defiling science

slapdash.rb

class AttributeError < RuntimeError; end
def foo
  return @foo if defined?(@foo)
  raise AttributeError
end

foo

@foo = :foo

foo

foo = nil

foo

#[1] pry(main)> class AttributeError < RuntimeError; end
#=> nil
#[2] pry(main)> def foo
#[2] pry(main)*   return @foo if defined?(@foo)  
#[2] pry(main)*     raise AttributeError  
#[2] pry(main)*   end  
#=> nil
#[3] pry(main)> 
#[4] pry(main)> foo
#AttributeError: AttributeError
#from (pry):4:in `foo'
#[5] pry(main)> 
#[6] pry(main)> @foo = :foo
#=> :foo
#[7] pry(main)> 
#[8] pry(main)> foo
#=> :foo
#[9] pry(main)>
#[10] pry(main)> foo = nil
#=> nil
#[11] pry(main)> foo
#=> nil