Question

我想创建相当于：

def toTupleN[A1, ..., AN, L <: HList](l: L): TupleN[A1, ..., AN]

使用toTupleN的代码只有在N中只有一个l值组合才可以编译时才能编译。其他任何东西都应该生成编译时错误。应考虑可用的隐式转换。请注意，l的大小或其中的值排序没有限制。

实施例：

val l = 23 :: (1, "wibble") :: (2, "wobble") :: "foo" :: HNil
// l: shapeless.::[Int,shapeless.::[(Int, String),shapeless.::[(Int, String),shapeless.::[String,shapeless.HNil]]]] = 23 :: (1,wibble) :: (2,wobble) :: foo :: HNil

val t2: (String, Int) = toTuple2(l)
// t2: (String, Int) = (foo,23)

val nope: (String, String) = toTuple2(l)
// Compiler error because no combination of l's values can create nope

val nein: ((Int, String)) = toTuple2(l)
// Another compiler error because there is more than one way l's values can create nein

此问题来自answer到以下question。此问题中更通用的机制可用于创建数据结构并使用FunctionN#tupled调用任何标准函数（其参数属于不同类型）。

更新

使用子类型定义所需行为的一些示例：

trait A
trait B extends A
trait C extends A

val a: A
val b: B
val c: C

toTuple2[(A, Int)](5 :: b :: HNil)      // (b, 5): subtypes match supertypes when there is no exact match
toTuple2[(A, Int)](5 :: b :: a :: HNil) // (a, 5): only one exact match is available
toTuple2[(A, Int)](5 :: a :: a :: HNil) // compile error: more than one exact match is available
toTuple2[(A, Int)](5 :: b :: c :: HNil) // compile error: more than one inexact match is available

Answer 1

我还没有能够按照您想要的方式进行目标类型推断工作，但作为补偿，我通过无形的Generic推广到任意产品类型，

import shapeless._, ops.hlist._, test._

object Demo {
  trait UniqueSelect[L <: HList, M <: HList] {
    def apply(l: L): M
  }

  object UniqueSelect {
    implicit def hnil[L <: HList]: UniqueSelect[L, HNil] =
      new UniqueSelect[L, HNil] {
        def apply(l: L): HNil = HNil
      }

    implicit def hcons[L <: HList, H, T <: HList, S <: HList]
      (implicit
        pt: Partition.Aux[L, H, H :: HNil, S],
        ust: UniqueSelect[S, T]
      ): UniqueSelect[L, H :: T] =
      new UniqueSelect[L, H :: T] {
        def apply(l: L): H :: T = {
          val (h :: HNil, s) = pt(l)
          h :: ust(s)
        }
      }
  }

  def toProductUniquely[P <: Product] = new ToProductUniquely[P]
  class ToProductUniquely[P <: Product] {
    def apply[L <: HList, M <: HList](l: L)
      (implicit gen: Generic.Aux[P, M], up: UniqueSelect[L, M]): P =
        gen.from(up(l))
  }

  val l = 23 :: (1, "wibble") :: (2, "wobble") :: "foo" :: HNil

  val t2 = toProductUniquely[(String, Int)](l)
  typed[(String, Int)](t2)
  assert(t2 == ("foo", 23))

  illTyped("""
  toProductUniquely[(String, String)](l)
  """)

  illTyped("""
  toProductUniquely[Tuple1[(Int, String)]](l)
  """)
}

更新1

如果我们说类型为A和B <: A，那么从{{A中选择A :: B :: HNil时，添加对所请求类型的子类型满足的选择的支持是相当简单的1}}含糊不清，因为两个元素都符合A。这可以通过在SubtypeUnifier的先前定义中向证人添加hcons来完成，

import shapeless._, ops.hlist._, test._

object Demo extends App {
  trait UniqueSelect[L <: HList, M <: HList] {
    def apply(l: L): M
  }

  object UniqueSelect {
    implicit def hnil[L <: HList]: UniqueSelect[L, HNil] =
      new UniqueSelect[L, HNil] {
        def apply(l: L): HNil = HNil
      }

