在Scala中对密封特征进行迭代? [英] Iteration over a sealed trait in Scala?
问题描述
我只是想知道是否有可能迭代Scala中的密封特性? 如果没有,为什么不可能呢?由于特质是密封的,应该可以吗?
I just wanted to know if it is possible to iterate over a sealed trait in Scala? If not, why is it not possible? Since the trait is sealed it should be possible no?
我想做的是这样的:
sealed trait ResizedImageKey {
/**
* Get the dimensions to use on the resized image associated with this key
*/
def getDimension(originalDimension: Dimension): Dimension
}
case class Dimension(width: Int, height: Int)
case object Large extends ResizedImageKey {
def getDimension(originalDimension: Dimension) = Dimension(1000,1000)
}
case object Medium extends ResizedImageKey{
def getDimension(originalDimension: Dimension) = Dimension(500,500)
}
case object Small extends ResizedImageKey{
def getDimension(originalDimension: Dimension) = Dimension(100,100)
}
我可以通过给枚举值提供一个实现来用Java完成. Scala中有与之等效的东西吗?
What I want can be done in Java by giving an implementation to the enum values. Is there an equivalent in Scala?
推荐答案
在我看来,这实际上是2.10宏的合适用例:您想访问您知道编译器具有但未公开的信息,并且宏为您提供了一种(合理的)简便的方法来窥视内部.请在此处中查看我的答案,以获取相关的示例(但现在有点过时了),或者仅使用类似的示例:
This is actually in my opinion an appropriate use case for 2.10 macros: you want access to information that you know the compiler has, but isn't exposing, and macros give you a (reasonably) easy way to peek inside. See my answer here for a related (but now slightly out-of-date) example, or just use something like this:
import language.experimental.macros
import scala.reflect.macros.Context
object SealedExample {
def values[A]: Set[A] = macro values_impl[A]
def values_impl[A: c.WeakTypeTag](c: Context) = {
import c.universe._
val symbol = weakTypeOf[A].typeSymbol
if (!symbol.isClass) c.abort(
c.enclosingPosition,
"Can only enumerate values of a sealed trait or class."
) else if (!symbol.asClass.isSealed) c.abort(
c.enclosingPosition,
"Can only enumerate values of a sealed trait or class."
) else {
val children = symbol.asClass.knownDirectSubclasses.toList
if (!children.forall(_.isModuleClass)) c.abort(
c.enclosingPosition,
"All children must be objects."
) else c.Expr[Set[A]] {
def sourceModuleRef(sym: Symbol) = Ident(
sym.asInstanceOf[
scala.reflect.internal.Symbols#Symbol
].sourceModule.asInstanceOf[Symbol]
)
Apply(
Select(
reify(Set).tree,
newTermName("apply")
),
children.map(sourceModuleRef(_))
)
}
}
}
}
现在我们可以编写以下内容:
Now we can write the following:
scala> val keys: Set[ResizedImageKey] = SealedExample.values[ResizedImageKey]
keys: Set[ResizedImageKey] = Set(Large, Medium, Small)
这完全是安全的-如果您请求的类型不是密封的,具有非对象子代的类型等,则会出现编译时错误.
And this is all perfectly safe—you'll get a compile-time error if you ask for values of a type that isn't sealed, has non-object children, etc.
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