继承和自递归类型推断

Question

（斯卡拉 2.11.8）

我有一个特征GenTableLike 具有复杂的自递归类型签名，它定义了用于连接兼容表实现的方法++。我也有一个层次结构GenTableLike >: KeyTable >: MapKeyTable。但是，++'ing 两个 MapKeyTable 无法推断自递归类型。

这里有一些简化的 sn-p，其中问题仍然重现：

trait GenTableLike[RKT,
                   CKT,
                   +A,
                   +SelfType[+A2] <: GenTableLike[RKT, CKT, A2, SelfType, TransposedType],
                   +TransposedType[+A2] <: GenTableLike[CKT, RKT, A2, TransposedType, SelfType]] {
  def ++[B >: A,
         T2 <: GenTableLike[RKT, CKT, B, ST, TT],
         ST[+A2] <: GenTableLike[RKT, CKT, A2, ST, TT],
         TT[+A2] <: GenTableLike[CKT, RKT, A2, TT, ST]](that: T2): SelfType[B] = ???
}

trait KeyTable[RKT, CKT, +A]
  extends GenTableLike[RKT,
                       CKT,
                       A,
                       KeyTable.Curried[RKT, CKT]#Self,
                       KeyTable.Curried[CKT, RKT]#Self]

object KeyTable {
  /** Type helper for defining self-recursive type */
  type Curried[RKT, CKT] = {
    type Self[+A] = KeyTable[RKT, CKT, A]
  }
}

class MapKeyTable[RKT, CKT, +A]
  extends KeyTable[RKT, CKT, A]
  with GenTableLike[RKT,
                    CKT,
                    A,
                    MapKeyTable.Curried[RKT, CKT]#Self,
                    MapKeyTable.Curried[CKT, RKT]#Self]

object MapKeyTable {
  /** Type helper for defining self-recursive type */
  type Curried[RKT, CKT] = {
    type Self[+A] = MapKeyTable[RKT, CKT, A]
  }
}

val t1: MapKeyTable[Int, String, Int] = ???
val t2: MapKeyTable[Int, String, Any] = ???

// The following works
t1.++[Any, MapKeyTable[Int, String, Any], ({ type ST[+A2] = MapKeyTable[Int, String, A2] })#ST, ({ type TT[+A2] = MapKeyTable[String, Int, A2] })#TT](t2)
t1.++[Any, MapKeyTable[Int, String, Any], MapKeyTable.Curried[Int, String]#Self, MapKeyTable.Curried[String, Int]#Self](t2)
// Error: inferred type arguments [Int,MapKeyTable[Int,String,Any],Nothing,Nothing] do not conform to method ++'s type parameter bounds [B >: Int,T2 <: GenTableLike[Int,String,B,ST,TT],ST[+A2] <: GenTableLike[Int,String,A2,ST,TT],TT[+A2] <: GenTableLike[String,Int,A2,TT,ST]]
t1 ++ t2

我在这里做错了吗？

注意事项：

自类型和转置类型用于定义函数返回类型。我还有一个 IndexedTable 实现定义如下，所以我不能修改 self-type 来接受 3 个参数

trait IndexedTable[+A]
    extends GenTableLike[Int,
                         Int,
                         A,
                         IndexedTable,
                         IndexedTable]

class IndexedSeqTable[+A]
    extends IndexedTable[A]
    with GenTableLike[Int,
                      Int,
                      A,
                      IndexedSeqTable,
                      IndexedSeqTable]

