Search code examples
scalaparser-combinators

How to parse to a type in Scala

I'm trying to write a parser in Scala that gradually builds up a concrete type hierarchy. I started with:

private def word   = regex(new Regex("[a-zA-Z][a-zA-Z0-9-]*")) 
private def quicktoken: Parser[Quicktoken] = "/" ~> word <~ "/" <~ (space?) ^^ { new Quicktoken(_) }

which is fine. /hello/ will get parsed to a quicktoken

Now I want to add the quicktoken to a composite expression. I have a class

class MatchTokenPart(word:String,quicktoken:RewriteWord){
}

I would have thought that I could write...

private def matchTokenPartContent: Parser[MatchTokenPart] = word<~equals~quicktoken ^^ { case word~quicktoken => new MatchTokenPart(word, quicktoken)}

but it doesn't work. It says that word is of type Option[String] and quicktoken of type String. What am I missing?

Solution

  • Another precedence issue: a <~ b ~ c is interpreted as a <~ (b ~ c), not (a <~ b) ~ c. This is because infix operators starting with < have lower precedence than ones starting with ~, (see the list in 6.12.3 of the language specification).

    You want (word <~ equals) ~ quicktoken, so you'll need to supply the parentheses.