CODEWITHSUNDEEP
scala
Daniel Shin
Daniel Shin

Reputation: 5206

Scala use dependent type in type signature of function?

I'm not sure if the phrasing of the question title was appropriate, so I'll elaborate.

sealed trait RedisKey[A] {
  type valueType = A
  def name: String
}
case object FirstName extends RedisKey[String] { val name = "first_name" }
case object Age extends RedisKey[Int] { val name = "age" }

trait Redis {
  def fetch(key: RedisKey)// : key.valueType
}

I need to constrain the return type of fetch by RedisKey's dependent type valueType (not sure if this is the right term). But obviously, the above wont work since I need a concrete instance of RedisKey before I can access its dependent type.

Is it possible to achieve this in any way?

Upvotes: 1

Views: 109

Answers (2)

planetenkiller
planetenkiller

Reputation: 714

You can do it with dependent type too: 

sealed trait RedisKey {
  type valueType
  def name: String
}
case object FirstName extends RedisKey { type valueType = String; val name = "first_name" }
case object Age extends RedisKey { type valueType = Int;  val name = "age" }

trait Redis {
  def fetch(key: RedisKey): key.valueType
}
(null: Redis).fetch(Age): Int
(null: Redis).fetch(FirstName): String

The last two lines are there to test return type of the method in the repl.

Upvotes: 6

Daniel Shin
Daniel Shin

Reputation: 5206

Turns out after about 5 minutes after posting this question, I realized I was overthinking this.

The following works without dependent type hackery. 

sealed trait RedisKey[A] {
  def name: String
}
case object FirstName extends RedisKey[String] { val name = "first_name" }
case object Age extends RedisKey[Int] { val name = "age" }

trait Redis {
  def fetch[A](key: RedisKey[A]): A
}

Upvotes: 1

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