Julia Language Immutable Types


The simplest composite type is an immutable type. Instances of immutable types, like tuples, are values. Their fields cannot be changed after they are created. In many ways, an immutable type is like a Tuple with names for the type itself and for each field.

Singleton types

Composite types, by definition, contain a number of simpler types. In Julia, this number can be zero; that is, an immutable type is allowed to contain no fields. This is comparable to the empty tuple ().

Why might this be useful? Such immutable types are known as "singleton types", as only one instance of them could ever exist. The values of such types are known as "singleton values". The standard library Base contains many such singleton types. Here is a brief list:

  • Void, the type of nothing. We can verify that Void.instance (which is special syntax for retrieving the singleton value of a singleton type) is indeed nothing.
  • Any media type, such as MIME"text/plain", is a singleton type with a single instance, MIME("text/plain").
  • The Irrational{:π}, Irrational{:e}, Irrational{:φ}, and similar types are singleton types, and their singleton instances are the irrational values π = 3.1415926535897..., etc.
  • The iterator size traits Base.HasLength, Base.HasShape, Base.IsInfinite, and Base.SizeUnknown are all singleton types.
  • In version 0.5 and later, each function is a singleton instance of a singleton type! Like any other singleton value, we can recover the function sin, for example, from typeof(sin).instance.

Because they contain nothing, singleton types are incredibly lightweight, and they can frequently be optimized away by the compiler to have no runtime overhead. Thus, they are perfect for traits, special tag values, and for things like functions that one would like to specialize on.

To define a singleton type,

julia> immutable MySingleton end

To define custom printing for the singleton type,

julia> Base.show(io::IO, ::MySingleton) = print(io, "sing")

To access the singleton instance,

julia> MySingleton.instance

Often, one assigns this to a constant:

julia> const sing = MySingleton.instance

Wrapper types

If zero-field immutable types are interesting and useful, then perhaps one-field immutable types are even more useful. Such types are commonly called "wrapper types" because they wrap some underlying data, providing an alternative interface to said data. An example of a wrapper type in Base is String. We will define a similar type to String, named MyString. This type will be backed by a vector (one-dimensional array) of bytes (UInt8).

First, the type definition itself and some customized showing:

immutable MyString <: AbstractString

function Base.show(io::IO, s::MyString)
    print(io, "MyString: ")
    write(io, s.data)

Now our MyString type is ready for use! We can feed it some raw UTF-8 data, and it displays as we like it to:

julia> MyString([0x48,0x65,0x6c,0x6c,0x6f,0x2c,0x20,0x57,0x6f,0x72,0x6c,0x64,0x21])
MyString: Hello, World!

Obviously, this string type needs a lot of work before it becomes as usable as the Base.String type.

True composite types

Perhaps most commonly, many immutable types contain more than one field. An example is the standard library Rational{T} type, which contains two fieds: a num field for the numerator, and a den field for the denominator. It is fairly straightforward to emulate this type design:

immutable MyRational{T}
    MyRational(n, d) = (g = gcd(n, d); new(n÷g, d÷g))
MyRational{T}(n::T, d::T) = MyRational{T}(n, d)

We have successfully implemented a constructor that simplifies our rational numbers:

julia> MyRational(10, 6)