系统技术非业余研究

在Erlang-china的邮件列表上看到这样的问题:

我的服务经常发生这样的错误,举例:
Error in process <0.33.0> with exit value: {badarg,[{erlang,’++’,[undefined,[{“37”}]]},{groups,doWork,1},
{groups,doWork,1},{groups,manage_clients,1}]}

大意明白,但问题是我使用匹配机制时没考虑到多个函数”doWork/1″出错无法定位到其中一个,这该如何是好?
Erlang是否会像其它语言一样提示某一行出错?

这个问题确实很常见, Erlang的运行期没有给出出错的具体行数, 这给我们定位问题带来了很大的麻烦.

有先驱给出了这样的解决方案 http://mryufeng.javaeye.com/blog/368507 但是这个模块已经很老了, 过时不维护了.

这里我给出另外一个方案, 利用erlang现有的模块来实现的: cover + dbg

cover的工作原理可以参考这篇文章 http://mryufeng.javaeye.com/blog/482204.

原理就是cover编译过的模块会在每行执行前, 先执行ets:update_counter(cover_internal_data_table,{bump,Mod,Fun,1,1,Line},1) 来更新模块某行的执行次数.
那么我们只要截取 ets:update_counter这个动作, 我们就知道改模块最后的执行行, 也就是异常所在的行.

Ok, 原理介绍完毕, 上菜.

[root@centos ~]# cat line.erl

-module(line).
-export([dbg/1]).
-include_lib("stdlib/include/ms_transform.hrl").

dbg(Mod)->
    cover:compile(Mod),
    dbg:tracer(),
    dbg:p(all, [call]),
    dbg:tpl(ets,
            update_counter,
            dbg:fun2ms(fun([_,{bump,Mod,_,_,_,_},1]) ->
                               return_trace()
                       end)),
    ok.

[root@centos ~]# cat hello.erl

-module(hello).
-export([start/0]).

start()->
    a=a,
    A=2,
    C=3,
    A=C-1,
    C=A+1,
    io:format("hello world~n",[]),
    test(C),
    ok.


test(C)->
    A=4,
    A=C,  % Error is on this line.
    ok.

我们可以看到这个hello模块会在hello:test发生异常, A=C这个地方是具体位置. 现在让我们找到行号:

[root@centos ~]# erl
Erlang R13B02 (erts-5.7.3) [source] [64-bit] [smp:2:2] [rq:2] [async-threads:0] [hipe] [kernel-poll:false]

Eshell V5.7.3  (abort with ^G)
1> line:dbg(hello).
ok
2> hello:start().
hello world
** exception error: no match of right hand side value 3
     in function  hello:test/1
     in call from hello:start/0
4> (<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,5},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,6},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,7},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,8},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,9},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,10},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,start,0,1,11},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,test,1,1,16},1)
(<0.34.0>) returned from ets:update_counter/3 -> 1
(<0.34.0>) call ets:update_counter(cover_internal_data_table,{bump,hello,test,1,1,17},1)   %这里我们看到出错的行号
(<0.34.0>) returned from ets:update_counter/3 -> 1
3> 
BREAK: (a)bort (c)ontinue (p)roc info (i)nfo (l)oaded
       (v)ersion (k)ill (D)b-tables (d)istribution

我们可以看到最后一次执行hello模块的行数是17.

Bingo!

原文地址：http://hyperstruct.net/2007/6/26/literal-xml-in-erlang-with-parse-transform-2

One of the things I dislike about Erlang is that it severely impairs bragging opportunities. Yesterday I wrote a module that allows writing literal XML in the source and have it parsed into Erlang structures at compile time—sort of like E4X minus the manipulation goodies at runtime (at least for now).

You write:

Doc = '<greeting>Hello!</greeting>',
io:format("~p~n", [Doc]).

And it prints…

{xmlElement,greeting,
            greeting,
            [],
            {xmlNamespace,[],[]},
            [],
            1,
            [],
            [{xmlText,[{greeting,1}],1,[],"Hello!",text}],
            [],
            "/tmp",
            undeclared}

In most languages I’m familiar with, this would have granted the author instant Yacc-demigod status. With Erlang… it was less than 40 LOC. Hardly something you’d wear at a party.

Anyway, this code owes everything to Philip’s writings. It also uses parse_transform/2, and “programmers are strongly advised not to engage in parse transformations and no support is offered for problems encountered”. So unless you, like me, are still at the kid-in-a-candy-shop stage of Erlang experience, think twice before using this in production, ok?

The code is here http://repo.hyperstruct.net/inline_xml/