An array can take elements of different types, so, we can have, for example, arrays of the following types: Array{Int64,1}, Array{Int8,1}, Array{Float64,1}, or Array{ASCIIString, 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
type Point{T}
  x::T
  y::T
end

(This is conceptually similar to generic types in Java or templates in C++).

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: issubtype(Point{String}, Point) that return true. However, this is not the case when comparing...