Explain use of abstract types

[BSchilperoort]

In the section on types and multiple dispatch, abstract types are mentioned as a way to implement a class-like abstraction (if you really want to do oop-like programming).

However, it does not explain the actual use of abstract types in Julia, such as;

• default implementations for concrete types
• parametric types in structs that allow for different backends (e.g. GPU arrays and Vec by using DenseArray).