[Erlang 0111] Erlang Abstract Format , Part 2

   上回书,我们说到飞天玉虎蒋伯芳来到蜈蚣岭,不是,重来,上回咱们说到可以在Erlang Shell里面手工构造,加载并调用一个模块.在那个demo里面,我把多个Form单独生成出来,最后放在一起做compile:forms,是不是可以简单点?我们先看完整的module代码,erl_scan:string之后是什么样子的:

 

erl_syntax

 

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Eshell V5.10.2  (abort with ^G) 1> Code = "-module(t).\n-export([say/0]).\n\nsay() -> \"Hello world!!\". ". "-module(t).\n-export([say/0]).\n\nsay() -> \"Hello world!!\". " 2> {ok,Tokens,_} =erl_scan:string(Code). {ok,[{‘-‘,1},      {atom,1,module},      {‘(‘,1},      {atom,1,t},      {‘)‘,1},      {dot,1},      {‘-‘,2},      {atom,2,export},      {‘(‘,2},      {‘[‘,2},      {atom,2,say},      {‘/‘,2},      {integer,2,0},      {‘]‘,2},      {‘)‘,2},      {dot,2},      {atom,4,say},      {‘(‘,4},      {‘)‘,4},      {‘->‘,4},      {string,4,"Hello world!!"},      {dot,4}],     4}

　　

 

 可以看到上面的list里面包含了若干Form,erl_scan可以逐行解析代码:

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4> erl_scan:tokens([],Code,1). {done,{ok,[{‘-‘,1},            {atom,1,module},            {‘(‘,1},            {atom,1,t},            {‘)‘,1},            {dot,1}],           2},       "-export([say/0]).\n\nsay() -> \"Hello world!!\". "}

　　

 

当然还有更简单的方式,dynamic_compile 项目把这些事情都做了,还考虑了更为复杂的情况,比如头文件,宏,注释,record之类的, https://github.com/JacobVorreuter/dynamic_compile/blob/master/src/dynamic_compile.erl 下面就是dynamic_compile使用的一个例子: 

 

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Eshell V5.10.2  (abort with ^G) 1> Code = "-module(t).\n-export([say/0]).\n\nsay() -> \"Hello world!!\". ". "-module(t).\n-export([say/0]).\n\nsay() -> \"Hello world!!\". " 2> dynamic_compile:load_from_string(Code). {module,t} 3> t:say(). "Hello world!!" 4>

　　

 

上面拼字符串的方法看起来丑?你可以选择erl_syntax,下面我们用比较"优雅"的方法去创建t模块:

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Eshell V5.10.2  (abort with ^G) 1> M  = erl_syntax:attribute(erl_syntax:atom(module),[erl_syntax:atom(t)]). {tree,attribute,       {attr,0,[],none},       {attribute,{tree,atom,{attr,0,[],none},module},                  [{tree,atom,{attr,0,[],none},t}]}} 2> MF =  erl_syntax:revert(M). {attribute,0,module,t} 3> E = erl_syntax:attribute(erl_syntax:atom(export),[erl_syntax:list([erl_syntax:arity_qualifier(erl_syntax:atom(say),erl_syntax:integer(0))])]). {tree,attribute,     {attr,0,[],none},     {attribute,         {tree,atom,{attr,0,[],none},export},         [{tree,list,              {attr,0,[],none},              {list,                  [{tree,arity_qualifier,                       {attr,0,[],none},                       {arity_qualifier,                           {tree,atom,{attr,0,[],none},say},                           {tree,integer,{attr,0,[],none},0}}}],                  none}}]}} 4> ExportForm = erl_syntax:revert(E). {attribute,0,export,[{say,0}]} 5> 5> 5> Clause= erl_syntax:clause([],[],[erl_syntax:atom(hello_world)]). {tree,clause,       {attr,0,[],none},       {clause,[],none,[{tree,atom,{attr,0,[],none},hello_world}]}} 6> 6> Function =  erl_syntax:function(erl_syntax:atom(say),[Clause]). {tree,function,       {attr,0,[],none},       {func,{tree,atom,{attr,0,[],none},say},             [{tree,clause,                    {attr,0,[],none},                    {clause,[],none,                            [{tree,atom,{attr,0,[],none},hello_world}]}}]}} 7> FunctionForm = erl_syntax:revert(Function). {function,0,say,0,[{clause,0,[],[],[{atom,0,hello_world}]}]} 8>  {ok, Mod, Bin1} = compile:forms([MF,ExportForm, FunctionForm]). {ok,t,     <<70,79,82,49,0,0,1,208,66,69,65,77,65,116,111,109,0,0,0,       57,0,0,0,6,1,116,...>>} 9> t:say(). ** exception error: undefined function t:say/0 10> code:load_binary(Mod, [], Bin1). {module,t} 11> t:say(). hello_world 12>

　　

 

 Erlang Shared Data using mochiglobal 里面mochiglobal 就是用这种方法实现的,不过,不过,又会有人提意见了,这编写难度也太大了,能折中一下吗?好吧,请出下一位嘉宾smerl

 

 

   我曾经介绍过开源项目smerl,其定位就是Simple Metaprogramming for Erlang, 我们可以从这份代码里面学到erl_scan erl_parse erl_eval更灵活的应用,项目地址:http://code.google.com/p/smerl/ 

 

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test_smerl() ->       M1 = smerl:new(foo),       {ok, M2} = smerl:add_func(M1, "bar() -> 1 + 1."),       smerl:compile(M2),       foo:bar(),   % returns 2``       smerl:has_func(M2, bar, 0). % returns true

　　

 

parse_transform

 

  在上篇文章里面,我需要反复生成 Abstract Format,开始手工搞了一下,后来不胜其烦就使用了一下parse_transform.这东西是干什么用的呢?

 

{parse_transform,Module}
Causes the parse transformation function Module:parse_transform/2 to be applied to the parsed code before the code is checked for errors.

 

 对,它就是允许你在检查错误之前插入自己的逻辑,把那些"奇怪的东西"变成正常的东西,当然你可以选择什么都不做,仅仅打印,看代码:

 

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-module(print_form). -export([parse_transform/2]).   parse_transform(Forms, _Options) ->   io:format("forms: ~p~n", [Forms]),   Forms.

　　

 

下面,我们写一个简单的模块a.erl,然后编译它,看输出:

 

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[root@nimbus demo]# cat a.erl -module(a). -compile({parse_transform,print_form}). -export([test/0]).    test()->   "hello world!".   [root@nimbus demo]# erlc -o . -pa . a.erl  forms: [{attribute,1,file,{"a.erl",1}},         {attribute,1,module,a},         {attribute,3,export,[{test,0}]},         {function,5,test,0,[{clause,5,[],[],[{string,6,"hello world!"}]}]},         {eof,10}]

　　

好吧,感冒,鼻子堵得难受,先到这里吧

 

 

 

参考资料:

 

[0] erl_syntax  http://erlang.org/doc/man/erl_syntax.html

 

[1] Erlang: How to Create and Compile Module in Run-time

http://vas.io/blog/2013/03/23/erlang-how-to-create-and-compile-module-in-run-time/

 

[2] Hacking-Erlang http://zh.scribd.com/doc/22451864/Hacking-Erlang

 

 

最后小图一张,最早在山东卫视凌晨剧场看过第一季,现在已经14季了, 老员工只有Nicky ,Sara ,Jim了: