Class variables

Class variables are associated to classes instead of instances. They are prefixed with two "at" signs (@@). For example:

class Counter
  @@instances = 0

  def initialize
    @@instances += 1
  end

  def self.instances
    @@instances
  end
end

Counter.instances # => 0
Counter.new
Counter.new
Counter.new
Counter.instances # => 3

Class variables can be read and written from class methods or instance methods.

When initialized with a default value, the type inference algorithm can often implicitly determine the type of the variable.

module InferredTypes
  @@integer = 1 # : Int32
  @@string = "" # : String
end

It won't be able to infer the type of complex expressions involving methods calls, for example. In these cases, the variable needs to be typed explicitly.

def generate_foo
  1
end

module NonInferableType
  @@untyped = generate_foo() # Error: can't infer the type of class variable '@@foo' of NonInferableTypes
  @@typed : Int32 = generate_foo()
end

Class variables are inherited by subclasses with this meaning: their type is the same, but each class has a different runtime value. For example:

class Parent
  @@numbers = [] of Int32

  def self.numbers
    @@numbers
  end
end

class Child < Parent
end

Parent.numbers # => []
Child.numbers  # => []

Parent.numbers << 1
Parent.numbers # => [1]
Child.numbers  # => []

Class variables can also be associated to modules and structs. Like above, they are inherited by including/subclassing types.