如何降低约束HList

Question

如何修复下面的代码以使其工作？

object Foo {
  object sum extends Poly {
    implicit def caseFoo = use((f1: Int, f2: Int) => f1 + f2)
  }

  def foo[L <: HList : <<:[Int]#λ](l: L): Int = {
    l.reduceLeft(sum)
    // Error: could not find implicit value for parameter reducer: shapeless.ops.hlist.LeftReducer[L,com.struct.Foo.sum.type] 
    //   l.reduceLeft(sum)
    // Error: not enough arguments for method reduceLeft: (implicit reducer: shapeless.ops.hlist.LeftReducer[L,com.struct.Foo.sum.type])reducer.Out. 
    //   Unspecified value parameter reducer.
  }
}

另外。如何改变它，以减少任何数字的HList？

更新：

请考虑我的问题的更充分的例子：

import shapeless._

trait Bar {
  def foo: Int
}
case class Foo[L <: HList](l: L) extends Bar {
  object sum extends Poly {
    implicit def caseFoo[A: Numeric] = use((f1: A, f2: A) => f1 + f2)
    //Error: type mismatch;
    //found   : A
    //required: String
  }
  override def foo(implicit reducer: LeftReducer[L, sum.type]): Int = reducer(l)
  //Error: type mismatch;
  //found   : reducer.Out
  //required: Int
}

更新：

package com.test

import shapeless.ops.hlist.LeftReducer
import shapeless.{HList, HNil, Poly}

trait Bar {
  def foo: Int
}

case class Foo[L <: HList](list: L) extends Bar {
  import Numeric.Implicits._
  object sum extends Poly {
    implicit def caseFoo[A: Numeric] = use((f1: A, f2: A) => f1 + f2)
  }

  class MkFoo[T] {
    def apply(l: L)(implicit reducer: LeftReducer.Aux[L, sum.type, T]): T = reducer(l)
  }

  override def foo: Int = (new MkFoo[Int]())(list)
  // Error: could not find implicit value for parameter reducer: shapeless.ops.hlist.LeftReducer.Aux[L,Foo.this.sum.type,Int]

}

object Testing {
  def main(args: Array[String]): Unit = {
    val b:Bar  = Foo(1 :: 2 :: 3 :: HNil)
    println(b.foo)
  }

}

Answer 1

这工作：

package com.test

import shapeless.{::, Generic, HList, HNil, Lazy}


trait Bar {
  def foo: Int
}

case class Foo[L <: HList](list: L)(implicit ev: SizeCalculator[L]) extends Bar {
  override def foo: Int = ev.size(list)
}

object Testing {
  def main(args: Array[String]): Unit = {
    val b: Bar = Foo(1 :: 2 :: 3 :: HNil)
    println(b.foo)
  }
}


sealed trait SizeCalculator[T] {
  def size(value: T): Int
}

object SizeCalculator {

  // "Summoner" method
  def apply[A](implicit enc: SizeCalculator[A]): SizeCalculator[A] = enc

  // "Constructor" method
  def instance[A](func: A => Int): SizeCalculator[A] = new SizeCalculator[A] {
    def size(value: A): Int = func(value)
  }

  import Numeric.Implicits._

  implicit def numericEncoder[A: Numeric]: SizeCalculator[A] = new SizeCalculator[A] {
    override def size(value: A): Int = value.toInt()
  }

  implicit def hnilEncoder: SizeCalculator[HNil] = instance(hnil => 0)

  implicit def hlistEncoder[H, T <: HList](
                                            implicit
                                            hInstance: Lazy[SizeCalculator[H]],
                                            tInstance: SizeCalculator[T]
                                          ): SizeCalculator[H :: T] = instance {
    case h :: t =>
      hInstance.value.size(h) + tInstance.size(t)
  }

  implicit def genericInstance[A, R](
                                      implicit
                                      generic: Generic.Aux[A, R],
                                      rInstance: Lazy[SizeCalculator[R]]
                                    ): SizeCalculator[A] = instance { value => rInstance.value.size(generic.to(value)) }


  def computeSize[A](value: A)(implicit enc: SizeCalculator[A]): Int = enc.size(value)
}

Answer 2

从某种意义上说，LUBConstraint.<<:类型是非建设性的，因为很明显，所有HList成员都有某种类型，但不能从中派生任何东西。

要使用reduceLeft方法，需要专门的LeftReducer操作提供程序。您可以在您的方法中直接要求它。

import shapeless._, ops.hlist._

object Foo {
  import Numeric.Implicits._
  object sum extends Poly {
    implicit def caseFoo[A: Numeric] = use((f1: A, f2: A) => f1 + f2)
  }

  def foo[L <: HList](l: L)(implicit reducer: LeftReducer[L, sum.type]): reducer.Out = 
    reducer(l)
}


Foo.foo(1 :: 2 :: 3 :: HNil) // res0: Int = 6

Foo.foo(1.0 :: 2.0 :: 3.0 :: HNil) // res1: Double = 6.0

更新

如果您需要直接的证据，说明您的结果将是某种结果类型，则可以使用附加的类型参数，但这需要通过类型化的Maker模式分离类型参数。

object Foo {
  import Numeric.Implicits._
  object sum extends Poly {
    implicit def caseFoo[A: Numeric] = use((f1: A, f2: A) => f1 + f2)
  }

  def foo[T] = new MkFoo[T]

  class MkFoo[T] {
    def apply[L <: HList](l: L)(implicit reducer: LeftReducer.Aux[L, sum.type, T]): T = reducer(l)
  }
}

现在

Foo.foo[Int](1 :: 2 :: 3 :: HNil)
Foo.foo[Double](1.0 :: 2.0 :: 3.0 :: HNil)

仍然会产生正确的结果，而

Foo.foo[Double](1 :: 2 :: 3 :: HNil)

将在编译时失败。