= Julia Data Types =

While Julia is a dynamically typed language, values have a concrete type.

<<TableOfContents>>

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== Abstract Types ==

Abstract types should not be used directly. They are supertypes that categorize behaviors. They are however useful for constructing a novel data type, or for typing the arguments to a function.

The primary set of abstract types, with their hierarchy of supertypes and subtypes indicated by indentation, is:
 * `Number`
   * `Real`
     * `AbstractFloat`
     * `Integer`
       * `Signed`
       * `Unsigned`
 * `AbstractChar`
 * `AbstractString`
 * `AbstractArray`
   * `AbstractVector`
   * `AbstractMatrix`
   * `AbstractRange`

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== Numeric Types ==

The set of primitive types that descend from the `Number` abstract type are:
 * `Bool`
 * `Int8`
 * `Int16`
 * `Int32`
 * `Int64`
 * `Int128`
 * `UInt8`
 * `UInt16`
 * `UInt32`
 * `UInt64`
 * `UInt128`
 * `Float16`
 * `Float32`
 * `Float64`

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== String Types ==

`String` is a subtype of `AbstractString` that implicitly is encoded in UTF-8.

Packages may create novel subtypes of `AbstractString`, so consider typing string arguments for functions as `AbstractString` rather than `String`.

`Char` is a subtype of `AbstractChar`, and is a 32-bit representation a Unicode character.

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== Composite Types ==

Structs are a composite data type. Struct values can be instantiated like:

{{{
julia> struct Foo
           bar
           baz
           qux
       end

julia> foo = Foo("Hello, world.", 23, 1.5)
Foo("Hello, world.", 23, 1.5)
}}}

Fields are accessed by the dot operator.

{{{
julia> foo.bar
"Hello, world."
}}}

By default, struct objects are immutable.

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== Collection Types ==

Values can be collected in a variety of series types.

The `Array` type is a general purpose series of values, all of the same type. Both [[Julia/Vectors|Vector]] and [[Julia/Matrices|Matrix]] types descend from `AbstractArray`.

The range operator (`:`) creates collections that have some type descending from `AbstractRange`, which also descends from `AbstractArray`. The differences between e.g. `UnitRange` and `OrdinalRange` relate to the available optimizations.

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== Nothing Type ===

The singleton constant `nothing` is of type `Nothing`.



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