团队的兄弟遇到一个问题,如何把字符串动态解析成为Erlang数据结构(Erlang term)? 比如"[{1,2},{2,3},{3,4}]"转成[{1,2},{2,3},{3,4}]
From String To Erlang Term
http://blog.yufeng.info/archives/tag/erl_eval 余锋在<erlang动态解释>这篇文章中给出了解决方案,他分析的是init.erl模块;按照文章的思路我们在shell里面试一下:
Eshell V5.8.2 (abort with ^G)
1> {ok, Scan1, _} = erl_scan:string("[a,b,c].").
{ok,[{'[',1},{atom,1,a},{',',1},{atom,1,b},{',',1},{atom,1,c},{']',1},{dot,1}],1}
2> {ok,P}=erl_parse:parse_exprs(Scan1).
{ok,[{cons,1,{atom,1,a},{cons,1,{atom,1,b},{cons,1,{atom,1,c},{nil,1}}}}]}
3> erl_eval:exprs(P, []) .
{value,[a,b,c],[]}
4>
红色标注的部分就是我们想要的结果,注意erl_scan:string(Exp).接受的参数是一个合法的表达式,必须以.结尾,代表一个表达式的结束,否则文法检查过不去;看一下输出的结果里面.符号被解析为{dot,1};这个问题最关键的部分就已经解决了,还有一个相关的问题就是如何把一个[{1,2},{2,3},{3,4}]转成字符串?这个当然要在io_lib里面去寻找答案,可以这样做: lists:flatten(io_lib:write([{1,2},{2,3},{3,4}])). 结果为"[{1,2},{2,3},{3,4}]"我们做一个完整的例子:
1> lists:flatten(io_lib:write([{1,2},{2,3},{3,4}])).
"[{1,2},{2,3},{3,4}]"
2> S= lists:flatten(io_lib:write([{1,2},{2,3},{3,4}])).
"[{1,2},{2,3},{3,4}]"
3> E=S++".". %%添加结束符
"[{1,2},{2,3},{3,4}]."
4> {ok, Scan1, _} = erl_scan:string(E).
{ok,[{'[',1},{'{',1},{integer,1,1},{',',1},{integer,1,2},{'}',1},{',',1},{'{',1},{integer,1,2},{',',1},{integer,1,3},{'}',1},{',',1},{'{',1},{integer,1,3},{',',1},{integer,1,4},{'}',1},{']',1},{dot,1}], 1}
5> {ok,P}=erl_parse:parse_exprs(Scan1).
{ok,[{cons,1,{tuple,1,[{integer,1,1},{integer,1,2}]}, {cons,1,{tuple,1,[{integer,1,2},{integer,1,3}]},
{cons,1,{tuple,1,[{integer,1,3},{integer,1,4}]},{nil,1}}}}]}
6> erl_eval:exprs(P, []) .
{value,[{1,2},{2,3},{3,4}],[]}
7>
仅仅是解析Erlang Term 也可以这样：
list_to_term(String) ->
{ok, T, _} = erl_scan:string(String++"."),
case erl_parse:parse_term(T) of
{ok, Term} ->
Term;
{error, Error} ->
Error
end.
From String To Erlang Code
这个问题可以泛化为字符串解析为Erlang代码并执行?同样的问题在.net中,我们可以这样解决:
using System;
using IronPython.Hosting; //缺少引用的去下载一个: http://ironpython.codeplex.com/
using Microsoft.Scripting.Hosting;
namespace ConsoleApplication2
{
class Program
{
static void Main(string[] args)
{
ScriptEngine engine = Python.CreateEngine();
var Result = engine.Execute("2*(1+3)");
Console.WriteLine(Result);
Console.ReadLine();
}
}
}
还是使用上面的erl_scan erl_parse erl_eval基础设施,我们可以很容易实现:
Eshell V5.8.2 (abort with ^G)
1> E=fun(S) ->
{ok,Scanned,_} = erl_scan:string(S),
{ok,Parsed} = erl_parse:parse_exprs(Scanned),
erl_eval:exprs(Parsed,[]) end.
#Fun<erl_eval.6.13229925>
2> E("R=1+2*3.").
{value,7,[{'R',7}]}
3> E("A=2,B=3,A+B.").