    implicit def hcons[L <: HList, M <: HList, H, T <: HList, S <: HList]
      (implicit
        su: SubtypeUnifier.Aux[L, H, M],
        pt: Partition.Aux[M, H, H :: HNil, S],
        upt: UniqueSelect[S, T]
      ): UniqueSelect[L, H :: T] =
      new UniqueSelect[L, H :: T] {
        def apply(l: L): H :: T = {
          val (h :: HNil, s) = pt(su(l))
          h :: upt(s)
        }
      }
  }

  def toProductUniquely[P <: Product] = new ToProductUniquely[P]
  class ToProductUniquely[P <: Product] {
    def apply[L <: HList, M <: HList](l: L)
      (implicit gen: Generic.Aux[P, M], up: UniqueSelect[L, M]): P =
        gen.from(up(l))
  }

  class A
  class B extends A
  class C

  val ac = new A :: new C :: HNil
  val bc = new B :: new C :: HNil
  val abc = new A :: new B :: new C :: HNil

  // Exact match
  val tac = toProductUniquely[(A, C)](ac)
  typed[(A, C)](tac)

  // Subtype
  val tbc = toProductUniquely[(A, C)](bc)
  typed[(A, C)](tbc)

  // Exact match again
  val tabc = toProductUniquely[(B, C)](abc)
  typed[(B, C)](tabc)

  // Ambiguous due to both elements conforming to A
  illTyped("""
  toProductUniquely[(A, C)](abc)
  """)
}

更新2

我们还可以提供统一语义，它优先选择完全匹配，然后回退到更新问题中描述的唯一子类型。我们通过组合上述两个解决方案中的实例来实现这一点：来自第一个正常优先级的完全匹配实例和低优先级的子类型匹配实例，

import shapeless._, ops.hlist._, test._

object Demo extends App {
  trait UniqueSelect[L <: HList, M <: HList] {
    def apply(l: L): M
  }

  object UniqueSelect extends UniqueSelect0 {
    implicit def hnil[L <: HList]: UniqueSelect[L, HNil] =
      new UniqueSelect[L, HNil] {
        def apply(l: L): HNil = HNil
      }

    implicit def hconsExact[L <: HList, H, T <: HList, S <: HList]
      (implicit
        pt: Partition.Aux[L, H, H :: HNil, S],
        upt: UniqueSelect[S, T]
      ): UniqueSelect[L, H :: T] =
      new UniqueSelect[L, H :: T] {
        def apply(l: L): H :: T = {
          val (h :: HNil, s) = pt(l)
          h :: upt(s)
        }
      }
  }

  trait UniqueSelect0 {
    implicit def hconsSubtype[L <: HList, M <: HList, H, T <: HList, S <: HList]
      (implicit
        su: SubtypeUnifier.Aux[L, H, M],
        pt: Partition.Aux[M, H, H :: HNil, S],
        upt: UniqueSelect[S, T]
      ): UniqueSelect[L, H :: T] =
      new UniqueSelect[L, H :: T] {
        def apply(l: L): H :: T = {
          val (h :: HNil, s) = pt(su(l))
          h :: upt(s)
        }
      }
  }

  def toProductUniquely[P <: Product] = new ToProductUniquely[P]
  class ToProductUniquely[P <: Product] {
    def apply[L <: HList, M <: HList](l: L)
      (implicit gen: Generic.Aux[P, M], up: UniqueSelect[L, M]): P = gen.from(up(l))
  }

  trait A
  trait B extends A
  trait C extends A

  val a: A = new A {}
  val b: B = new B {}
  val c: C = new C {}

  // (b, 5): subtypes match supertypes when there is no exact match
  toProductUniquely[(A, Int)](5 :: b :: HNil)

  // (a, 5): only one exact match is available
  toProductUniquely[(A, Int)](5 :: b :: a :: HNil)

  // compile error: more than one exact match is available
  illTyped("""
  toProductUniquely[(A, Int)](5 :: a :: a :: HNil)
  """)

  // compile error: more than one inexact match is available
  illTyped("""
  toProductUniquely[(A, Int)](5 :: b :: c :: HNil)
  """)
}

无形HList到TupleN，其中元组形状不需要与HList形状完全匹配

1 个答案:

更新1

更新2