Erlang 中的递归下降解析器

Question

我正在尝试在 Erlang 中构建 RDP。到目前为止，我已经阅读并处理了一个令牌文件，我将把它传递给函数，例如 [2,6,3,7,3,2,4,6,3,2,4,4,99]（示例应该工作的输入），我需要确保可以通过从默认规则 [bexp0] 转换为一些匹配的终端列表来派生每个字符（或一组）。

get_terminal_list() ->
    [1,2,3,4,5,6,7,8,9,10,11,12,99].

get_prod_list() ->
[bexp0,bexp,bexp1,bterm].

get_sym_list(Prod) ->
    case Prod of
        bexp0 -> [[bexp,99]];
        bexp  -> [[bterm,bexp1]];
        bexp1 -> [[5,bexp,bexp1],[6,bexp,bexp1]];
        bterm -> [[3,bexp,4],[2],[8],[9],[2,10,1],[2,12,1],[2,11,1],[7,bterm]]
    end.

get_sym_list 显示了正在使用的语法 - 其中每个 int 代表一个终端字符，每个子列表是一个集合，即 bterm-> [[7,bterm]] 表示 bterm 可以变成终端 '7' 后跟 non-终端“bterm”。

现在我正在努力以某种方式实现这一点：

Check if first set of rule has some terminal
 if so, check which side, reduce list of tokens from same side until first occurrence of this token,  
     parse this new list (w/o token) with rest of the set of this rule (also without the matched terminal). 
       return {success|failure, list_of_tokens, list_of_rules}  
   if success -> check with new list_of_tokens, default_rule
   if failure, check with old list_of_tokens, and new list_of_rules.

如果规则列表为空，我假设将达到最终状态 - 因此我们已经用尽了所有可能的生产，因此无效，或者
标记列表为空，因此我们已将每个标记/标记集与某个规则匹配

Answer 1

这可能会做你想要的：

-module(parse).
-export([parse1/0, parse1/1, parse2/0, parse2/1]).

parse1() ->
    parse([bexp], [2,6,3,7,3,2,4,6,3,2,4,4,99], fun get_sym_list1/1).

parse1(Input) ->
    parse([bexp], Input, fun get_sym_list1/1).


parse2() ->
    parse([bexp0], [2,6,3,7,3,2,4,6,3,2,4,4,99], fun get_sym_list2/1).

parse2(Input) ->
    parse([bexp0], Input, fun get_sym_list2/1).


parse([], [], _) ->
    true;
parse([], _, _) ->
    false;
parse([X | TX], [X | TY], Fun) ->
    io:format("+ Current:~w\tTokens:~w Input:~w~n", [X, TX, TY]),
    parse(TX, TY, Fun);
parse([X | TX], Input, Fun) ->
    io:format("  Current:~w\tTokens:~w Input:~w~n", [X, TX, Input]),
    case lists:member(X, get_terminal_list()) of
        true -> false;
        false -> lists:any(fun(T) -> parse(T ++ TX, Input, Fun) end, Fun(X))
    end.

get_terminal_list() ->
    [1,2,3,4,5,6,7,8,9,10,11,12,99].

get_sym_list1(Prod) ->
    case Prod of
        bexp    -> [[bexp1],[bterm],[bterm,bexp2]];
        bexp1   -> [[99]];
        bexp2   -> [[5,bterm,bexp2],[6,bterm,bexp2]];
        bterm   -> [[bfactor],[7,bterm]];
        bfactor -> [[3,bexp,4],[bconst],[2,10,1],[2,12,1],[2,11,1],[2]];
        bconst  -> [[8],[9]]
    end.

get_sym_list2(Prod) ->
    case Prod of
        bexp0 -> [[bterm,bexp1]];
        bexp  -> [[bterm,bexp1]];
        bexp1 -> [[5,u1],[6,bexp,bexp1],[99]];
        bterm -> [[u1,4],[2],[8],[9],[2,10,1],[2,12,1],[2,11,1],[7,bterm]];
        u1    -> [[3,bexp]]
    end.