{value,5,[{'A',2},{'B',3}]}
那如果是"M=K+L."这样的表达式呢?如何呢?
4> E("M=K+L.").
** exception error: {unbound_var,'K'}
我们直接在shell里面执行报的错误一致,只不过错误信息格式变化一下:* 1: variable 'K' is unbound,那么如何动态给变量绑定数值呢?这样我们可以更灵活控制表达式和值, http://www.trapexit.org/String_Eval 给了一个这样的例子:
-module(test).
-export([test/0]).
test()->
%% Create a code string with unbound variables 'A' and 'B'
String="Results=A+B/2.",
%% Scan the code into tokens
{ok,ErlTokens,_}=erl_scan:string(String),
io:format("ErlTokens are ~p~n",[ErlTokens]),
%% Now parse the tokens into the abstract form
{ok,ErlAbsForm}=erl_parse:parse_exprs(ErlTokens),
io:format("ErlAbsForm are ~p~n",[ErlAbsForm]),
%% Now we need to bind values to variable 'A' and 'B'
Bindings=erl_eval:add_binding('A',20,erl_eval:new_bindings()),
NewBindings=erl_eval:add_binding('B',45,Bindings),
io:format("The bindings are ~p~n",[erl_eval:bindings(NewBindings)]),
%% Now evaluate the string
io:format("Going into erl_eval:exprs~n",[]),
{value,Value,_}=erl_eval:exprs(ErlAbsForm,NewBindings),
io:format("Value is ~p~n",[Value]).
|
You can compile and run this in the shell:
(arrian@psyduck)17> c(test).
{ok,test}
(arrian@psyduck)18> test:test().
ErlTokens are [{var,1,'Results'},
{'=',1},
{var,1,'A'},
{'+',1},
{var,1,'B'},
{'/',1},
{integer,1,2},
{dot,1}]
ErlAbsForm are [{match,1,
{var,1,'Results'},
{op,1,
'+',
{var,1,'A'},
{op,1,'/',{var,1,'B'},{integer,1,2}}}}]
The bindings are [{'A',20},{'B',45}]
Going into erl_eval:exprs
Value is 42.5000
ok
(arrian@psyduck)19>
|
Note: If you bind variables that don't exist in the code string/token set/abstract form then when you erl_eval the abstract form will simply silently ignore your additional bindings
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
最后顺便提一句,如何把{abc}作为字符串写入到文件中呢? lists:flatten(io_lib:format("~p",[{a,b,c}])).
2012年9月27日16:54:11 更新 添加一段类似的代码
%%% %%% distel_ie - an interactive erlang shell %%% %%% Some of the code has shamelessly been stolen from Luke Gorrie %%% [luke@bluetail.com] - ripped from its elegance and replaced by bugs. %%% It just goes to show that you can't trust anyone these days. And %%% as if that wasn't enough, I'll even blame Luke: "He _made_ me do it!" %%% %%% So, without any remorse, I hereby declare this code to be: %%% %%% copyright (c) 2002 david wallin [david.wallin@ul.ie]. %%% %%% (it's probably going to be released onto an unexpecting public under %%% some sort of BSD license). -module(distel_ie). -export([ evaluate/2, test1/0, test2/0, test3/0, test4/0, test5/0, ensure_registered/0, start/0, start/1, init/1, loop/1 ]). -compile(export_all). -define(FMT(X), list_to_binary(lists:flatten(io_lib:format("~p", [X])))). %% %% ensure_registered/0 ensure_registered() -> case whereis(distel_ie) of undefined -> start() ; Pid -> group_leader(group_leader(), Pid), ok end. %% %% start/0 start() -> start([]). %% %% start/1 start(Options) -> spawn(?MODULE, init, [Options]). %% %% init/1 init(Options) -> register(distel_ie, self()), Defs = ets:new(definitions, [set]), Line = 12, Bindings = [], State = {Defs, Line, Bindings}, loop(State). %% %% loop/1 loop({Defs, Line, Bindings}) -> receive {evaluate, Emacs, String} -> case catch evaluate(String, {Defs, Line, Bindings}) of {'EXIT', Rsn} -> Emacs ! {ok, list_to_binary( io_lib:format("EXIT: ~p", [Rsn]))}, ?MODULE:loop({Defs, Line, Bindings}); {Result, {NL, NB}} -> Emacs ! Result, ?MODULE:loop({Defs, NL, NB}) end; forget_bindings -> put(distel_ie_bindings, []), ?MODULE:loop({Defs, Line, []}) ; Unknown -> io:format("distel_ie: unknown message recvd '~p'\n", [Unknown]), ?MODULE:loop({Defs, Line, Bindings}) end. %% %% evaluate/2 evaluate(String, {Defs, Line, Bindings}) -> case parse_expr(String) of %% ok, so it is an expression : {ok, Parse} -> RemoteParse = add_remote_call_info(Parse, Defs), case catch erl_eval:exprs(RemoteParse, Bindings) of {value, V, NewBinds} -> {{ok, ?FMT(V)}, {Line, NewBinds}}; Error -> {?FMT(Error), {Line, Bindings}} end; %% try and treat it as a form / definition instead : Other -> case parse_form(String) of {ok, Parse} -> {ok, Name, Arity} = get_function_name(Parse), ets:insert(Defs, {Name, Parse}), FunTrees = lists:flatten( lists:reverse(ets:match(Defs,{'_', '$1'}))), %% Line isn't really used yet NewLine = Line, compile_load(FunTrees), Def = list_to_binary(atom_to_list(Name) ++ "/" ++ integer_to_list(Arity)), {{ok, Def}, {NewLine, Bindings}} ; Error -> {{error, ?FMT({Error, Other})}, {Line, Bindings}} end end. %% %% parse_expr/1 parse_expr(String) -> case erl_scan:string(String) of {ok, Tokens, _} -> catch erl_parse:parse_exprs(Tokens) ; {error, {_Line, erl_parse, Rsn}} -> {error, lists:flatten(Rsn)}; {error, Error, _} -> {error, Error} end. %% %% parse_form/1 parse_form(String) -> case erl_scan:string(String) of {ok, Tokens, _} -> catch erl_parse:parse_form(Tokens) ; {error, {_Line, erl_parse, Rsn}} -> {error, lists:flatten(Rsn)}; {error, Error, _} -> {error, Error} end. %% %% defun/1 defun(String) -> {ok, Tokens, _} = erl_scan:string(String), {ok, Parse} = erl_parse:parse_form(Tokens), compile_load([Parse]). %% %% compile_load/1 compile_load(Parse) -> Header = [{attribute,9,module,distel_ie_internal}, {attribute,11,compile,export_all}, {attribute,12,export,[]}], EOF = [{eof,20}], SyntaxTree = Header ++ Parse ++ EOF, {ok, Mod, Binary} = compile:forms(SyntaxTree), File = "heltigenomfelfelsomfansomentyskindianenbefjadradgerman", code:load_binary(Mod, File, Binary). %% %% add_remote_call_info/2 %% %% TODO: this is gonna need more work, e.g. it needs to recurse into %% lists (cons) and tuples ... +more add_remote_call_info([], _Defs) -> [] ; add_remote_call_info({var, L, Var}, Defs) -> {var, L, Var} ; add_remote_call_info({atom, L, Atom}, Defs) -> {atom, L, Atom} ; add_remote_call_info({integer, L, Value}, Defs) -> {integer, L, Value} ; add_remote_call_info({string, L, String}, Defs) -> {string, L, String} ; add_remote_call_info([{call, L, {atom, L2, Name}, Body} | Rs], Defs) -> B = add_remote_call_info(Body, Defs), IsBuiltin = erlang:is_builtin(erlang, Name, length(B)), Call = case IsBuiltin of true -> {call, L, {atom, L2, Name}, B} ; false -> Arity = length(Body), case find_module(Name, Arity) of {ok, Mod} -> {call, L, {remote, L2, {atom, L2, Mod}, {atom, L2, Name}}, B} ; {error, _} -> {call, L, {atom, L2, Name}, B} end end, [Call | add_remote_call_info(Rs, Defs)] ; add_remote_call_info([{tuple, L, Values} | Rs], Defs) -> F = fun(X) -> add_remote_call_info(X, Defs) end, [{tuple, L, lists:map(F, Values)} | add_remote_call_info(Rs, Defs)] ; add_remote_call_info([{Type, L, Hdr, Body} | Rs], Defs) when