如何应用隐式参数的隐式转换？

Question

我想在SQL方法参数中隐式地将连接转换为JDBC连接的隐式连接参数。我有这段代码，它会抛出编译错误。

class JDBCConnection

class Connection(val connection: JDBCConnection)
object Connection {
  implicit def Connection2JDBCConnection(connection: Connection) = connection.connection
}

object DB {
  def withTransaction[A](block: (Connection => A)) = block(new Connection(new JDBCConnection))
}

object Main {
  def SQL(query: String)(implicit connection: JDBCConnection) = println("SQL:" + query)

  def main(args: Array[String]) = {
    DB.withTransaction { implicit connection =>
      SQL("Hello world")
    }
  }
}



Error:(20, 10) could not find implicit value for parameter connection: JDBCConnection
      SQL("Hello world")
         ^
Error:(20, 10) not enough arguments for method SQL: (implicit connection: JDBCConnection)Unit.
Unspecified value parameter connection.
      SQL("Hello world")

我怎么才能解决这个问题？

我试图使用一个参数作为隐式，但仍然得到一个编译错误。

class Connection(val connection: JDBCConnection)
object Connection {
  implicit def Connection2JDBCConnection(implicit connection: Connection) = connection.connection
}


Error:(20, 10) not enough arguments for method SQL: (implicit connection: JDBCConnection)Unit.
Unspecified value parameter connection.
      SQL("Hello world")
         ^
Error:(20, 10) could not find implicit value for parameter connection: JDBCConnection
      SQL("Hello world")
         ^

Answer 1

只有在显式使用Connection时，才会使用带有显式参数的隐式转换。如果您希望它隐式地工作，请让您的Connection2JDBCConnection接受一个implicit connection: Connection。