如何使用 Shapeless 从对象中获取 Option[T]

Posted 2023-03-23

【中文标题】如何使用 Shapeless 从对象中获取 Option[T]

【英文标题】：How to get Option[T] from object using Shapeless

【发布时间】：2021-11-18 08:03:42

【问题描述】：

我试图在getAuthor 方法中从book 获取Option[Author]：

  import shapeless.ops.record._

  case class Book(title: String, author: Option[Author])
  case class Author(name: String)
  val book = Book("Types and Programming Languages", Option(Author("Benjamin Pierce")))

  val w = Witness('author)
  def getAuthor[T1, T2 <: HList, T3](t: T1)(implicit gen: LabelledGeneric.Aux[T1, T2], s: Selector.Aux[T2, w.T, Option[T3]]): s.Out = 
    val repr = gen.to(t)
    val opt = s.apply(repr)
    opt.foreach 
      ??? // do something
    
    opt
  

  val author = getAuthor(book)
  println(author)

但此代码会导致以下编译错误：

could not find implicit value for parameter s: shapeless.ops.record.Selector.Aux[T2,shapeless.DuckTyping.w.T,Option[T3]] (No field shapeless.DuckTyping.w.T in record T2)
  val author = getAuthor(book)

我想避免编译错误并将Option[T]指定为Selector#apply返回值。

有人可以帮忙吗？

【参考方案1】：

你似乎过度约束了隐式。

尝试修改方法签名

def getAuthor[T1, T2 <: HList, T3](t: T1)(implicit
  gen: LabelledGeneric.Aux[T1, T2],
  s: Selector.Aux[T2, w.T, T3],
  ev: T3 <:< Option[_]
): s.Out /* T3 */

HList foldLeft with tuple as zero

Why is this implicit resolution failing?

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