CODEWITHSUNDEEP
scalaclassscala.jsdependent-type
Simon
Simon

Reputation: 6462

Scala: create a dependent type variable from Any

I would like to create a variable with a dependent type.

I have

val a: Any = 5

and I would like to have

val b: [TypeOfA] = a

TypeOfA must be the subtype of Any (Int, String or MyClass) and not Any.
I cannot use scala reflection since this code is part of a scalajs code (which is not a the JVM).

What I have tried:

trait DepValue{
  type V
  val value: V
} 
def mk[T](x: T) = new DepValue{ 
  type V = T
  val value = x
}

But I don't get the result I want:

val x: Any = 5
magic(mk(x)) 
res70: Any = 5

I would like to have res70: Int = 5 It works well when the type is not Any:

val y = 5 
y: Int = 5
magic(mk(y)) 
res72: Int = 5

Is there any way do to that?

Upvotes: 0

Views: 50

Answers (1)

Justin du Coeur
Justin du Coeur

Reputation: 2659

I think you're getting runtime and compile-time types mixed up. Your example is failing because when you say:

val x: Any = 5
magic(mk(x)) 
res70: Any = 5

You are explicitly telling the compiler that the type of x is Any, not Int. So it's doing what you're telling it to: at compile-time, all it knows is that this is an Any. (If you just said val x = 5, it would work.)

I think what you're asking for is that the compiler should somehow infer that the type of x is Int. But because you told it that it's Any, the compiler just plain doesn't know that any more at compile-time. The system only knows about that at runtime, which is much too late.

None of this really has much to do with dependent types per se -- it's more about the fact that you're hiding the type information, and the compiler can't just suss it out again...

Upvotes: 1

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