In this hierarchy, some types, such as Number, Integer, and Signed, are abstract, which means that they have no concrete objects or values of their own. Instead, objects or values are of concrete types given by the result of applying typeof(value), such as Int8, Float64, and String. For example, the concrete type of the value 5 is Int64 given by typeof(5) (on a 64-bit machine). However, a value also has the type of all of its supertypes; for example, isa(5, Number) returns true (we introduced the isa function in the Types section of Chapter 2, Variables, Types, and Operations).
Concrete types have no subtypes and might only have abstract types as their supertypes. Schematically, we can differentiate them as follows: ...
