Lexical and dynamic scoping in Ruby

gistfile1.rb

# Within the context of a single instance, instance variables have dynamic
# scoping semantics.  But local variables do not; they are lexically scoped.

class DynamicScopingExample
  def some_method
    @x = :foo
    some_other_method
  end
  def some_other_method
    @x
  end
end

p DynamicScopingExample.new.some_method
# prints :foo



class LexicalScopingExample
  def some_method
    x = :foo
    some_other_method
  end
  def some_other_method
    x
  end
end

p LexicalScopingExample.new.some_method
# raises undefined local variable or method `x' for #<LexicalScopingExample:0x100565ff8> (NameError)

What you cite as dynamic scope isn't dynamic scope. It's an assignment, a side-effect, that will live on after the scope ends. Here's proof: if this were true dyamic scope (or, for that matter, lexical scope), then calling DynamicScopingExample.new.some_method would return :foo. However, it returns :bar even though some_other_method scope is long gone.

class DynamicScopingExample
  def some_method
    @x = :foo
    some_other_method
    @x
  end
  def some_other_method
    @x = :bar
  end
end