带有类型参数的case类的模式匹配不起作用

Question

我在相应的伴随对象中有一些带有unapply的case类，我将其用于模式匹配，如下所示：

trait DAO [T] {
  def create: Int
  def insert(model: List[T]): Int
  def update(id: String, name: String): Int
  def delete(id: String): Int
  def all: List[T]
  def find(id: String): List[T]
  def findSome(id: String): List[T]
}
sealed trait Service[A]
final case class Create[A](implicit val dao: DAO[A]) extends Service[Int]
final case class Insert[A](model: List[A], implicit val dao: DAO[A]) extends Service[Int]
object Create_{
  def unapply[A](in: Create [A])= Some(in.dao)
}
object Insert_ {
  def unapply[A](in: Insert[A]) = Some((in.model,in.dao))
}
case class F(id:String)
implicit val abc = new DAO[F] {
  def create = 1
  def insert(model: List[F]) = 1
  def update(id: String, name: String) = 1
  def delete(id: String) = c1
  def all = List(F("Test"))
  def find(id: String) = List(F("Test"))
  def findSome(id: String) = List(F("Test"))
   }
def b[A](in: Service[A]) = {
  in match {
    case Create_(dao) => dao.create
    case Insert_(model, dao) => dao.insert(model)
  }
}

当我加载上述文件时，REPL会抱怨以下错误：

scala> :load ..../tools/scala/macros/test.scala
Loading ...../tools/scala/macros/test.scala...
defined trait DAO
defined trait Service
defined class Create
defined class Insert
defined class F
defined object Create_
defined object Insert_
abc: DAO[F] = $anon$1@3e776a48
<console>:65: error: type mismatch;
found   : List[Any]
required: List[A] where type +A
case  Insert_(model, dao) => dao.insert(model)

它不可编译。

Answer 1

问题就在这里：

Insert[+A,B](model: List[A], implicit val dao:DAO[B])

您允许模型和DAO的列表元素使用不同的类型，但DAO需要一种类型：

  def insert(model:List[T]):Int

您可以通过要求插入的类型相同来解决此问题。然后保证为您的DAO传递正确类型的模型。

Insert[A](model: List[A], implicit val dao:DAO[A])

Answer 2

剩下的问题是case class Create[A](implicit val dao: DAO[A])与case class Create[A]()(implicit val dao: DAO[A])相同。所以case Create(dao)是非法的，它应该是case Create()。使用case Create_(dao)或从Create类定义中删除implicit val。

Answer 3

解决方案是

 def b[A](in: Service[A]) = {
  in match {
    case Create_(dao) => dao.create
    case Insert_(model:List[A], dao:DAO[A]) => dao.insert(model)
 }

仍然有警告！！！

 scala> :load .../tools/scala/macros/Test.scala
 Loading .../tools/scala/macros/Test.scala...
 defined trait DAO
import scalaz.{Free, $tilde$greater, Id, Coyoneda}
defined trait Service
defined trait Command
defined type alias Functor
defined class Create
defined class Insert
defined class Execute
defined class F
defined object Create_
defined object Insert_
defined object Execute_
lift: [A](service: Service[A])scalaz.Free[Functor,A]
abc: DAO[F] = $anon$1@2a071108
<console>:219: warning: abstract type pattern A is unchecked since it is   eliminated by erasure
            case  Insert_(model:List[A], dao:DAO[A]) => dao.insert(model)
                                ^
 <console>:219: warning: abstract type A in type pattern DAO[A] is unchecked since it is eliminated by erasure
            case  Insert_(model:List[A], dao:DAO[A]) => dao.insert(model)
                                       ^
<console>:217: warning: match may not be exhaustive.
It would fail on the following input: Insert(_, _)
          in match {
             ^
 b: [A](in: Service[A])Int