Erlang的常驻模块与功能模块
Residence module
The module where a process has its tail-recursive loop function(s).
If the tail-recursive loop functions are implemented in several modules,
all those modules are residence modules for the process.
Functional module
A module which is not a residence module for any process.
Note that for a process implemented using an OTP behaviour,
the behaviour module is the residence module for that process.
The callback module is a functional module.
常驻模块是指loop()函数所在模块，其他模块为功能模块。
对于OTP行为，行为模块是常驻模块，回调模块是功能模块。
行为模块已由OTP实现。