callable

Callable module

A callable object in Ruby is one that implements the #call method and can be invoked using the .(...) syntax, e.g.:

class Adder
  def self.call(x, y)
    x + y
  end
end

Adder.(2, 3) #= 5

Alternatively, an object might implement a #to_proc so it could be passed to arbitrary methods as a block:

class Square
  def self.to_proc
    proc { |x| x * X }
  end
end

[1, 2, 3].map(&Square) #=> [1, 4, 9]

In the present example, the imported module implements both #call and #to_proc and the application makes use of both to calculate factorials.