如何解决使用 @uncheckedVariance 强制 Scala 抽象类型的协方差
在很多地方都很好地介绍了差异主题,但不幸的是,我没有找到任何关于抽象类型的主题。
然后为了理解集合,我写了以下内容
trait Collectionops[+A,+C] {
this: C =>
type CC[B] <: BasicColl[B] // Invariant position
def filter(f: A => Boolean): C
def map[B](f: A => B): CC[B] // CC is in a covariant position but declared invariant via abstract type
}
trait BasicColl[+A] extends Collectionops[A,BasicColl[A]] {
type CC[B] = BasicColl[B] // this is the problem,the equality
// But if i let inequality: type CC[B] <: BasicColl[B]
}
// Do not compile due to prevIoUs equality and invariance on CC
trait BasicColl2[+A] extends BasicColl[A] with Collectionops[A,BasicColl2[A]] {
type CC[B] = BasicColl2[B]
// But if i let the prevIoUs inequality i can writte type CC[B] <: BasicColl2[B]
}
trait Test_With_Abstract_Type_Equality {
// Only BasicColl extist
def coll1: BasicColl[Int]
// We have exact type due to equality of CC with BasicColl
def mapped: BasicColl[Double] = coll1.map(_.todouble)
// But no specialized inhéritence possible
}
trait Test_With_Abstract_Type_Inequality {
def coll1: BasicColl[Int]
// We have abstract type due to inequality of CC with BasicColl
def mappedWithAbstractTypeDef: BasicColl[Int]#CC[Double] = coll1.map(_.todouble)
// BasicColl2 can exist
def coll2: BasicColl2[Int]
// We have abstract type due to inequality of CC with BasicColl2
def mappedWithAbstractTypeDef2: BasicColl2[Int]#CC[Double] = coll2.map(_.todouble)
}
Scala 模式不使用抽象类型,而是直接使用协变参数化类型 CC,如下所示
trait IterableOnceOps[+A,+CC[_],+C]
协方差具有促进特征混合的优势。
但是在类型参数级别使用这种类型的更高级会阻止对具有超过 1 个参数化类型的类的任何扩展,其中使用抽象类型允许扩展到更复杂的类,但代价是将抽象类型作为输出。 在这里,我尝试过这一点,但我对它的风险没有足够的信心。
// RedeFinition
trait Collectionops[+A,+C] {
this: C =>
type CC[B] <: BasicColl[B] // Invariant position
def filter(f: A => Boolean): C
def map[B](f: A => B): CC[B] // but CC is in a covariant position
}
// RedeFinition
trait BasicColl[+A] extends Collectionops[A,BasicColl[A]]
trait CollectionopsLocked1[+A,+CCC[B] <: BasicColl[B],+C] extends Collectionops[A,C] {
this: C =>
type CC[B] = CCC[B] @uncheckedVariance // this is my question
// If i use CC ONLY in covariant position is there any risk ?
}
trait BasicColl2[+A] extends BasicColl[A] with CollectionopsLocked1[A,BasicColl2,BasicColl2[A]]
trait BasicColl3[+A] extends BasicColl2[A] with CollectionopsLocked1[A,BasicColl3,BasicColl3[A]]
trait Test {
def coll1: BasicColl[Int]
def mappedWithAbstractTypeDef: BasicColl[Int]#CC[Double] = coll1.map(_.todouble)
// BasicColl2 can exist
def coll2: BasicColl2[Int]
def mapped2: BasicColl2[Double] = coll2.map(_.todouble)
// BasicColl3 can exist
def coll3: BasicColl3[Int]
def mapped3: BasicColl3[Double] = coll3.map(_.todouble)
}
更一般地说,在协方差/逆变合法的地方使用 @uncheckedVariance
是否有任何风险?
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