传递给类的类型参数的条件

Question

Suppose I have MyCoolClass which accepts a type parameter T . Depending on T we want to define aFun in this way:

If ( T is the same type as B ) aFun should be set to () => new B

else If ( T is the same type as C ) aFun should be set to () => new C

otherwise show an error.

class MyCoolClass[T]{ 
   def aFun = ...  // fill this part 
}

class A { 
 // something here 
}

class B extends A { 
 // something more here
}

class C extends A { 
 // yet more things here 
}

How would you go about doing this?

Answer 1

You can't do that this way. What you can do is instead restrict T s in MyCoolClass to require a instance of a type class that would know how to instantiate T :

trait Cons[T] { def cons:T }
class MyCoolClass[T](implicit ev:Cons[T]) {
 def aFun = ev.cons
}

Now what we need is define A and B and Cons[A] and Cons[B] (**)

class A
class B
object A { implicit def consA:Cons[A] = new Cons[A] { def cons = new A } }
object B { implicit def consA:Cons[B] = new Cons[B] { def cons = new B } }

Now in the REPL:

scala> new MyCoolClass[A].aFun
res0: A = A@1521d074

scala> new MyCoolClass[B].aFun
res1: B = B@19cdb1cb

(**) If you happen to try that in the REPL remember to copy and paste it in a :paste command, otherwise object A and B won't really be companion objects and this own't work