为什么这个 Scala 隐式转换不起作用？

Question

更新：我已经接受了答案，但我仍然很好奇为什么我尝试的方法不起作用（以了解 Scala 隐式行为）。任何其他答案将不胜感激。

（希望这个问题在没有太多 Circumflex 知识的情况下可以回答，但以防万一，here's 记录的源代码参考。）

我正在尝试在 Circumflex ORM 库上添加一些便利功能，但在尝试使用 Scala 隐式转换时遇到了一些障碍。下面，为什么隐式转换不触发？我怀疑与子类化和/或递归类型参数存在一些复杂的交互。

import ru.circumflex.orm._

// I subclass Record and Table to add my own convenience methods etc. (not pasted, irrelevant)
abstract class XRecord[PK, R <: XRecord[PK, R]] extends Record[PK, R] { this: R => }
trait XTable[PK, R <: XRecord[PK, R]] extends Table[PK, R] { this: R => }

// Example entity.
class Org extends XRecord[Long,Org] {
  val id = "id".BIGINT.NOT_NULL.AUTO_INCREMENT
  def PRIMARY_KEY = id
  def relation = Org
}
object Org extends Org with XTable[Long,Org]

object Test extends App {
  // I want this conversion to work for all Records, not just XRecords.
  // Need implicit f to be able to accept XRecord, a subclass of Record.
  implicit def toRichRelationNode[PK, R <: Record[PK,R], RR](xs: RR)(implicit f: RR => RelationNode[PK,R]) =
    new { def GET(f: RelationNode[PK,R] => Predicate) = 0 }

  // This works.
  toRichRelationNode(Org) GET (_.id EQ 1)

  // This doesn't:
  // "No implicit view available from Org.type => ru.circumflex.orm.RelationNode[PK,R]."
  Org GET (_.id EQ 1)
}

Answer 1

坦率地说，我从来没有使用过隐式参数，也从来没有遇到过强迫我使用它们的条件（Circumflex 本身没有隐式参数，就我而言，效果很好）。

无论如何，我已经能够重现您的场景并使其发挥作用。然而，它需要 2 次隐式才能使一切顺利。代码如下：

// The helper which holds custom methods, it will accept any subclass of R
class NodeHelper[PK, R <: Record[PK, R]](val node: RelationNode[PK, R]) {
  def GET(f: R => Predicate): Option[R] = node.criteria.add(f(node)).unique()
} 

// Now the implicits and usage scenario

object Tester {

  implicit def nodeToHelper[PK, R <: Record[PK, R]](node: RelationNode[PK, R]): NodeHelper[PK, R] = new NodeHelper(node)

  implicit def tableToHelper[PK, R <: Record[PK, R]](table: Table[PK, R]): NodeHelper[PK, R] = new NodeHelper(table.AS("this"))

  // Testing with table
  println(User GET (_.cn EQ "patrick"))
  // Testing with node
  println(User.AS("u") GET (_.cn EQ "patrick"))
}

// And finally, the test model we've mentioned above

class User extends Record[Long, User] {
  def PRIMARY_KEY = id
  val id = "id".BIGINT.NOT_NULL.AUTO_INCREMENT
  val cn = "cn".TEXT.NOT_NULL
  def relation = User
}

object User extends User with Table[Long, User]

希望对你有帮助。