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)

Are instance variables actually lexically scoped or do they just appear that way because the calling environment is the same as the defining environment?

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