Scala Generics - ADT中的通用提取器方法

Question

I have an ADT like;

sealed trait Dimension
case object Customer extends Dimension
case object Brand extends Dimension
case object Product extends Dimension

Now this ADT represents values in a data class

case class Record(custId: Long, prodId: Int, brand: String)

How could I write an 'dimension extractor' such that if I pass in any Dimension type and an instance of Record I get the value it represents?

In other words I have a Seq[Dimension](Brand,Customer).foreach{d => println(d.extract(aRecord))}

First thought was to do something like;

trait DimExtractor[A,B] {
  val extract = A => B
}

then change the ADT:

case object Brand extends Dimension with DimExtractor[Record, String] {
  val extract = (r: Record) => r.brand
}

Is there a way to implicitly pick up the type String based on the return type of r.brand? Maybe making this a def?

But this doesn't work because a Seq[Dimension with DimExtractor] wont compile because it needs type params... But I have lots of DimExtractor[Record, ?] (ie one for Brand which returns a String, on for Customer which returns a long etc...

I could make it a DimExtractor[Record, Any]... but again this doesnt seem right.

Whats a better way to do this?

Answer 1

You have 3 subclass (object) of the Dimension trait and the extractor's return value depends on the argument type (consider both a and b as arguments of method in a.method(b) ). You can use a path-dependent type.

sealed trait Dimension {
}

trait DimExtractor[R] {
  type Ret
  def extract(r: R): Ret
}

case object Customer extends Dimension with DimExtractor[Record] {
  type Ret = Long
  def extract(r: Record): Ret = r.custId
}
case object Brand extends Dimension with DimExtractor[Record] {
  type Ret = String
  def extract(r: Record): Ret = r.brand
}
case object Product extends Dimension with DimExtractor[Record] {
  type Ret = Int
  def extract(r: Record): Ret = r.prodId
}

case class Record(custId: Long, prodId: Int, brand: String)

Now you can write your example as

val aRecord = Record(1, 1, "asd")
Seq[DimExtractor[Record]](Brand, Customer).foreach { d => println(d.extract(aRecord)) }