明智与否,我正在编写一个我只想接受 Scala 单例的方法,即通过对象"而不是类或特征的构造实例实现的对象.它应该接受任何类型的 Scala 单例,所以MySingleton.type"不会.
Wisely or not, I'm writing a method that I'd like to accept only Scala singletons, i.e. objects implemented via "object" rather than constructed instances of a class or trait. It should accept Scala singletons of any type, so "MySingleton.type" won't do.
我发现了一个非常奇怪的构造scala.Singleton",它没有在 api 文档中记录,但似乎可以解决问题:
I came upon the very strange construct "scala.Singleton", which is not documented in the api docs, but seems to do the trick:
scala> def check( obj : Singleton ) = obj check: (obj: Singleton)Singleton scala> check( Predef ) res0: Singleton = scala.Predef$@4d3e9963 scala> check ( new java.lang.Object() ) <console>:9: error: type mismatch; found : java.lang.Object required: Singleton check ( new java.lang.Object() ) scala> check( Map ) res3: Singleton = scala.collection.immutable.Map$@6808aa2d scala> check( Map.empty[Any,Any] ) <console>:9: error: type mismatch; found : scala.collection.immutable.Map[Any,Any] required: Singleton check( Map.empty[Any,Any] )然而,令人费解的是(对我来说),字符串文字被接受为单例,而显式构造的字符串不是:
However, rather inexplicably (to me), String literals are accepted as Singletons while explicitly constructed Strings are not:
scala> check( "foo" ) res7: Singleton = foo scala> check( new String("foo") ) <console>:9: error: type mismatch; found : java.lang.String required: Singleton check( new String("foo") )为什么字符串文字符合单例?我是否误解了 Singleton 类型应该指定的内容?
Why do String literals conform to Singleton? Am I misunderstanding what the Singleton type is supposed to specify?
推荐答案首先,什么是单例类型?如果您认为类型是一组值,那么单例类型就是只有一个元素的集合.
Firstly, what is a singleton type? If you take the view that a type is a set of values, a singleton type is a set with exactly one element.
最常见的是,顶级对象可以包含在这样的集合中.
Most commonly, a top level object can inhabit such a set.
scala> object X defined module X scala> X: X.type res41: X.type = X$@131d1cb scala> res41: Singleton res42: Singleton = X$@131d1cb更一般的,stable值可以形成单例类型.
More generally, and stable value can form a singleton type.
scala> object X { val y: String = "boo" } defined module X scala> X.y: X.y.type res44: X.y.type = boo scala> res44: Singleton res45: Singleton = boo如果 y 是 def 或 var,它不再符合条件,因为随着时间的推移值可能不一样,所以编译器不能保证单例类型对一且一的值进行分类.
If y is a def or a var, it no longer qualifies, as the value might not be the same over time, so the compiler can't guarantee that the singleton type classifies one-and-only-one value.
scala> object X { def y: String = "boo" } defined module X scala> X.y: X.y.type <console>:12: error: stable identifier required, but X.y found. X.y: X.y.type ^ scala> object X { var y: String = "boo" } defined module X scala> X.y: X.y.type <console>:12: error: stable identifier required, but X.y found. X.y: X.y.type ^还有一个限制:AnyVals 不能形成单例类型,因为语言规范特别将它们限制为 AnyRef.
One more limitation: AnyVals can't form singleton types, because the language specification specifically restricts them to AnyRef.
Paul Phillips 一直在策划一个 branch,它允许您为文字.
Paul Phillips has been curating a branch which allows you to express a singleton type for literals.
val xs: Stream[0.type](0) val ys: Stream[0.type](0, 1) // does not compile val x = xs.head // inferred type is 0.type, we statically know that this can only be 0!更多推荐
为什么字符串文字符合 Scala Singleton
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