returning meaningful error messages from a parser written with Scala Parser Combinators

I try to write a parser in scala using Parser Combinators. If I match recursively,

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  })

then I get good errormessages whenever there is a fault in a statement. However, this is ugly long code. So I'd like to write this:

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

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

This code works, but only prints meaningful messages if there is an error in the FIRST statement. If it is in a later statement, the message becomes painfully unusable, because the parser wants to see the whole erroneous statement replaced by the "end" token:

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

 let b : string = x(3,b,"WHAT???",!ERRORHERE!开发者_如何学Go,7 ) 

 ^ 

My question: is there a way to get rep and repsep working in combination with meaningful error messages, that place the caret on the right place instead of on the begin of the repeating fragment?

---

You can do it by combining a "home made" rep method with non-backtracking inside statements. For example:

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
                  ^

---

Ah, found the solution! It turns out that you need to use the function phrase on your main parser to return a new parser that is less inclined to track back. (I wonder what it exactly means, perhaps that if it finds a line break it will not track back?) tracks the last position on wich an failure occured.

changed:

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] ...

into:

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] ...