lager与lager_transform

lager是一个erlang的日志库，详解介绍参考 这里。

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，直接翻看官方文档。

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);
    _ ->
        ok
end.

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。