Scala 如何在Poly1中操作字段类型?
我想在带标签的泛型上使用无形状contrib(+scalaz)的序列,但首先我需要映射字段类型 在那个例子中有可能创建缺少的f函数吗Scala 如何在Poly1中操作字段类型?,scala,shapeless,Scala,Shapeless,我想在带标签的泛型上使用无形状contrib(+scalaz)的序列,但首先我需要映射字段类型 在那个例子中有可能创建缺少的f函数吗 object TestLabelledGeneric { import shapeless._ import singleton._ val a = "name" ->> Option("hello") :: "y" ->> Option(1) :: HNil val b = Option("name" ->
object TestLabelledGeneric {
import shapeless._
import singleton._
val a = "name" ->> Option("hello") :: "y" ->> Option(1) :: HNil
val b = Option("name" ->> "hello") :: Option("y" ->> 1) :: HNil
val f = ???
val name = Witness('name); val age = Witness('age)
def assertTypedEquals[A](expected: A, actual: A): Unit = assert(expected == actual)
assertTypedEquals[b.type](b, a.map(f))
}
解决了,谢谢@travis brown
以下是在我的机器上运行的版本:
object TestLabelledGeneric {
import shapeless._
import singleton._
val a = "name" ->> Option("hello") :: "y" ->> Option(1) :: HNil
val b = Option("name" ->> "hello") :: Option("y" ->> 1) :: HNil
import labelled.{ FieldType, field }
object f extends Poly1 {
implicit def kv[K, V]: Case.Aux[
FieldType[K, Option[V]],
Option[FieldType[K, V]]
] =
at(_.map(field[K](_)))
}
// If I try to use Witness.`"name"`.T directly in Res, I have a "not accessible type" error
val name = Witness.`"name"`
val y = Witness.`"y"`
type Res = Option[FieldType[name.T, String]] :: Option[FieldType[y.T ,Int]] :: HNil
def assertTypedEquals[A](expected: A, actual: A): Unit = assert(expected == actual)
assertTypedEquals[Res](b, a.map(f))
}
您想将
字段类型[K,选项[V]]
转换为选项[FieldType[K,V]]]
FieldType[K,Option[V]]
是Option[V]
的一个子类型,您可以使用shapeless.labeled.field
将V
转换为FieldType[K,V]
然后您可以将此操作放入Poly1
:
import shapeless._, labelled.{ FieldType, field }, syntax.singleton._
object f extends Poly1 {
implicit def kv[K, V]: Case.Aux[
FieldType[K, Option[V]],
Option[FieldType[K, V]]
] =
at(_.map(field[K](_)))
}
这并不能完全满足您的需要,因为a.map(f)
的类型不是b.type
(这是b
的单例类型)。您可以确认它确实做了您想要的事情,不过:
scala> type Res =
| Option[FieldType[Witness.`"name"`.T, String]] ::
| Option[FieldType[Witness.`"y"`.T, Int]] :: HNil
defined type alias Res
scala> assertTypedEquals[Res](b, a.map(f))
是的,他们是一样的