Question

I’m toying with Scala’s Parser library. I am trying to write a parser for a format where a length is specified followed by a message of that length. For example:

x.parseAll(x.message, "5helloworld") // result: "hello", remaining: "world"

I’m not sure how to do this using combinators. My mind first goes to:

def message = length ~ body

But obviously body depends on length, and I don’t know how to do that :p

Instead you could just define a message Parser as a single Parser (not combination of Parsers) and I think that is doable (although I haven’t looked if a single Parser can pull several elem?).

Anyways, I’m a scala noob, I just find this awesome 🙂

Answer 1

You should use into for that, or its abbreviation, >>:

scala> object T extends RegexParsers {
     |   def length: Parser[String] = """\d+""".r
     |   def message: Parser[String] = length >> { length => """\w{%d}""".format(length.toInt).r }
     | }
defined module T

scala> T.parseAll(T.message, "5helloworld")
res0: T.ParseResult[String] =
[1.7] failure: string matching regex `\z' expected but `w' found

5helloworld
      ^

scala> T.parse(T.message, "5helloworld")
res1: T.ParseResult[String] = [1.7] parsed: hello

Be careful with precedence when using it. If you add an “~ remainder” after the function above, for instance, Scala will interpret it as length >> ({ length => ...} ~ remainder) instead of (length >> { length => ...}) ~ remainder.