[英]How to enforce a Collection (Seq, List etc.) to accept only positive integers - Scala
[英]Scala Seq - accept only elements of the same subtype
假设我有一个类似以下的类型层次结构:
trait Color
case class Red(r: String) extends Color
case class Green(g: String) extends Color
是否有可能创建一个接受Seq[Color]
的方法,该Seq[Color]
包含全部为Red
元素,或者全部为Green
,但不是两者都包含?
例如,在以下代码中:
def process[T](colors: Seq[T]) = colors.size
process(Seq(Red("a"), Green("g")))
[T]
应该是什么,以便上面不进行类型检查?
编辑
最初的问题如下:我正在尝试为嵌套查询设计JSON API。 我想出了以下设计:
trait QueryUnit
case class SimpleQuery(query: String, metadata: Metadata)
case class ComplexQuery(Map[String, Seq[SimpleQuery])
case class API(
query: Map[String, Seq[QueryUnit]]
)
Map的元素将是conjuctions(ANDs),而Seq的元素将是析取(OR)。 我不想将Simple与ComplexQueries混合使用,我计划在Seq [QueryUnit]上进行模式匹配。
试试这个(部分基于Miles Sabin的博客文章 ):
// Your domain
trait QueryUnit
case class SimpleQuery(query: String, metadata: AnyRef) extends QueryUnit
case class ComplexQuery(map: Map[String, Seq[SimpleQuery]]) extends QueryUnit
// End of your domain
// Something which has type parameter, so we can add QueryUnit, ...
trait WrongArg[T]
// Create ambiguous implicits for QueryUnit
implicit def v0: WrongArg[QueryUnit] = ???
implicit def v2: WrongArg[QueryUnit] = ???
// And valid values for the concrete subclasses
implicit val simpleQWrongArg: WrongArg[SimpleQuery] = new WrongArg[SimpleQuery] {}
implicit val complexQWrongArg: WrongArg[ComplexQuery] = new WrongArg[ComplexQuery] {}
case class API[QU <: QueryUnit](
query: Map[String, Seq[QU]]
// Require an evidence that we are getting the correct type
)(implicit w: WrongArg[QU]) {
}
API/*[SimpleQuery]*/(Map("" -> Seq(SimpleQuery("", ""))))
API/*[ComplexQuery]*/(Map("" -> Seq(ComplexQuery(Map.empty))))
// Fails to compile because of ambiguous implicits
//API(Map("s" -> Seq(SimpleQuery("", "")), "c" -> Seq(ComplexQuery(Map.empty))))
它不理想(名称错误,没有@implicitNotFound
注释)。
基本思想是我们为基类创建模糊的含义,但不为子类创建含糊不清的含义。 由于隐式解析规则,这将仅针对子类进行编译,但不针对基类进行编译。
清理版本:
@annotation.implicitNotFound("Base QueryUnit is not supported, you cannot mix SimpleQuery and ComplexQuery")
trait Handler[T] extends (T => Unit)
object API {
implicit def baseQueryUnitIsNotSupported0: Handler[QueryUnit] = ???
implicit def baseQueryUnitIsNotSupported1: Handler[QueryUnit] = ???
implicit val simpleQWrongArg: Handler[SimpleQuery] = new Handler[SimpleQuery] {
override def apply(s: SimpleQuery): Unit = {}
}
implicit val complexQWrongArg: Handler[ComplexQuery] = new Handler[ComplexQuery] {
override def apply(s: ComplexQuery): Unit = {}
}
}
case class API[QU <: QueryUnit](query: Map[String, Seq[QU]])(
implicit handler: Handler[QU]) {
// Do something with handler for each input
}
// Usage
import API._
import scala.annotation.implicitNotFound
API/*[SimpleQuery]*/(Map("" -> Seq(SimpleQuery("", ""))))
API/*[ComplexQuery]*/(Map("" -> Seq(ComplexQuery(Map.empty))))
// Error:(56, 71) Base QueryUnit is not supported, you cannot mix SimpleQuery and ComplexQuery
//API(Map("s" -> Seq(SimpleQuery("", "")), "c" -> Seq(ComplexQuery(Map.empty))))
或者使用类型边界并implicitly
:
case class API[QU <: QueryUnit: Handler](query: Map[String, Seq[QU]]) {
def doSomething: Unit = for {(_, vs) <- query
v <- vs} {
val handler: Handler[QU] = implicitly[Handler[QU]]
handler(v)
}
}
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