如何隐式找出无形HList开头的类型 [英] How to implicitly figure out the type at the head of a shapeless HList

问题描述

让我说以下内容:

case class TestField(value: String)
case class TestField2(value: String)

implicit class ProductExtensions[T <: Product](val value T) extends AnyVal {

  def mapTo[R <: Product](implicit tGen: Generic.Aux[T, String :: HNil], rGen: Generic.Aux[R, String :: HNil]: R = ???

}

val testField2 = TestField("my value").mapTo[TestField2]
// TestField2("my value")

是否可以生成" mapTo 函数以用于除 String 之外的其他类型，而无需指定类型?

Can I "genersize" the mapTo function to work for types other than String without having to specify the type?

请注意 TestField 或 TestField2 实现AnyVal(我也不希望它们这样做)，所以我不能使用 Unwrapped .

Note TestField nor TestField2 implement AnyVal (nor do I want them to), so I can't use Unwrapped.

修改

@Dmytro_Mitin答案在上面的示例中有效，但是如果我将示例扩展到此:

@Dmytro_Mitin answer works in my example above, but if I extend the example to this:

implicit class ProductExtensions[T <: Product](val value T) extends AnyVal {

  def mapTo[R <: Product](implicit tGen: Generic.Aux[T, String :: HNil], rGen: Generic.Aux[R, String :: HNil], o: OtherImplicit[String]): R = ???

}

...所以我有点想让它工作(但没有):

...so I am kind of looking for this to work (but it doesn't):

implicit class ProductExtensions[T <: Product, U](val value T) extends AnyVal {

  def mapTo[R <: Product](implicit tGen: Generic.Aux[T, U :: HNil], rGen: Generic.Aux[R, U :: HNil], o: OtherImplicit[U]): R = ???

}

有什么想法吗?

推荐答案

此处是通用版本

implicit class ProductExtensions[T <: Product, L <: HList](val value: T) extends AnyVal {
  def mapTo[R <: Product](implicit tGen: Generic.Aux[T, L], rGen: Generic.Aux[R, L]): R = rGen.from(tGen.to(value))
}

《无形型宇航员指南》.6.3案例研究:案例类迁移 https://books.underscore.io/shapeless-guide/shapeless-guide.html#sec:ops:migration

The Type Astronaut’s Guide to Shapeless. 6.3 Case study: case class migrations https://books.underscore.io/shapeless-guide/shapeless-guide.html#sec:ops:migration

新版本

import shapeless.ops.hlist.IsHCons

implicit class ProductExtensions[T <: Product, L <: HList, U, L1 <: HList](val value: T) extends AnyVal {
  def mapTo[R <: Product](implicit 
                          tGen: Generic.Aux[T, L], 
                          rGen: Generic.Aux[R, L], 
                          isHCons: IsHCons.Aux[L, U, L1], 
                          o: OtherImplicit[U]
                         ): R = rGen.from(tGen.to(value))
}

4.3链接依赖函数 https://books.underscore.io/shapeless-guide/shapeless-guide.html#sec:type-level-programming:chaining

4.3 Chaining dependent functions https://books.underscore.io/shapeless-guide/shapeless-guide.html#sec:type-level-programming:chaining

找不到Scala无形状的Generic.Aux隐式参数取消申请

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