[英]Map the types of a shapeless HList
我一直在嘗試從scala的shapeless
包中映射HList
的類型,而無法訪問它們的值。
以下成功映射了HList的值
import shapeless._
import shapeless.Poly._
import ops.hlist.Mapper
import ops.hlist.Mapper._
trait Person {
type Value
val v : Value
}
case class StringPerson extends Person {
type Value = String
val v = "I like strings"
}
case class IntPerson extends Person {
type Value = Int
val v = 42
}
object what_is_going_on {
object test_value_op {
val stringPerson = StringPerson()
val intPerson = IntPerson()
trait lpvfun extends Poly1 {
implicit def default[A <: Person] = at[A](_.v)
}
object vfun extends lpvfun {}
// Use these to generate compiler errors if the mapped type is not what we'd expect:
type TestListType = StringPerson :: IntPerson :: HNil
type TestListExpectedMappedType = String :: Int :: HNil
// Input:
val testList : TestListType = stringPerson :: intPerson :: HNil
// Output:
val mappedList : TestListExpectedMappedType = testList map vfun
// Get the actual mapped type
type TestListActualMappedType = mappedList.type
// This compiles......
val mappedList1 : TestListActualMappedType = mappedList
// .... but weirdly this line doesn't. That isn't the point of this question, but I'd be very grateful for an answer.
//implicitly[TestListActualMappedType =:= TestListExpectedMappedType]
}
}
涼! 除了由於某種原因不能implicitly[A =:= B]
使用implicitly[A =:= B]
,還已經映射了HList
的值,因此也映射了它們的類型。
現在,假設我們沒有HList
值,但是我們知道它的類型。 我們如何映射其類型?
我根據此處 map
的定義嘗試了以下方法:
object test_type_op {
type TestListType = StringPerson :: IntPerson :: HNil
type TestListExpectedMappedType = String :: Int :: HNil
// Attempt 1 does not work, compiler cannot prove =:=
type MappedType = Mapper[vfun.type, TestListType]#Out
implicitly[MappedType =:= TestListExpectedMappedType]
// Attempt 2 does not work, compiler cannot prove =:=
class GetMapper {
implicit val mapper : Mapper[vfun.type, TestListType]
implicitly[mapper.Out =:= TestListExpectedMappedType]
}
}
如何在不訪問其值的情況下獲取映射的HList
的類型? 有沒有一種調試方法,為什么編譯器無法證明某些內容? 謝謝您的閱讀。
在TestListActualMappedType
的情況下,您獲得了mappedList
的單例類型,這與推斷的mappedList
類型mappedList
。 您可以看到完全相同的問題,而無需涉及Shapeless:
scala> val x = "foo"
x: String = foo
scala> implicitly[x.type =:= String]
<console>:13: error: Cannot prove that x.type =:= String.
implicitly[x.type =:= String]
^
您可以要求提供證據,證明x.type
是String
的子類型,或者可以使用shapeless.test.typed
,在您的情況下看起來像這樣:
import shapeless._, ops.hlist.Mapper
trait Person {
type Value
val v : Value
}
case class StringPerson() extends Person {
type Value = String
val v = "I like strings"
}
case class IntPerson() extends Person {
type Value = Int
val v = 42
}
trait lpvfun extends Poly1 {
implicit def default[A <: Person] = at[A](_.v)
}
object vfun extends lpvfun {}
val stringPerson = StringPerson()
val intPerson = IntPerson()
val testList = stringPerson :: intPerson :: HNil
val mappedList = testList map vfun
shapeless.test.typed[String :: Int :: HNil](mappedList)
不過,這與真正地指定類型相比並沒有多大好處。
您可以要求提供證據,證明諸如Mapper
類的類型類的輸出類型是您期望的特定輸入類型的類型:
scala> val m = Mapper[vfun.type, StringPerson :: IntPerson :: HNil]
m: shapeless.ops.hlist.Mapper[vfun.type,shapeless.::[StringPerson,shapeless.::[IntPerson,shapeless.HNil]]]{type Out = shapeless.::[String,shapeless.::[Int,shapeless.HNil]]} = shapeless.ops.hlist$Mapper$$anon$5@6f3598cd
scala> implicitly[m.Out =:= (String :: Int :: HNil)]
res1: =:=[m.Out,shapeless.::[String,shapeless.::[Int,shapeless.HNil]]] = <function1>
這更可能有用,但同樣取決於您要說服自己的確切方式。
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