An array can take elements of different types. Therefore, we can have, for example, arrays of the following types: Array{Int64,1}, Array{Int8,1}, Array{Float64,1}, or Array{String, 1}, and so on. That is why an Array is a parametric type; its elements can be of any arbitrary type T, written as Array{T, 1}.
In general, types can take type parameters, so that type declarations actually introduce a whole family of new types. Returning to the Point example of the previous section, we can generalize it to the following:
# see the code in Chapter 6\parametric.jl
mutable struct Point{T}
x::T
y::T
end
This abstract type creates a whole family of new possible concrete types (but they are only compiled as needed at runtime), such as Point{Int64}, Point{Float64}, and Point{String}.
These are all subtypes of Point: Point{String} <: Point returns true. However, this is not the case when...