为什么字符串文字符合 Scala Singleton [英] Why do String literals conform to Scala Singleton
问题描述
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:AnyVal
s 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: AnyVal
s 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!
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