C＃中的antlr语法/解析器无效的转换问题

Question

我正在使用Anlr 3.5从下面包含的语法生成解析器和词法分析器。 该语法用于读取字符串，以便将它们转换为对象图形以供以后评估。 但是当我尝试使用unaryExpression子句时，我遇到了问题，这样做会抛出一个类转换异常报告：

无法将类型为'operandExpression_return'的对象强制转换为'unaryExpression_return'。

导致这种情况的一个示例输入是

([p1 eq \"p1Value\"]) and [p2 eq \"p2Value\"] or [p3 > \"p3\"\"Val\"\"ue\"]

它似乎是明确触发异常的括号（unaryExpression）。 运行相同的语句减去括号似乎正确解析，括号内的子表达式也是如此。

我可以通过异常清楚地看到正在使用错误的类型。 我只是不遵循为什么解析器选择它以及语法需要什么校正来避免这个错误。

值得注意的是，每个expression子句的两个分支都有多个匹配的替代品与每个binaryOperator分支，但从我的理解，这应该是好的，只会导致效率较低的贪婪解析器，而不会导致我看到的错误。

语法使用：

grammar QueryExpressionGrammar;

options {
    language=CSharp3;
    TokenLabelType=CommonToken;
    output=AST;
    ASTLabelType=CommonTree;
}

@lexer::namespace{namespace}
@parser::namespace{namespace}

@parser::members {
public static string CleanQuotedString(string input)
{
    return input == null
        ? null
        : input.Substring(1, input.Length - 2).Replace("\"\"", "\"");
}
}

/*------------------------------------------------------------------
 * PARSER RULES
 *------------------------------------------------------------------*/

public parse returns [IQueryExpression value]
    : exp=expression EOF {$value = $exp.value;}
    ;

expression returns [IQueryExpression value]
    : lhs=operandExpression { $value = $lhs.value; } ( op=binaryOperator rhs=expression {$value = new BinaryOperationQueryExpression($lhs.value, $op.value, $rhs.value);} )*
    | lhs=unaryExpression { $value = $lhs.value; } ( op=binaryOperator rhs=expression {$value = new BinaryOperationQueryExpression($lhs.value, $op.value, $rhs.value);} )*
    ;

binaryOperator returns [BinaryOperator value]
    : BinaryOperatorAnd {$value = BinaryOperator.And;}
    | BinaryOperatorOr {$value = BinaryOperator.Or;}
    ;

unaryExpression  returns [IQueryExpression value]
    : UnaryOperatorNot sub=expression {$value = new UnaryOperationQueryExpression(UnaryOperator.Not, $sub.value);}
    | OPEN_PAREN sub=expression CLOSE_PAREN {$value = new UnaryOperationQueryExpression(UnaryOperator.Paren, $sub.value);}
    ;

operandExpression returns [IQueryExpression value]
    : OPEN_BRACKET p=PROPERTY OperandQueryExpressionOperatorEq v=QUOTED_STRING CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.Eq, CleanQuotedString($v.text));}
    | OPEN_BRACKET p=PROPERTY OperandQueryExpressionOperatorLt v=QUOTED_STRING CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.Lt, CleanQuotedString($v.text));}
    | OPEN_BRACKET p=PROPERTY OperandQueryExpressionOperatorLe v=QUOTED_STRING CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.Le, CleanQuotedString($v.text));}
    | OPEN_BRACKET p=PROPERTY OperandQueryExpressionOperatorGt v=QUOTED_STRING CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.Gt, CleanQuotedString($v.text));}
    | OPEN_BRACKET p=PROPERTY OperandQueryExpressionOperatorGe v=QUOTED_STRING CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.Ge, CleanQuotedString($v.text));}
    | OPEN_BRACKET p=PROPERTY OperandQueryExpressionOperatorLike v=QUOTED_STRING CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.Like, CleanQuotedString($v.text));}
    | OPEN_BRACKET p=PROPERTY OperandQueryExpressionOperatorIlike v=QUOTED_STRING CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.Ilike, CleanQuotedString($v.text));}
    | OPEN_BRACKET p=PROPERTY NonValueBasedOperandQueryExpressionOperatorIsNull CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.IsNull, null);}
    | OPEN_BRACKET p=PROPERTY NonValueBasedOperandQueryExpressionOperatorNotNull CLOSE_BRACKET {$value = new OperandQueryExpression($p.text, OperandQueryExpressionOperator.NotNull, null);}
    ;

