Valid type casting of both covariant and contravariant class at runtime in Scala

问题

I wrote a class implementing the command design pattern:

class MyCommand[-T, +R](val name: String, val execute: T => R)

, prepared two command and stored it in a MutableList:

val commands = new mutable.MutableList[MyCommand[Nothing, Any]]
commands += new MyCommand[String, String]("lower", s => s.toLowerCase())
commands += new MyCommand[Date, Long]("time", d => d.getTime)

Then I have two data to be executed:

val data = Array("StRiNG", new Date())

The problem for me is that I don't know how to determine which datum is applicable to the command:

data.foreach {
  d => commands.foreach {
    c =>
    // println(c.execute(d)) if d is applicable to c.execute().
  }
}

what I tried is pattern matching with type specification, but it yields syntax error:

c.execute match {
  case m: (d.getClass => Any) => println(c.execute(d))
}

Help me :(

回答1:

I am pretty sure that there are better ways of solving this problem, but this might work for you. I tested it with Scala 2.9.2.

MyCommand takes a Manifest as an additional implicit argument, which gives us access to the class that represents the from-type of the execute function at runtime:

class MyCommand[-T: Manifest, +R](val name: String, val execute: T => R) {
  val fromClass = manifest[T].erasure
}

The list of commands is basically as in your original post, as is the list of data:

val commands = List(
  new MyCommand[String, String]("lower", _.toLowerCase()),
  new MyCommand[Date, Long]("time", _.getTime)
)

val data = List("StRiNG", new Date())

Matching data to commands relies on the runtime representation of the involved types as classes, and a rather ugly cast. The cast is especially ugly because it does not give a precise return type, i.e., additional matches or casts are necessary if you need to know the precise return type of the command's return value.

data foreach { d =>
  commands foreach { c =>
    println("data: %s (%s), command takes: %s"
           .format(d, d.getClass.getSimpleName, c.fromClass.getSimpleName))

    if (d.getClass.isAssignableFrom(c.fromClass)) {
      println("    cmd(data) = " + c.execute.asInstanceOf[Any => Any](d))
    }
  }
}

The output is:

data: StRiNG (String), command takes: String
    cmd(data) = string
data: StRiNG (String), command takes: Date
data: Sun Aug 05 14:46:17 CEST 2012 (Date), command takes: String
data: Sun Aug 05 14:46:17 CEST 2012 (Date), command takes: Date
    cmd(data) = 1344170777681

来源：https://stackoverflow.com/questions/11815908/valid-type-casting-of-both-covariant-and-contravariant-class-at-runtime-in-scala

标签

scala

types

covariance

contravariance