As a simple rule, beginner Scala programmers should try to omit type declarations which seem to be easy to deduce from the context, and see if the compiler agrees.

• 对于初学者，可以遵循这样一条原则：当上下文看起来比较容易推导出数据类型时，就应该忽略类型声明，并尝试是否能够编译通过，不行就修改。