推断的类型参数

Question

为什么在编译此代码段时出现错误？

trait ID[R <: Record[R] with KeyedRecord[Long]] { 

    this: R => 

    val idField = new LongField(this) 
} 

错误：

inferred type arguments [ID[R] with R] do not conform to class LongField's 
type parameter bounds [OwnerType <: net.liftweb.record.Record[OwnerType]] 

我该如何解决这个问题？

---

LongField定义：

class LongField[OwnerType <: Record[OwnerType]](rec: OwnerType) 
    extends Field[Long, OwnerType] with MandatoryTypedField[Long] with LongTypedField { 

Answer 1

转换

val idField = new LongField(this) 

到

val idField = new LongField[R](this) 

如果不指定类型R，那么LongField无法检查的典型e是否为Record[OwnerType]的共变体。明确提及它应该解决目的。

PS：我不知道其他类的声明再次确认，但下面的声明的工作原理：

case class Record[R] 
class KeyedRecord[R] extends Record[R] 
class LongField[OwnerType <: Record[OwnerType]](rec: OwnerType) 
trait ID[R <: Record[R] with KeyedRecord[Long]] { 
    this: R => 
    val idField = new LongField[R](this) 
}