Reuse parser within another parser with Scala pars

问题:

I have a parser for arithmetic expressions:

object FormulaParser extends JavaTokenParsers {
  def apply(input: String) = parseAll(formula, input)
  // ...

  val formula: Parser[Formula] = 
    comparison | comparable | concatenable | term | factor
}

I need to parse a different language that can contain formulas. Let's say I need to parse something like X < formula. Unfortunately I cannot reuse FormulaParser.formula in my new parser:

object ConditionParser extends JavaTokenParsers {
  def apply(input: String) = parseAll(condition, input)
  // ...

  val condition: Parser[Condition] = 
    "X" ~ ("<=" | "<") ~ FormulaParser.formula ^^ { ... }  // doesn't work
}

because the parser on the left-hand side of ~ is an instance of ConditionParser.Parser, so its ~ method expects something with that same type, not something of type FormulaParser.Parser.

The whole point of using parser combinators is, well, combining parsers! It seems silly to me that my first attempt didn't work, although I understand why it happens (we are reusing base parsers by extending a base trait).

Is there a simple way to combine parsers defined in different types?

回答1:

In order to reuse parsers, you need to use inheritance. So if you make FormulaParsers a class or a trait, ConditionParser can inherit from it and reuse its parsers.

This is also how you're already reusing the parsers defined in JavaTokenParsers.