/*------------------------------------------------------------------
 * LEXER RULES
 *------------------------------------------------------------------*/

UnaryOperatorNot
    : 'not'
    | '!'
    ;

BinaryOperatorAnd
    : 'and'
    | '&&'
    | '&'
    ;

BinaryOperatorOr
    : 'or'
    | '||'
    | '|'
    ;

// OperandQueryExpressionOperator that uses a comparison value
OperandQueryExpressionOperatorEq
    : 'eq'
    | '=='
    | '='
    ;

OperandQueryExpressionOperatorLt
    : 'lt'
    | '<'
    ;

OperandQueryExpressionOperatorLe
    : 'le'
    | '<='
    ;

OperandQueryExpressionOperatorGt
    : 'gt'
    | '>'
    ;

OperandQueryExpressionOperatorGe
    : 'ge'
    | '>='
    ;

OperandQueryExpressionOperatorLike
    : 'like'
    ;

OperandQueryExpressionOperatorIlike
    : 'ilike'
    ;

// OperandQueryExpressionOperator that does not use a comparison value
NonValueBasedOperandQueryExpressionOperatorIsNull
    : 'null'
    | 'isnull'
    ;

NonValueBasedOperandQueryExpressionOperatorNotNull
    : 'notnull'
    ;

OPEN_BRACKET: '[';
CLOSE_BRACKET:  ']';
OPEN_PAREN: '(';
CLOSE_PAREN: ')';
PROPERTY: LETTER (ALPHA_NUMERIC | SPECIAL)*; // property definition is in line with the definition of a property definition for c#
QUOTED_STRING: QUOTE (~QUOTE | (QUOTE QUOTE))* QUOTE; // values are characters, or if they contain quotes they are escaped by double quoting and surrounding value in quotes

fragment DIGIT: ('0'..'9');
fragment LETTER: (('a'..'z')|('A'..'Z'));   
fragment ALPHA_NUMERIC: (LETTER|DIGIT);
fragment SPECIAL: ('_'|'-');    
fragment QUOTE: '\u0022';

WHITESPACE: (' ' | '\t' | '\n' | '\r'){ Skip(); }; // valid whitespace characters

私有QueryExpressionGrammarParser.expression_return表达式（）方法中解析器内异常抛出代码的片段

...
switch (alt3)
{
case 1:
    DebugEnterAlt(1);
    // QueryExpressionGrammar.g:37:4: lhs= operandExpression (op= binaryOperator rhs= expression )*
    {
    root_0 = (CommonTree)adaptor.Nil();

    DebugLocation(37, 7);
    PushFollow(Follow._operandExpression_in_expression111);
    lhs=operandExpression();
    PopFollow();

    adaptor.AddChild(root_0, lhs.Tree);
    DebugLocation(37, 26);


    /// v v v Exception throwing on line below v v v
     retval.value = (lhs!=null?((QueryExpressionGrammarParser.unaryExpression_return)lhs).value:default(IQueryExpression)); 


    DebugLocation(37, 51);
    // QueryExpressionGrammar.g:37:51: (op= binaryOperator rhs= expression )*
    try { DebugEnterSubRule(1);
...

Answer 1

所以这是我们在评论中找到的解决方案。 改变这个：

expression returns [IQueryExpression value]
    : lhs=operandExpression { $value = $lhs.value; } ( op=binaryOperator rhs=expression {$value = new BinaryOperationQueryExpression($lhs.value, $op.value, $rhs.value);} )*
    | ulhs=unaryExpression { $value = $ulhs.value; } ( op=binaryOperator rhs=expression {$value = new BinaryOperationQueryExpression($ulhs.value, $op.value, $rhs.value);} )*
    ;