Diverging Implicits for Case Class w/ Case Class Argument - scala

Question

Given the following from Travis Brown's educational and well-written Type classes and generic derivation:
case class Person(name: String, age: Double)
trait Parser[A] {
def apply(s: String): Option[A]
}
implicit val hnilParser: Parser[HNil] = new Parser[HNil] {
def apply(s: String): Option[HNil] = if(s.isEmpty) Some(HNil) else None
}
implicit def hconsParser[H: Parser, T <: HList: Parser]: Parser[H :: T] = new Parser[H :: T] {
def apply(s: String): Option[H :: T] = s.split(",").toList match {
case cell +: rest => for {
head <- implicitly[Parser[H]].apply(cell)
tail <- implicitly[Parser[T]].apply(rest.mkString(","))
} yield head :: tail
}
}
implicit val stringParser: Parser[String] = new Parser[String] {
def apply(s: String): Option[String] = Some(s)
}
implicit val intParser: Parser[Int] = new Parser[Int] {
def apply(s: String): Option[Int] = Try(s.toInt).toOption
}
implicit val doubleParser: Parser[Double] = new Parser[Double] {
def apply(s: String): Option[Double] = Try(s.toDouble).toOption
}
implicit val booleanParser: Parser[Boolean] = new Parser[Boolean] {
def apply(s: String): Option[Boolean] = Try(s.toBoolean).toOption
}
implicit def caseClassParser[A, R <: HList](implicit gen: Generic[A] { type Repr = R },
reprParser: Parser[R]): Parser[A] =
new Parser[A] {
def apply(s: String): Option[A] = reprParser.apply(s).map(gen.from)
}
object Parser {
def apply[A](s: String)(implicit parser: Parser[A]): Option[A] = parser(s)
}
implicit val stringParser: Parser[String] = new Parser[String] {
def apply(s: String): Option[String] = Some(s)
}
implicit val intParser: Parser[Int] = new Parser[Int] {
def apply(s: String): Option[Int] = Try(s.toInt).toOption
}
implicit val doubleParser: Parser[Double] = new Parser[Double] {
def apply(s: String): Option[Double] = Try(s.toDouble).toOption
}
I was curious to try to get a Parser[X] where X is a case class with a Person argument, i.e. a case class:
case class PersonWrapper(person: Person, x: Int)
Yet I get an error:
scala> Parser[PersonWrapper]("kevin,66,42")
<console>:15: error: diverging implicit expansion for type net.Test.Parser[net.Test.PersonWrapper]
starting with method caseClassParser in object Test
Parser[PersonWrapper]("kevin,66,42")
^
First, why does this divergent implicit error occur?
Secondly, is it possible to use the above code to get a Parser[PersonWrapper]?

Answer 1

Secondly, is it possible to use the above code to get a Parser[PersonWrapper]?
No, just skip to the end of the article:
scala> case class BookBook(b1: Book, b2: Book)
defined class BookBook
scala> Parser[BookBook]("Hamlet,Shakespeare")
res7: Option[BookBook] = None
Our format doesn’t support any kind of nesting (at least we haven’t said anything about nesting, and it wouldn’t be trivial), so we don’t actually know how to parse a string into a BookBook...
The problem is that cell in case cell +: rest will only ever be a string with no commas that is passed to implicitly[Parser[H]].apply(cell). For PersonWrapper, this means that the very first cell will attempt to do this:
implicitly[Parser[PersonWrapper]].apply("kevin")
Which will obviously fail to parse. In order to get nested parsers to work, you would need some way to group the cells together prior to applying a Parser[H] to them.