Reuse parser within another parser with Scala parser combinators

Question

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?

sepp2k · Accepted Answer · 2016-10-21 00:36:10Z

3

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.

answered Oct 21, 2016 at 0:36

sepp2k

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1 Comment

Roberto Bonvallet Over a year ago

Thanks! I ended up creating a BaseParsers trait and having both parsers inherit from it.

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