lager是一个erlang的日志库，详解介绍参考 。

在使用lager进行日志记录时， 只需要调用lager:Level/2,3即可(Level包括debug,info,notice,warning等等).例如:

lager:info("hello world").

在编译时使用lager写日志的模块都需要指定编译选项:

-compile([{parse_transform, lager_transform}]).

如果使用的是rebar， 也可以在配置文件中增加对应的参数, 而不需要在每个模块中都增加编译选项.

{erl_opts, [{parse_transform, lager_transform}]}.

上述都是使用的简单介绍，那么重点是在lager模块中找不到debug/2,debug/3,info/2等函数的实现。

实际上是erlang将编译源文件生成的Abstract code交由lager_transform模块的parse_transform函数再次进行处理，这个函数会将debug/2, debug/3这些函数直接转换为Abstract code。这也就是需要指定编译选项{parse_transform, lager_transform}的原因。

什么是Abstract code? 其格式是怎样的? 这里不多解释，直接翻看。

代码 lager:info("hello world"). 通过转换后的Abstract code为:

[{'case',34,           {tuple,34,            [{call,34,{atom,34,whereis},[{atom,34,lager_event}]},             {call,34,              {remote,34,{atom,34,lager_config},{atom,34,get}},              [{atom,34,loglevel},{tuple,34,[{integer,34,0},{nil,34}]}]}]},           [{clause,34,             [{tuple,34,[{atom,34,undefined},{var,34,'_'}]}],             [],             [{call,34,               {'fun',34,                {clauses,                 [{clause,34,[],[],                   [{tuple,34,                     [{atom,34,error},{atom,34,lager_not_running}]}]}]}},               []}]},            {clause,34,             [{tuple,34,               [{var,34,'__Pidtest34'},                {tuple,34,                 [{var,34,'__Leveltest34'},{var,34,'__Tracestest34'}]}]}],             [[{op,34,'orelse',                {op,34,'/=',                 {op,34,'band',{var,34,'__Leveltest34'},{integer,34,64}},                 {integer,34,0}},                {op,34,'/=',{var,34,'__Tracestest34'},{nil,34}}}]],             [{call,34,               {remote,34,{atom,34,lager},{atom,34,do_log}},               [{atom,34,info},                {cons,34,                 {tuple,34,[{atom,34,application},{atom,34,upu}]},                 {cons,34,                  {tuple,34,[{atom,34,module},{atom,34,test}]},                  {cons,34,                   {tuple,34,[{atom,34,function},{atom,34,start}]},                   {cons,34,                    {tuple,34,[{atom,34,line},{integer,34,34}]},                    {cons,34,                     {tuple,34,                      [{atom,34,pid},                       {call,34,                        {atom,34,pid_to_list},                        [{call,34,{atom,34,self},[]}]}]},                     {cons,34,                      {tuple,34,[{atom,34,node},{call,34,{atom,34,node},[]}]},                      {call,34,{remote,34,{atom,34,lager},{atom,34,md}},[]}}}}}}},                {string,34,"hello world"},                {atom,34,none},                {integer,34,4096},                {integer,34,64},                {var,34,'__Leveltest34'},                {var,34,'__Tracestest34'},                {var,34,'__Pidtest34'}]}]},            {clause,34,[{var,34,'_'}],[],[{atom,34,ok}]}]}]}]}

翻译成erlang代码为:

case {whereis(lager_event), lager_config:get(loglevel, {0,[]})} of    {undefined, _} ->        fun() ->            {error, lager_not_running}        end();    {__Pidtest15, {__Leveltest15, __Tracestest15}}        when (__Leveltest15 band 64) /= 0 orelse             __Tracetest /= [] ->            lager:do_log(info, [{application, App},                                {module, Module},                                {function, Function},                                {line, LINE},                                {pid, pid_to_list(self())},                                {node, node()} | lager:md()],                         "hello world",                         none, 4096, 64                         __Leveltest15, __Tracestest15, __Pidtest15);    _ ->        okend.

注: 上述代码中 App, Module, Function, Line分别表示实际应用的名称，模块的名称，函数的名称，所在行数。