I'm failing to figure out how (if at all) you can set a default value for a type -parameter in Scala.
Currently I have a method similar to this:
def getStage[T <: Stage](key: String): T = {
// Do fancy stuff that returns something
}
But what I'd like to do is provide an implementation of getStage
that takes no value for T
and uses a default value instead. I tried to just define another method and overload the parameters, but it only leads to one of the methods being completely overriden by the other one. If I have not been clear what I'm trying to do is something like this:
def getStage[T<:Stage = Stage[_]](key: String): T = {
}
I hope it's clear what I'm asking for. Does anyone know how something like this could be achieved?
You can do this kind of thing in a type-safe way using type classes. For example, suppose you've got this type class:
trait Default[A] { def apply(): A }
And the following type hierarchy:
trait Stage
case class FooStage(foo: String) extends Stage
case class BarStage(bar: Int) extends Stage
And some instances:
trait LowPriorityStageInstances {
implicit object barStageDefault extends Default[BarStage] {
def apply() = BarStage(13)
}
}
object Stage extends LowPriorityStageInstances {
implicit object stageDefault extends Default[Stage] {
def apply() = FooStage("foo")
}
}
Then you can write your method like this:
def getStage[T <: Stage: Default](key: String): T =
implicitly[Default[T]].apply()
And it works like this:
scala> getStage("")
res0: Stage = FooStage(foo)
scala> getStage[BarStage]("")
res1: BarStage = BarStage(13)
Which I think is more or less what you want.
The technical post webpages of this site follow the CC BY-SA 4.0 protocol. If you need to reprint, please indicate the site URL or the original address.Any question please contact:yoyou2525@163.com.