Scala define type from multiple generic traits

Question

How can I fix this code :

trait A[A,B]{
  def f(a:A):B
}
trait B[A,B]{
  def g(a:A):B
}

type C = A[String,Int] with B[String,Double]

//works fine
new A[String,Int] with B[String,Double] {
  def f(a:String):Int = 1
  def g(a:String):Double = 2.0
}

//error
new C {
  def f(a:String):Int = 1
  def g(a:String):Double = 2.0
}

The exception i got is :

Error:(41, 6) class type required but A$A42.this.A[String,Int] with A$A42.this.B[String,Double] found 
new C {
    ^

Any Idea how to solve that and what is the reason for that ?

The simplest solution would be to define C as a trait instead of a type alias : trait C extends A[String,Int] with B[String,Double]. — Peter Neyens
– Peter Neyens, Commented Oct 29, 2015 at 16:41
The generic part is a red herring here—you'd see the same result with trait A and trait B. — Travis Brown
– Travis Brown, Commented Oct 29, 2015 at 17:15
OK, so what is the problem ? how can i solve it without traits? i want to dive into it and understand the meaning of what i write. what is the difference between trait and type ? — David H
– David H, Commented Oct 29, 2015 at 17:31

Victor Moroz · Accepted Answer · 2015-10-29 18:11:10Z

2

My guess why it doesn't work (and probably shouldn't): type C = ... defines something which is not a class type, I wonder what it is. But when you pass it to new it expects new class_type with trait_type ..., so you are trying to replace only one thing, namely class_type with C. If you define C without with it will work.

You can also write:

type C1 = A[String,Int]
type C2 = B[String,Double]
new C1 with C2 {
  def f(a:String):Int = 1
  def g(a:String):Double = 2.0
}

answered Oct 29, 2015 at 18:11

Victor Moroz

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