Answer 1

将SelfType 和TransposeType 转换为抽象类型怎么样？这更简单且有效：

import scala.language.higherKinds

trait GenTableLike[RKT, CKT, +A] {
  type SelfType[+A2] <: GenTableLike[RKT, CKT, A2]
  type TransposedType[+A2] <: GenTableLike[CKT, RKT, A2]

  def ++[B >: A](that: GenTableLike[RKT, CKT, B]): SelfType[B]
}

trait KeyTable[RKT, CKT, +A]
  extends GenTableLike[RKT, CKT, A] {
  override type SelfType[+A2] <: KeyTable[RKT, CKT, A2]
  override type TransposedType[+A2] <: KeyTable[CKT, RKT, A2]
}

class MapKeyTable[RKT, CKT, +A]
  extends KeyTable[RKT, CKT, A] {
  override type SelfType[+A2] = MapKeyTable[RKT, CKT, A2]
  override type TransposedType[+A2] = MapKeyTable[CKT, RKT, A2]

  override def ++[B >: A](that: GenTableLike[RKT, CKT, B]): MapKeyTable[RKT, CKT, B] =
    new MapKeyTable[RKT, CKT, B]
}

val t1 = new MapKeyTable[Int, String, Int]
val t2 = new MapKeyTable[Int, String, Any]

// The following works
t1.++[Any](t2)
t1 ++ t2


trait IndexedTable[+A]
  extends GenTableLike[Int, Int, A] {
  override type SelfType[+A2] <: IndexedTable[A2]
  override type TransposedType[+A2] <: IndexedTable[A2]
}

class IndexedSeqTable[+A]
  extends IndexedTable[A] {
  override type SelfType[+A2] = IndexedSeqTable[A2]
  override type TransposedType[+A2] = IndexedSeqTable[A2]

  override def ++[B >: A](that: GenTableLike[Int, Int, B]): IndexedSeqTable[B] = new IndexedSeqTable[B]
}

2 月 15 日更新以回应 Alex 的评论：

递归类型 + 继承很棘手，如果可能，我总是避免使用它 :-)。
如果 SelfType 和 TransposeType 仅用作函数返回类型，如何完全消除 SelfType 和 TransposeType 并使用隐式构建器，例如：

import scala.language.higherKinds

trait GenTableLike[RKT, CKT, +A] {

  def value: A

  def ++[B >: A](that: GenTableLike[RKT, CKT, B])(implicit builder: GenTableLike.Builder[this.type, B]): builder.Self = {
    builder.buildSelf(that.value)
  }

  def transpose(implicit builder: GenTableLike.Builder[this.type, A]) = builder.buildTranspose(value)

}

object GenTableLike {

  trait Builder[-This, -A] {
    type Self
    type Transpose

    def buildSelf(a: A): Self

    def buildTranspose(a: A): Transpose
  }

}

trait KeyTable[RKT, CKT, +A]
  extends GenTableLike[RKT, CKT, A] {
}

class MapKeyTable[RKT, CKT, +A](override val value: A)
  extends KeyTable[RKT, CKT, A] {
}

object MapKeyTable {

  implicit def builder[RKT, CKT, A] = new GenTableLike.Builder[MapKeyTable[RKT, CKT, A], A] {
    override type Self = MapKeyTable[RKT, CKT, A]
    override type Transpose = MapKeyTable[CKT, RKT, A]

    override def buildSelf(a: A): Self = new MapKeyTable[RKT, CKT, A](a)

    override def buildTranspose(a: A): Transpose = new MapKeyTable[CKT, RKT, A](a)
  }

}

class MapKeyTableEx[RKT, CKT, +A](override val value: A)
  extends MapKeyTable[RKT, CKT, A](value)

object MapKeyTableEx {
  implicit def builder[RKT, CKT, A] = new GenTableLike.Builder[MapKeyTableEx[RKT, CKT, A], A] {
    override type Self = MapKeyTableEx[RKT, CKT, A]
    override type Transpose = MapKeyTableEx[CKT, RKT, A]

    override def buildSelf(a: A): Self = new MapKeyTableEx[RKT, CKT, A](a)

    override def buildTranspose(a: A): Transpose = new MapKeyTableEx[CKT, RKT, A](a)
  }

}


val t1 = new MapKeyTable[Int, String, Int](1)
val t2 = new MapKeyTable[Int, String, Any]("b")
val t3 = new MapKeyTableEx[Int, String, Int](1)
val t4 = new MapKeyTableEx[Int, String, Any]("b")

