Scala中带有case类的模式匹配

Question

我正在为远程存储设计一个模型，最终得到了：

sealed trait StorageTag
case object Gcs extends StorageTag
case object S3 extends StorageTag
sealed trait StorageFile[T <: StorageTag]
final case class GcsFile(bucket: String, path: String) extends StorageFile[Gcs.type]
final case class S3File(bucket: String, path: String) extends StorageFile[S3.type]
sealed trait StorageConfig[T <: StorageTag]
final case class GcsConfig(keyPath: String) extends StorageConfig[Gcs.type]
final case class S3Config(keyPath: String) extends StorageConfig[S3.type]
def open[T <: StorageTag](storageFile: StorageFile[T], storageConfig: StorageConfig[T]): OutputStream =
(storageFile, storageConfig) match {
case (f: S3File, c: S3Config) => //
case (f: GcsFile, c: GcsConfig) => //
}

但是Scala编译器发出以下警告：

Warning:(39, 5) match may not be exhaustive.
It would fail on the following inputs: (GcsFile(_, _), S3Config(_)), (S3File(_, _), GcsConfig(_))
(storageFile, storageConfig) match {

但在我的具体情况下，用GcsConfig打开S3File显然是没有意义的，反之亦然。有没有办法改进模型？

我个人不喜欢在像GcsFile和S3Config这样不真实的情况下抛出异常或将其作为MatchError的想法。

Answer 1

您需要向编译器提供一些关于允许哪些对的信息。通过将对storageFile: StorageFile[T], storageConfig: StorageConfig[T]传递给open方法，您总是面临有人用错误的par调用open方法的风险，并且您将不得不处理异常情况。为了使其以类型安全的方式工作，您需要传递预定义的类型，该类型"知道"允许哪些对。

例如：

sealed trait StorageTag
case object Gcs extends StorageTag
case object S3 extends StorageTag
sealed trait StorageFile[T <: StorageTag]
final case class GcsFile(bucket: String, path: String) extends StorageFile[Gcs.type]
final case class S3File(bucket: String, path: String) extends StorageFile[S3.type]
sealed trait StorageConfig[T <: StorageTag]
final case class GcsConfig(keyPath: String) extends StorageConfig[Gcs.type]
final case class S3Config(keyPath: String) extends StorageConfig[S3.type]
sealed trait FileConfPair
case class S3Conf(f: S3File, c: S3Config) extends FileConfPair
case class ScsConf(f: GcsFile, c: GcsConfig) extends FileConfPair
def open[T <: StorageTag](fp: FileConfPair): OutputStream =
fp match {
case S3Conf(f: S3File, c: S3Config) => ???
case ScsConf(f: GcsFile, c: GcsConfig) => ???
}

Answer 2

Scala编译器抱怨，这是正确的，你没有涵盖所有的可能性。我认为你有两种选择。

1基于泛型类型的模式匹配(如本题所述：Scala中泛型类型上的模式匹配(

def open[T <: StorageTag](storageFile: StorageFile[T], storageConfig: StorageConfig[T]): OutputStream =
(storageFile, storageConfig) match {
case x if typeOf[T] <:< typeOf[Gcs]  => //
case x if typeOf[T] <:< typeOf[S3]   => //
}

2最简单的方法是提取2个服务类的逻辑作为点@Bogdan Vakulenko

Answer 3

我想提出另一种解决这个问题的方法
根据我对您模型的理解，您真正需要的是确定您必须使用哪个远程存储来执行正确的open逻辑
因此，您可以提供这方面的隐含证据，例如：

