无形 HList 到 TupleN，其中元组形状不需要与 HList 形状完全匹配

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 l 中恰好有一个 N 值组合可以从中创建元组时才能编译。其他任何东西都应该产生编译时错误。应考虑可用的隐式转换。请注意，l 的大小或其中值的顺序没有限制。

示例：

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

这个问题来自 to the following 。这个问题中更通用的机制可用于创建数据结构和使用 FunctionN#tupled.

调用任何标准函数（其参数类型不同）

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

我无法按照您想要的方式进行目标类型推断，但作为补偿，我已经通过 shapeless 的 Generic、

泛化到任意产品类型

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

如果我们说我们有类型 A 和 B <: A 然后选择 A 来自A :: B :: HNil 是不明确的，因为两个元素都符合 A。这可以通过在 hcons、

的先前定义中的 witnesses 添加 SubtypeUnifier 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

我们还可以提供一种统一语义，它优先考虑完全匹配，然后回退到您更新后的问题中所述的唯一子类型。我们通过组合上述两个解决方案的实例来做到这一点：第一个正常优先级的精确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

shapeless HList to TupleN where the tuple shape need not exactly match the HList shape

scala

tuples

hlist

shapeless

更新 1

更新 2