Why won't Scala parser combinators propagate failure inside a repeated (rep) clause?

Question

If I have simple parser defs like this:

  def term: Parser[String] = """[a-zA-Z"']+""".r ^^ { _.toString }
  def intWhole: Parser[String] = wholeNumber ^^ { w => w }

  def simpleTerm: Parser[String] = term >> { 
    case t:String => failure("Oops!") 
  }

If I parse against simpleTerm (with any string) it fails as expected with my "Oops!" message.

Now if I add this:

  def repTerm: Parser[Unit] = rep(simpleTerm | intWhole) ^^ { _ => Unit }

If I now parse against repTerm, again with just a non-numeric character string, what I'd hope to happen is have it fail with the same "Oops!" message--basically an aborted parse. What happens instead is that I get no error at all; just the returned Unit.

Is there a way to make parsing stop once it hits a failure, and return that failure, during a rep() clause?

Greg Greg · Accepted Answer · 2018-03-01T22:39:52

Looked at the code. There's a difference how rep() handles failure vs errors. failure() just tells a repeating sequence to stop, i.e. end of repeating clause. It's not a breakage necessarily. err() means something broke, and a rep() clause does propagate errors and stop further parsing.

Changing my failure() in code above to err() produces the desired result of stopping further parsing.

Why won't Scala parser combinators propagate failure inside a repeated (rep) clause?

1 Answers