除非

unless 在其条件为假值时评估 then 分支，如果存在，则评估else 分支。也就是说，它的行为与if相反。

unless some_condition
  expression_when_falsey
else
  expression_when_truthy
end

# The above is the same as:
if some_condition
  expression_when_truthy
else
  expression_when_falsey
end

# Can also be written as a suffix
close_door unless door_closed?