gured和if的区别

// guard
var a: String? = "ddd"
guard let b = a else { print("a is nil") }
print(b) //注意解包之后b的作用域仍然有效

//if
var a: String? = "ddd"
if let b = a {
    print(b) //解包之后b的只有在 { ... } 内才有作用域
}