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我尝试使用 Parser Combinators 在 scala 中编写解析器。如果我递归匹配，

def body: Parser[Body] =
("begin" ~> statementList  )  ^^ {
     case s => {   new Body(s); }
}

def statementList : Parser[List[Statement]] = 
  ("end" ^^ { _ => List() } )|
  (statement ~ statementList ^^ { case statement ~ statementList => statement :: statementList  })

然后，只要语句中有错误，我就会收到很好的错误消息。然而，这是丑陋的长代码。所以我想写这个：

def body: Parser[Body] =
("begin" ~> statementList <~ "end"  )  ^^ {
   case s => {   new Body(s); }
}

def statementList : Parser[List[Statement]] = 
    rep(statement)

此代码有效，但仅在 FIRST 语句中有错误时才打印有意义的消息。如果它在后面的语句中，则消息将变得非常不可用，因为解析器希望看到整个错误语句被“end”标记替换：

Exception in thread "main" java.lang.RuntimeException: [4.2] error: "end" expected but "let" found

 let b : string = x(3,b,"WHAT???",!ERRORHERE!,7 ) 

 ^

我的问题：有没有办法让rep和repsep与有意义的错误消息结合使用，将插入符号放在正确的位置而不是重复片段的开头？

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2 回答 2

1

啊，找到了解决办法！事实证明，您需要在主解析器上使用函数短语来返回一个不太倾向于回溯的新解析器。（我想知道它到底是什么意思，也许如果它发现一个换行符它就不会回溯？）跟踪发生故障的最后一个位置。

改变：

def parseCode(code: String): Program = {
 program(new lexical.Scanner(code)) match {
      case Success(program, _) => program
      case x: Failure => throw new RuntimeException(x.toString())
      case x: Error => throw new RuntimeException(x.toString())
  }

}

def program : Parser[Program] ...

进入：

def parseCode(code: String): Program = {
 phrase(program)(new lexical.Scanner(code)) match {
      case Success(program, _) => program
      case x: Failure => throw new RuntimeException(x.toString())
      case x: Error => throw new RuntimeException(x.toString())
  }

}


def program : Parser[Program] ...
于 2010-12-11T23:01:36.177 回答
1

您可以通过将“自制”rep方法与非回溯内部语句相结合来实现。例如：

scala> object X extends RegexParsers {
     |   def myrep[T](p: => Parser[T]): Parser[List[T]] = p ~! myrep(p) ^^ { case x ~ xs => x :: xs } | success(List())
     |   def t1 = "this" ~ "is" ~ "war"
     |   def t2 = "this" ~! "is" ~ "war"
     |   def t3 = "begin" ~ rep(t1) ~ "end"
     |   def t4 = "begin" ~ myrep(t2) ~ "end"
     | }
defined module X

scala> X.parse(X.t4, "begin this is war this is hell end")
res13: X.ParseResult[X.~[X.~[String,List[X.~[X.~[String,String],String]]],String]] =
[1.27] error: `war' expected but ` ' found

begin this is war this is hell end
                          ^

scala> X.parse(X.t3, "begin this is war this is hell end")
res14: X.ParseResult[X.~[X.~[String,List[X.~[X.~[String,String],String]]],String]] =
[1.19] failure: `end' expected but ` ' found

begin this is war this is hell end
                  ^
于 2010-12-11T23:40:20.837 回答