scala – 如何使用一元类型构造函数推断Shapeless记录值的内部类

我无法理解Shapeless记录选择器与 scala类型推断交互的方式.我正在尝试创建一个方法,可以通过键从Shapeless记录中获取字段,只有当字段的值具有某个一元类型构造函数时,在这种特殊情况下Vector [_],然后获取内部值推断出Vector中的V型,在本例中是Vector.apply().

我觉得我很亲密.这是有效的,具有Int的具体内部类型：

val record = ( "a" ->> Vector(0,2,4) ) :: ( "b" ->> Set(1,3,5) ) :: HNil

def getIntFromVectorField[L <: HList](l: L,fieldName:Witness,index:Int)(implicit 
  sel: Selector.Aux[L,fieldName.T,Vector[Int]]
):Int = l(fieldName).apply(index)

getIntFromVectorField(record,"a",1) // Returns 1
getIntFromVectorField(record,"b",0) // Does not compile,as intended

但如果我试图推断内部类型,它会失败：

def getValueFromVectorField[L <: HList,V](l:L,Vector[V]]
):V = l(fieldName).apply(index) // Compiles
getValueFromVectorField(record,1) // Why does this not compile?

这是完整的错误：

could not find implicit value for parameter sel: 
shapeless.ops.record.Selector[shapeless.::[scala.collection.immutable.Vector[Int] 
with shapeless.labelled.KeyTag[String("a"),scala.collection.immutable.Vector[Int]],shapeless.::[scala.collection.immutable.Set[Int] 
with shapeless.labelled.KeyTag[String("b"),scala.collection.immutable.Set[Int]],shapeless.HNil]],String("a")]{type Out = scala.collection.immutable.Vector[V]}

我能做的是这样的：

def getValueFromVectorField[L <: HList,T,T],unpack: Unpack1[T,Vector,V]
):V = l(fieldName) match { 
  case v:Vector[V] => v.apply(index) 
}
getValueFromVectorField(record,1) // Returns 1,Yay!
getValueFromVectorField(record,as intended

哪个应该是安全的,是吗？但模式匹配对于无形而言并不是非常惯用,我想知道为什么更简洁的推理方法不起作用.有更清洁的方法吗？

import shapeless._,ops.record.Selector,syntax.singleton._

def getValueFromVectorField[L <: HList,VS,V](
  l: L,fieldName: Witness,index: Int
)(implicit
  sel: Selector.Aux[L,VS],ev: VS <:< Vector[V]
): V = sel(l).apply(index)

val record = ( "a" ->> Vector(0,5) ) :: HNil

getValueFromVectorField(record,as intended