// The following works
t1 ++ t2
t1 ++ t3
t2 ++ t3
t3 ++ t4

t1.transpose
t2.transpose
t3.transpose
t4.transpose

Answer 2

我已经设法找到解决我的问题的方法，我认为它比@PH88 的建议更好。我从SelfType/TransposedType 中删除了约束，并在++ 参数类型中使用了ST forSome { type ST[+A2] } 语法，就像在https://issues.scala-lang.org/browse/SI-8039 的描述中一样

此解决方案的副作用：

1. 不能在GenTableLike 中定义其他方法的方法实现（因为它们将返回一个未知性质的SelfType） - 即不能在GenTableLike 代码中重用方法。这可以通过在GenTableLike 和KeyTable 和IndexedTable 之上提供额外的继承层来解决
2. 子类可以将非表定义为其自身/转置类型，但这似乎不是一个大问题 - 他们没有理由这样做。

结果代码：

trait GenTableLike[RKT,
                   CKT,
                   +A,
                   +SelfType[+A2],
                   +TransposedType[+A2]] {
  def ++[B >: A](that: GenTableLike[RKT,
                                    CKT,
                                    B,
                                    ST forSome { type ST[+A2] },
                                    TT forSome { type TT[+A2] }]): SelfType[B] = ???
}

// Common ancestor for all table classes
trait GenTable[RKT, CKT, +A]
  extends GenTableLike[RKT,
                       CKT,
                       A,
                       GenTable.Curried[RKT, CKT]#Self,
                       GenTable.Curried[CKT, RKT]#Self] {
  // Here we can implement common methods reusing other methods due to proper SelfType bounds
}

object GenTable {
  /** Type helper for defining self-recursive type */
  private type Curried[RKT, CKT] = {
    type Self[+A] = GenTable[RKT, CKT, A]
  }
}

trait KeyTable[RKT, CKT, +A]
  extends GenTable[RKT, CKT, A]
  with GenTableLike[RKT,
                    CKT,
                    A,
                    KeyTable.Curried[RKT, CKT]#Self,
                    KeyTable.Curried[CKT, RKT]#Self]

object KeyTable {
  /** Type helper for defining self-recursive type */
  type Curried[RKT, CKT] = {
    type Self[+A] = KeyTable[RKT, CKT, A]
  }
}

class MapKeyTable[RKT, CKT, +A]
  extends KeyTable[RKT, CKT, A]
  with GenTableLike[RKT,
                    CKT,
                    A,
                    MapKeyTable.Curried[RKT, CKT]#Self,
                    MapKeyTable.Curried[CKT, RKT]#Self] {
  override def ++[B >: A](that: GenTableLike[RKT,
                                             CKT,
                                             B,
                                             ST forSome { type ST[+A2] },
                                             TT forSome { type TT[+A2] }]): MapKeyTable[RKT, CKT, B] = {
    new MapKeyTable
  }
}

object MapKeyTable {
  /** Type helper for defining self-recursive type */
  type Curried[RKT, CKT] = {
    type Self[+A] = MapKeyTable[RKT, CKT, A]
  }
}

trait IndexedTable[+A]
  extends GenTable[Int, Int, A]
  with GenTableLike[Int,
                    Int,
                    A,
                    IndexedTable,
                    IndexedTable]

class IndexedSeqTable[+A]
    extends IndexedTable[A]
    with GenTableLike[Int,
                      Int,
                      A,
                      IndexedSeqTable,
                      IndexedSeqTable] {
  override def ++[B >: A](that: GenTableLike[Int,
                                             Int,
                                             B,
                                             ST forSome { type ST[+A2] },
                                             TT forSome { type TT[+A2] }]): IndexedSeqTable[B] = {
    new IndexedSeqTable
  }
}


// Usage

def s1: IndexedSeqTable[Int] = ???
def s2: IndexedSeqTable[Any] = ???

def t1: MapKeyTable[Int, Int, Int] = ???
def t2: MapKeyTable[Int, Int, Any] = ???

// All of this works with result being of proper type
t1.++[Any](t2)
t1 ++ t2
s1 ++ s2
t1 ++ s1
s1 ++ t1
s2 ++ t1