if var.responds_to?(...)

如果 if 的条件是一个 responds_to? 测试，那么在 then 分支中，变量的类型保证会被限制为响应该方法的类型

if a.responds_to?(:abs)
  # here a's type will be reduced to those responding to the 'abs' method
end

此外，在 else 分支中，变量的类型保证会被限制为不响应该方法的类型

a = some_condition ? 1 : "hello"
# a : Int32 | String

if a.responds_to?(:abs)
  # here a will be Int32, since Int32#abs exists but String#abs doesn't
else
  # here a will be String
end

以上操作不适用于实例变量或类变量。要操作它们，请先将它们分配给一个变量

if @a.responds_to?(:abs)
  # here @a is not guaranteed to respond to `abs`
end

a = @a
if a.responds_to?(:abs)
  # here a is guaranteed to respond to `abs`
end

# A bit shorter:
if (a = @a).responds_to?(:abs)
  # here a is guaranteed to respond to `abs`
end