[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

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}]