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scalaparsingparser-combinators

Scala's parser combinator: parse an option

Say I have a parser like this:

trait PT {}

trait StatementPT extends PT{}

...

class LoopConditionPT(val operation: String, val variable: IdentPT, val condition: Option[(String,ExpressionPT)]) extends StatementPT { ... }

The Option[(String,ExpressionPT)]) bug me for a day, I mean I can deal with Option[T], but Option[(T,T)]? I wrote this def loopCondition but it always raises error:

def loopCondition: Parser[LoopConditionPT] = "some string" ~ var ~ opt(("TIL"|"WILE") ~ expression) ^^ {
  case a ~ b ~ Some(c ~ d) => new LoopCondition(a, b, Option[(c, d)])
  case a ~ b ~ None => new LoopCondition(a, b, Option[("None", -1)])

Can someone help me fixes the loopCondition? Thanks.

Solution

  • [] are used for denoting types. When you want to create an Option - i.e. call Option.apply() - use parenthesis instead of square brackets. So instead of

    new LoopCondition(a, b, Option[(c, d)])

    say

    new LoopCondition(a, b, Option((c, d)))

    or you can use syntactic sugar for tuples and say

    new LoopCondition(a, b, Option(c -> d))