如何在Scala中的函数签名中匹配类型成员？

Question

For obscure reasons due to a specific library I'm using involving macros and serialization, I have chosen to try to use type members instead of type parameters for a class (background: Is there a way to use ClassTags with the Autowire library in Scala? )

sealed trait Command {
  type M <: CommandMetaData
  val meta: M
}

final case class SysCmdMetaData(
// ...
) extends CommandMetaData


// How to guarantee cmd.M <: SysCmdMetaData
def runInSystem(cmd: Command){
  //...
}

Is there a way to do this statically? I think using type classes and context bounds is one option, but I feel it is a bit heavy handed since I'm already using subclassing here. Perhaps the better option is just to make metadata a trait, and embrace object oriented more fully and create all kinds of variations of Command based on which traits are mixed in.

Answer 1

You can define the function argument with a refined type:

def runInSystem(cmd: Command { type M <: SysCmdMetaData }) = { }

Note that unlike type parameters, type members are not inferred by default . The following code will fail:

scala> val cmd: Command = new Command {
  type M = SysCmdMetaData
  val meta = SysCmdMetaData()
}

cmd: Command = $anon$1@830b8a5

scala> runInSystem(cmd)
<console>:15: error: type mismatch;
 found   : cmd.type (with underlying type Command)
 required: Command{type M <: SysCmdMetaData}
       runInSystem(cmd)
                   ^

You'd have to either:

• Use some named subclass of Command , where M is statically fixed to an appropriate type.

• Pass an anonymous subclass instance directly to runInSystem :

   runInSystem(new Command { type M = SysCmdMetaData val meta = SysCmdMetaData() }) 

• Manually prescribe the type member of the variable you pass to runInSystem :

   val cmd: Command { type M = SysCmdMetaData } = new Command { type M = SysCmdMetaData val meta = SysCmdMetaData() } runInSystem(cmd) 

• You can also define a type alias with a type parameter:

   object Command { type Aux[M0 <: CommandMetaData] = Command { type M = M0 } } val cmd: Command.Aux[SysCmdMetaData] = ??? runInSystem(cmd)