if !

! 运算符返回一个 Bool，该 Bool 是对一个值的 真值 取反的结果。

当在 if 中与变量、is_a?、responds_to? 或 nil? 结合使用时，编译器将相应地限制类型

a = some_condition ? nil : 3
if !a
  # here a is Nil because a is falsey in this branch
else
  # here a is Int32, because a is truthy in this branch
end

b = some_condition ? 1 : "x"
if !b.is_a?(Int32)
  # here b is String because it's not an Int32
end