sealed trait StorageTag extends Product with Serializable
implicit case object Gcs extends StorageTag
implicit case object S3 extends StorageTag
sealed trait StorageFile[T <: StorageTag] extends Product with Serializable { 
def bucket: String
def path: String
}
final case class GcsFile(bucket: String, path: String) extends StorageFile[Gcs.type]
final case class S3File(bucket: String, path: String) extends StorageFile[S3.type]
sealed trait StorageConfig[T <: StorageTag] extends Product with Serializable {
def keyPath: String
}
final case class GcsConfig(keyPath: String) extends StorageConfig[Gcs.type]
final case class S3Config(keyPath: String) extends StorageConfig[S3.type]
def open[T <: StorageTag](storageFile: StorageFile[T], storageConfig: StorageConfig[T])
(implicit tag: T):String = tag match {
case S3  =>
s"S3 -> bucket: '${storageFile.bucket}', path: '${storageFile.path}' | config keyPath: '${storageConfig.keyPath}'"
case Gcs =>
s"Gcs -> bucket: '${storageFile.bucket}', path: '${storageFile.path}' | config keyPath: '${storageConfig.keyPath}'"
}

现在，你可以用这种方法调用

open(S3File(bucket = "bucket", path = "path"), S3Config(keyPath = "keyPath"))
// res0: String = "S3 -> bucket: 'bucket', path: 'path' | config keyPath: 'keyPath'"
open(GcsFile(bucket = "bucket", path = "path"), GcsConfig(keyPath = "keyPath"))
// res1: String = "Gcs -> bucket: 'bucket', path: 'path' | config keyPath: 'keyPath'"
open(S3File(bucket = "bucket", path = "path"), GcsConfig(keyPath = "keyPath"))
// Compile time error!

请注意，只有当所有StorageFiles和StorageConfigs都具有相同的属性时，这种方法才会起作用
如果不是这样，您可以尝试以下操作：
但是，请注意，此代码不是完全类型安全的，可能会被欺骗

sealed trait StorageTag extends Product with Serializable
implicit case object Gcs extends StorageTag
implicit case object S3 extends StorageTag
sealed trait StorageFile[T <: StorageTag] extends Product with Serializable
final case class GcsFile(bucket: String, path: String, id: Int) extends StorageFile[Gcs.type]
final case class S3File(bucket: String, path: String) extends StorageFile[S3.type]
sealed trait StorageConfig[T <: StorageTag] extends Product with Serializable
final case class GcsConfig(keyPath: String, name: String) extends StorageConfig[Gcs.type]
final case class S3Config(keyPath: String) extends StorageConfig[S3.type]
def open[T <: StorageTag](storageFile: StorageFile[T], storageConfig: StorageConfig[T])
(implicit tag: T): String = tag match {
case S3 =>
// These lines are not checked in compile-time, you can put GcsFile instead, and it will compile and fail at run-time!!!
val S3File(bucket, path) = storageFile
val S3Config(keyPath) = storageConfig
s"S3 -> bucket: '${bucket}', path: '${path}' | config keyPath: '${keyPath}'"
case Gcs =>
val GcsFile(bucket, path, id) = storageFile
val GcsConfig(keyPath, name) = storageConfig
s"Gcs -> bucket: '${bucket}', path: '${path}', id: $id | config keyPath: '${keyPath}', name: 'name'"
}
open(S3File(bucket = "bucket", path = "path"), S3Config(keyPath = "keyPath"))
// res0: String = "S3 -> bucket: 'bucket', path: 'path' | config keyPath: 'keyPath'"
open(GcsFile(bucket = "bucket", path = "path", id = 0), GcsConfig(keyPath = "keyPath", name = "name"))
// res1: String = "Gcs -> bucket: 'bucket', path: 'path', id: 0 | config keyPath: 'keyPath', name: 'name'"
open(S3File(bucket = "bucket", path = "path"), GcsConfig(keyPath = "keyPath", name = "name"))
// Compile time error!
open(
GcsFile(bucket = "bucket", path = "path", id = 0).asInstanceOf[StorageFile[StorageTag]],
GcsConfig(keyPath = "keyPath", name = "name").asInstanceOf[StorageConfig[StorageTag]]
)(S3.asInstanceOf[StorageTag])
// Runtime error!!!!!!!