从数组到列表的隐式转换
如何编写从 Array[_] 到 List[_] 类型的隐式转换？我尝试了以下方法，但似乎不起作用。
scala> implicit def arrayToList[A : ClassManifest](a: Array[A]): List[A] = a.toList
<console>:5: error: type mismatch;
found : Array[A]
required: ?{val toList: ?}
Note that implicit conversions are not applicable because they are ambiguous:
both method arrayToList in object $iw of type [A](a: Array[A])(implicit evidence$1: ClassManifest[A])List[A]
and method genericArrayOps in object Predef of type [T](xs: Array[T])scala.collection.mutable.ArrayOps[T]
are possible conversion functions from Array[A] to ?{val toList: ?}
implicit def arrayToList[A : ClassManifest](a: Array[A]): List[A] = a.toList
^
How do I write an implicit conversion from Array[_] to List[_] type? I tried the following but it doesn't seem to work.
scala> implicit def arrayToList[A : ClassManifest](a: Array[A]): List[A] = a.toList
<console>:5: error: type mismatch;
found : Array[A]
required: ?{val toList: ?}
Note that implicit conversions are not applicable because they are ambiguous:
both method arrayToList in object $iw of type [A](a: Array[A])(implicit evidence$1: ClassManifest[A])List[A]
and method genericArrayOps in object Predef of type [T](xs: Array[T])scala.collection.mutable.ArrayOps[T]
are possible conversion functions from Array[A] to ?{val toList: ?}
implicit def arrayToList[A : ClassManifest](a: Array[A]): List[A] = a.toList
^
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似乎按预期工作。我的猜测是，
Predef中已经有一个genericArrayOps，它具有从Array[T] -> 进行隐式转换的签名。 ArrayOps[T]、ArrayOps[T]有一个方法.toList(): List[T]。您的方法具有签名Array[T] -> List[T]，这也使.toList[T]方法可用。主体要求使用该签名进行隐式转换。编译器不知道使用 arrayToList 会使该方法进入无限循环，因此会出现歧义错误。然而，类型推断返回类型似乎能够解决这个问题。隐式解析似乎与类型推断不太相符。另外值得注意的是，由于已经有一个隐式转换可以默认为您提供所需的内容，因此不需要从
Array到List进行隐式转换。Seems to work as expected. My guess is that there's already a
genericArrayOpsinPredefthat has the signature for implicit conversion fromArray[T] -> ArrayOps[T],ArrayOps[T]has a method.toList(): List[T]. Your method has the signatureArray[T] -> List[T], which also makes the method.toList[T]available. The body is asking for an implicit conversion with that signature. The compiler doesn't know that usingarrayToListwill make that method go into an infinite loop, hence the ambiguity error. However, type-inferencing the return type seems to be able to work around this problem. Implicits resolution don't jive very well with type-inference it seems.Also worth noting is that since there is already an implicit conversion that will get you what you want by default, there's no need to have an implicit conversion from
ArraytoList.从数组转换时不需要
Manifest或ClassManifest，因为Array是一种“集合”类型它在 JVM 上得到特殊处理并且不会经历类型擦除。这意味着您可以采用明显/微不足道的方法，不需要任何技巧：
不过有一个问题...鉴于
.toList已经是一个如此微不足道的操作，那么通过将其隐式化您可以获得什么？There's no need for a
ManifestorClassManifestwhen converting from arrays, asArrayis the one "collection" type that gets special treatment on the JVM and doesn't undergo type erasure.This means that you can go with the obvious/trivial approach, no trickery required:
One question though... Given that
.toListis already such a trivial operation, what do you gain by making it implicit?