的案例類瓦特/箱類參數

Question

鑑於發散Implicits從以下特拉維斯布朗的教育和良好的書面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 
    } 

我很好奇，試圖得到一個Parser[X]其中X是一個案例類與Person的說法，即一個案例類：

case class PersonWrapper(person: Person, x: Int) 

然而，我得到一個錯誤：

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") 
          ^

首先，爲什麼會出現這種發散的隱式錯誤？

其次，是否可以使用上面的代碼來獲得Parser[PersonWrapper]？

Answer 1

Secondly, is it possible to use the above code to get a Parser[PersonWrapper]?

沒有，只是跳到本文的結尾：

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...

的問題是，在cell將case cell +: rest永遠只能是沒有傳遞到implicitly[Parser[H]].apply(cell)逗號的字符串。對於PersonWrapper，這意味着第一個單元格將試圖做到這一點：

implicitly[Parser[PersonWrapper]].apply("kevin") 

這顯然無法解析。爲了使嵌套的解析器正常工作，在將Parser[H]應用於它們之前，需要一些方法將這些單元分組在一起。