of class Buffer has a type member T, but the definition of class Buffer does not reveal to what concrete type the member type T corresponds. Like value definitions, we can override type definitions in subclasses. This allows us to reveal more information about an abstract type by tightening the type bound (which describes possible concrete instantiations of the abstract type). In the following program we derive a class SeqBuffer which allows us to store only sequences in the buffer by stating that type T has to be a subtype of Seq[U] for a new abstract type U:


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