scala - 从字符串构造简单的 Scala 案例类，严格没有样板

Question

我寻求简洁的代码来从字符串(例如 csv 行)初始化简单的 Scala 案例类:

case class Person(name: String, age: Double)
case class Book(title: String, author: String, year: Int)
case class Country(name: String, population: Int, area: Double)

val amy = Creator.create[Person]("Amy,54.2")
val fred = Creator.create[Person]("Fred,23")
val hamlet = Creator.create[Book]("Hamlet,Shakespeare,1600")
val finland = Creator.create[Country]("Finland,4500000,338424")

什么是最简单的 Creator反对这样做？通过看到一个很好的解决方案，我会学到很多关于 Scala 的知识。

(注意伴随对象 Person 、 Book 和 Country 不应该被强制存在。那将是样板!)

Answer 1

我将使用 Shapeless 给出一个解决方案，该解决方案在给定一些关于类型安全性的合理约束(没有运行时异常、没有运行时反射等)的情况下尽可能简单。对于泛型推导:

import scala.util.Try
import shapeless._

trait Creator[A] { def apply(s: String): Option[A] }

object Creator {
  def create[A](s: String)(implicit c: Creator[A]): Option[A] = c(s)

  def instance[A](parse: String => Option[A]): Creator[A] = new Creator[A] {
    def apply(s: String): Option[A] = parse(s)
  }

  implicit val stringCreate: Creator[String] = instance(Some(_))
  implicit val intCreate: Creator[Int] = instance(s => Try(s.toInt).toOption)
  implicit val doubleCreate: Creator[Double] =
    instance(s => Try(s.toDouble).toOption)

  implicit val hnilCreator: Creator[HNil] =
    instance(s => if (s.isEmpty) Some(HNil) else None)

  private[this] val NextCell = "^([^,]+)(?:,(.+))?$".r

  implicit def hconsCreate[H: Creator, T <: HList: Creator]: Creator[H :: T] =
    instance {
      case NextCell(cell, rest) => for {
        h <- create[H](cell)
        t <- create[T](Option(rest).getOrElse(""))
      } yield h :: t
      case _ => None
    }

  implicit def caseClassCreate[C, R <: HList](implicit
    gen: Generic.Aux[C, R],
    rc: Creator[R]
  ): Creator[C] = instance(s => rc(s).map(gen.from))
}

这完全按照指定的方式工作(但请注意，这些值包含在 Option 中以表示解析操作可能失败的事实):

scala> case class Person(name: String, age: Double)
defined class Person

scala> case class Book(title: String, author: String, year: Int)
defined class Book

scala> case class Country(name: String, population: Int, area: Double)
defined class Country

scala> val amy = Creator.create[Person]("Amy,54.2")
amy: Option[Person] = Some(Person(Amy,54.2))

scala> val fred = Creator.create[Person]("Fred,23")
fred: Option[Person] = Some(Person(Fred,23.0))

scala> val hamlet = Creator.create[Book]("Hamlet,Shakespeare,1600")
hamlet: Option[Book] = Some(Book(Hamlet,Shakespeare,1600))

scala> val finland = Creator.create[Country]("Finland,4500000,338424")
finland: Option[Country] = Some(Country(Finland,4500000,338424.0))

Creator这是一个类型类，它提供了我们可以将字符串解析为给定类型的证据。我们必须为基本类型提供显式实例，例如 String , Int等，但我们可以使用 Shapeless 来泛型派生案例类的实例(假设我们有 Creator 实例用于它们的所有成员类型)。