list(Body) -> B = add_remote_call_info(Body, Defs), [{Type, L, Hdr, B} | add_remote_call_info(Rs, Defs)] ; add_remote_call_info([{Type, L, Hd, Tl} | Rs], Defs) -> [{Type, L, Hd, Tl} | add_remote_call_info(Rs, Defs)] ; add_remote_call_info([R | Rs], Defs) -> [add_remote_call_info(R, Defs) | add_remote_call_info(Rs, Defs) ]; add_remote_call_info(X, Defs) -> X. %% %% find_module/2 find_module(Function, Arity) -> Mods = [distel_ie_internal, distel_ie, c], F = fun(M) -> not is_exported(Function, Arity, M) end, case lists:dropwhile(F, Mods) of [] -> search_modules(Function, Arity, code:all_loaded()) ; [M | _] -> {ok, M} end. %% %% is_exported/3 is_exported(Function, Arity, Module) -> case code:is_loaded(Module) of false -> false; _ -> Info = Module:module_info(), {value, {exports, Exports}} = lists:keysearch(exports, 1, Info), lists:member({Function, Arity}, Exports) end. %% %% search_modules/3 search_modules(Function, Arity, []) -> {error, not_found}; search_modules(Function, Arity, [{M, _} | Ms]) -> case is_exported(Function, Arity, M) of true -> {ok, M} ; false -> search_modules(Function, Arity, Ms) end. %% %% get_function_name/1 get_function_name({function, _, Name, Arity, _}) -> {ok, Name, Arity} ; get_function_name(Unknown) -> {error, Unknown}. %%% ------------------------------------------------------------------- [tests] %% %% test1/0 test1() -> Defun = "sista(W) -> lists:last(W).", defun(Defun). %% %% test2/0 test2() -> Prefix = [ {attribute,9,module,compile_and_load_me}, {attribute,11,compile,export_all}, {attribute,12,export,[]} ], Postfix = [{eof,20}], String = "sista([]) -> [] ;\n\nsista(W) -> lists:last(W).\n", % String = "sista([], _) -> [] ;\n\nsista(W, _) -> lists:last(W).\n", {ok, Tokens, _} = erl_scan:string(String, 23), {ok, Tree} = erl_parse:parse_form(Tokens), io:format("tree : '~p'\n", [Tree]), SyntaxTree = Prefix ++ [Tree] ++ Postfix, io:format("syntaxtree : '~p'\n", [SyntaxTree]), {ok, Mod, Binary} = compile:forms(SyntaxTree), code:load_binary(Mod, "spam", Binary), compile_and_load_me:sista([1,2,galapremiere]). %% %% test3/0 test3() -> Sista = "sista(W) -> lists:last(W).\n", defun(Sista), distel_ie_internal:sista([1,2,galapremiere]), NySista = "en_till_sista(W) -> lists:last(W).\n", defun(NySista), distel_ie_internal:en_till_sista([1,2,onestepbeyond]). %% %% test4/0 test4() -> Defs = ets:new(definitions, [set]), Define = "nisse([]) -> [] ;\n\nnisse(W) -> lists:last(W).", Expr = "nisse([1,2,3]).", % Expr = "distel_ie_internal:nisse([1,2,3]).", % Expr = "lists:last([1,2,3]).", D = evaluate(Define, {Defs, 10, []}), io:format("nisse defined: '~p'\n", [D]), io:format("is_loaded: '~p'\n", [code:is_loaded(distel_ie_internal)]), evaluate(Expr, {Defs, 10, []}). %% %% test5/0 test5() -> Defs = ets:new(definitions, [set]), Define = "nisse([]) -> [] ;\n\nnisse(W) -> lists:last(W).", Expr = "nisse([1,2,3]).", evaluate(Define, {Defs, 10, []}), % ets:insert(Defs, {nisse, nil}), {ok, Tokens, _} = erl_scan:string(Expr, 23), {ok, Tree} = erl_parse:parse_exprs(Tokens), io:format("original tree : '~p'\n", [Tree]), RemoteTree = add_remote_call_info(Tree, Defs), io:format("remote tree : '~p'\n", [RemoteTree]).
--The End--
