[Erlang 0111] Erlang Abstract Format , Part 2

时间：2023-03-08 20:41:05
   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

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:
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: 
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:
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/ 
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:
-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:
[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:
[1] Erlang: How to Create and Compile Module in Run-time
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:
[Erlang 0111] Erlang Abstract Format , Part 2