Question

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，只要语句出错，我就会得到很好的错误消息。但是，这是一个丑陋的长代码。所以我想写一下：

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

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

此代码有效，但只有在FIRST语句中出现错误时才会打印有意义的消息。如果它在后面的语句中，则消息变得非常难以使用，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在开头重复片段？

Answer 1

啊，找到了解决办法！事实证明，您需要使用主解析器上的函数短语来返回~~不太倾向于追溯的新解析器。（我想知道它究竟意味着什么，或许如果找到一个换行符就不会追溯？）~~跟踪发生故障时的最后一个位置。

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] ...

成：

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] ...

Answer 2

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
                  ^

从使用Scala Parser Combinators编写的解析器返回有意义的错误消息

2 个答案: