了解潜在的冲突

Question

我有一个由 Menhir 构建的小型表达式解析器。我试图通过在以下位置编写恢复语法来恢复解析过程中括号不完整的表达式parser.mly：

%{ 
   open AST
%}

%token<int> LINT 
%token<string> ID
%token LPAREN RPAREN COMMA
%token EOF PLUS STAR EQ

%start<AST.expression> expressionEOF

%right LPAREN RPAREN
%nonassoc EQ
%left PLUS
%left STAR

%%

expressionEOF: e=expression EOF
{
  e
}

expression:
| x=LINT
{
  Int x
}
| x=identifier
{
  Read x
}
| e1=expression b=binop e2=expression
{
  Binop (b, e1, e2)
}
| e1=expression b=binop
(* for "2+", "2*3+" *)
{
  Binop (b, e1, FakeExpression)
}
| LPAREN e=expression RPAREN
{
  Paren e
}
| LPAREN RPAREN
(* for "()" *)
{
  Paren FakeExpression
}
| LPAREN
(* for "(" *)
{
  ParenMissingRparen FakeExpression
}
| LPAREN e=expression 
(* for "(1", "(1+2", "(1+2*3", "((1+2)" *)
{
  ParenMissingRparen e
}
| RPAREN
(* for ")" *)
{
  ExtraRparen FakeExpression
}
| e=expression RPAREN 
(* for "3)", "4))", "2+3)" *)
{
  ExtraRparen e
}

%inline binop:
  PLUS { Add   }
| STAR { Mul   }
| EQ   { Equal }

identifier: x=ID
{
  Id x
}

它适用于一组不完整的表达式。但是，menhir --explain parser.mly返回以下内容parser.conflict：

** Conflict (reduce/reduce) in state 10.
** Tokens involved: STAR RPAREN PLUS EQ EOF
** The following explanations concentrate on token STAR.
** This state is reached from expressionEOF after reading:

LPAREN expression RPAREN

** The derivations that appear below have the following common factor:
** (The question mark symbol (?) represents the spot where the derivations begin to differ.)

expressionEOF 
expression EOF 
expression STAR expression // lookahead token appears
(?)

** In state 10, looking ahead at STAR, reducing production
** expression -> LPAREN expression RPAREN
** is permitted because of the following sub-derivation:

LPAREN expression RPAREN . 

** In state 10, looking ahead at STAR, reducing production
** expression -> expression RPAREN
** is permitted because of the following sub-derivation:

LPAREN expression // lookahead token is inherited
       expression RPAREN . 

** Conflict (reduce/reduce) in state 3.
** Tokens involved: STAR RPAREN PLUS EQ EOF
** The following explanations concentrate on token STAR.
** This state is reached from expressionEOF after reading:

LPAREN RPAREN

** The derivations that appear below have the following common factor:
** (The question mark symbol (?) represents the spot where the derivations begin to differ.)

expressionEOF 
expression EOF 
expression STAR expression // lookahead token appears
(?)

** In state 3, looking ahead at STAR, reducing production
** expression -> LPAREN RPAREN
** is permitted because of the following sub-derivation:

LPAREN RPAREN . 

** In state 3, looking ahead at STAR, reducing production
** expression -> RPAREN
** is permitted because of the following sub-derivation:

LPAREN expression // lookahead token is inherited
       RPAREN . 

我不明白它试图解释什么。谁能告诉我可能存在哪些潜在冲突（优先举例）以及解决方案是什么？

Answer 1

你有：

expr: '(' expr ')'
    | '(' expr
    |     expr ')'

因此，您想要( x )匹配第一条规则：

expr: '(' expr ')'
    | '(' expr
    |     expr ')'

确实如此。但它也匹配另一种方式：

         expr
      -> '('  expr ')'  (rule 1)

而且它也这样匹配：

         expr
      -> expr       ')' (rule 3)
      -> '(' expr   ')' (rule 2)

由于您还让exprmatch(和),( )也可以通过多种方式进行匹配，包括 as expr ')'(with expr -> '(') 或'(' expr(with expr -> ')')。

“解决方案”是放弃尝试添加无效句子的识别。解析应该因语法错误而失败；一旦失败，您可以尝试使用 Menhir 的错误恢复机制来生成错误消息并继续解析。请参阅手册第 11 节。