但是，看起来语法或输入列表不正确，因为据我所知，旧语法和新语法都无法解析输入。它似乎工作正常，因为它会解析这样的输入：

41> parse:parse2([2,6,8,5,3,bterm,5,3,9,99,99]).
  Current:bexp0 Tokens:[] Input:[2,6,8,5,3,bterm,5,3,9,99,99]
  Current:bterm Tokens:[bexp1] Input:[2,6,8,5,3,bterm,5,3,9,99,99]
  Current:u1    Tokens:[4,bexp1] Input:[2,6,8,5,3,bterm,5,3,9,99,99]
  Current:3     Tokens:[bexp,4,bexp1] Input:[2,6,8,5,3,bterm,5,3,9,99,99]
+ Current:2     Tokens:[bexp1] Input:[6,8,5,3,bterm,5,3,9,99,99]
  Current:bexp1 Tokens:[] Input:[6,8,5,3,bterm,5,3,9,99,99]
  Current:5     Tokens:[u1] Input:[6,8,5,3,bterm,5,3,9,99,99]
+ Current:6     Tokens:[bexp,bexp1] Input:[8,5,3,bterm,5,3,9,99,99]
  Current:bexp  Tokens:[bexp1] Input:[8,5,3,bterm,5,3,9,99,99]
  Current:bterm Tokens:[bexp1,bexp1] Input:[8,5,3,bterm,5,3,9,99,99]
  Current:u1    Tokens:[4,bexp1,bexp1] Input:[8,5,3,bterm,5,3,9,99,99]
  Current:3     Tokens:[bexp,4,bexp1,bexp1] Input:[8,5,3,bterm,5,3,9,99,99]
  Current:2     Tokens:[bexp1,bexp1] Input:[8,5,3,bterm,5,3,9,99,99]
+ Current:8     Tokens:[bexp1,bexp1] Input:[5,3,bterm,5,3,9,99,99]
  Current:bexp1 Tokens:[bexp1] Input:[5,3,bterm,5,3,9,99,99]
+ Current:5     Tokens:[u1,bexp1] Input:[3,bterm,5,3,9,99,99]
  Current:u1    Tokens:[bexp1] Input:[3,bterm,5,3,9,99,99]
+ Current:3     Tokens:[bexp,bexp1] Input:[bterm,5,3,9,99,99]
  Current:bexp  Tokens:[bexp1] Input:[bterm,5,3,9,99,99]
+ Current:bterm Tokens:[bexp1,bexp1] Input:[5,3,9,99,99]
  Current:bexp1 Tokens:[bexp1] Input:[5,3,9,99,99]
+ Current:5     Tokens:[u1,bexp1] Input:[3,9,99,99]
  Current:u1    Tokens:[bexp1] Input:[3,9,99,99]
+ Current:3     Tokens:[bexp,bexp1] Input:[9,99,99]
  Current:bexp  Tokens:[bexp1] Input:[9,99,99]
  Current:bterm Tokens:[bexp1,bexp1] Input:[9,99,99]
  Current:u1    Tokens:[4,bexp1,bexp1] Input:[9,99,99]
  Current:3     Tokens:[bexp,4,bexp1,bexp1] Input:[9,99,99]
  Current:2     Tokens:[bexp1,bexp1] Input:[9,99,99]
  Current:8     Tokens:[bexp1,bexp1] Input:[9,99,99]
+ Current:9     Tokens:[bexp1,bexp1] Input:[99,99]
  Current:bexp1 Tokens:[bexp1] Input:[99,99]
  Current:5     Tokens:[u1,bexp1] Input:[99,99]
  Current:6     Tokens:[bexp,bexp1,bexp1] Input:[99,99]
+ Current:99    Tokens:[bexp1] Input:[99]
  Current:bexp1 Tokens:[] Input:[99]
  Current:5     Tokens:[u1] Input:[99]
  Current:6     Tokens:[bexp,bexp1] Input:[99]
+ Current:99    Tokens:[] Input:[]
true

顺便说一句，true 表示输入已被解析，false 表示尚未解析。