CODEWITHSUNDEEP
scalafunctiontypes
Alfredo Gimenez
Alfredo Gimenez

Reputation: 2224

Scala function that requires a type that extends both a class and a trait

I want to make a function that takes an input type S where S <: ParentClass and S also inherits SomeTrait. I created a solution using S <: ParentClass with SomeTrait and it compiles fine, however it rejects inputs that satisfy those conditions.

abstract class Units[T](v: T) { def getVal = v}

trait Dimension
trait Time extends Dimension

trait Quantity[T <: Dimension]
trait Instance[T <: Dimension] {
  def plus[S <: Units[_] with Quantity[T]](q: S)
}

case class Seconds(v: Double) extends Units(v) with Quantity[Time] {
}
case class Timestamp(i: Int) extends Units(i) with Instance[Time] {
  def plus[T <: Units[_] with Quantity[Time]](quantity: T) = Timestamp(2345/*placeholder value*/)
}

When I try to use this:

Timestamp(5).plus(Seconds(4))

I get the error:

<console>:46: error: inferred type arguments [Seconds] do not conform to method plus's type parameter bounds [T <: Units[_] with Quantity[Time]]
              Timestamp(5).plus(Seconds(4))
                           ^
<console>:46: error: type mismatch;
 found   : Seconds
 required: T
              Timestamp(5).plus(Seconds(4))

Bonus question: how do I get the value of an item with that type, as shown in the code?

Upvotes: 0

Views: 116

Answers (1)

Steve Waldman
Steve Waldman

Reputation: 14073

Your code typechecks for me, both in Scala 2.11.8 and in Scala 2.10.6.

> console
[info] Starting scala interpreter...
[info] 
Welcome to Scala 2.11.8 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_31).
Type in expressions for evaluation. Or try :help.

scala> abstract class Units[T](v: T) { def getVal = v}
defined class Units

scala> 

scala> trait Dimension
defined trait Dimension

scala> trait Time extends Dimension
defined trait Time

scala> 

scala> trait Quantity[T <: Dimension]
defined trait Quantity

scala> trait Instance[T <: Dimension] {
     |   def plus[S <: Units[_] with Quantity[T]](q: S)
     | }
defined trait Instance

scala> 

scala> case class Seconds(v: Double) extends Units(v) with Quantity[Time] {
     | }
defined class Seconds

scala> case class Timestamp(i: Int) extends Units(i) with Instance[Time] {
     |   def plus[T <: Units[_] with Quantity[Time]](quantity: T) = Timestamp(2345/*placeholder value*/)
     | }
defined class Timestamp

scala> Timestamp(5).plus(Seconds(4))

(A 2.10.6 REPL console is basically identical, so I'll skip it.)

Upvotes: 1

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