The type hierarchy – subtypes and supertypes

In Julia, every value has a type, for example, typeof(2) is Int64 (or Int32 on 32-bit systems). Julia has a lot of built-in types, in fact, a whole hierarchy starting from the type Any at the top. Every type in this structure also has a type, namely, DataType, so it is very consistent. typeof(Any), typeof(Int64), typeof(Complex{Int64}), and typeof(DataType) all return DataType. So, types in Julia are also objects; all concrete types, except tuple types, which are a tuple of the types of its arguments, are of type DataType.

This type hierarchy is like a tree; each type has one parent given by the supertype function:

• supertype(Int64) returns Signed
• supertype(Signed) returns Integer
• supertype(Integer) returns Real
• supertype(Real) returns Number
• supertype(Number) returns Any
• supertype(Any) returns Any

A type can have a lot of children or subtypes (a function from the InteractiveUtils package) as follows:

• subtypes(Integer) form 3-element Array{Any,1}, which contains Bool, Signed, and Unsigned
• subtypes(Signed) form 6-element Array{Any,1}, which contains BigInt, Int128, Int16, Int32, Int64, and Int8
• subtypes(Int64) is 0-element Array{Any,1}, which has no subtypes

To indicate the subtype relationship, the operator < is used: Bool <: Integer and Bool <: Any returns true, while Bool <: Char is false. The following is a visualization of part of this type tree: