格式一:
var variable_name [type] = [type] value
==> the second type we can ignore for the basic type, like int float32 string
==> the first type we can ignore, but make sure the compiler can get the correct type for the value
格式二:
variable_name := value
===> complier can get the type of the value and assgine the value to variable_name.
===> it includes declaration and assigne
