For updated code.
The compiler is very fair with complaints. Algorithm use one subclass of State as denoted and state successor may return any other subclass of State[M]
You may declare IntegerOne class
trait Abstract[T]
class IntegerOne extends Abstract[Int]
but the compiler have no clue that all instances of AbstractOne[Int] would be IntegerOne. It assume that there may be another class that also implements Abstract[Int]
class IntegerTwo extends Abstract[Int]
You may try to use implicit conversion to cast from Abstract[Int] to IntegerOne, but traits have no implicit view bounds as they have no value parameters at all.
Solution 0
So you may rewrite your Algorithm trait as an abstract class and use implicit conversion:
abstract class MyAlgorithm[MT <: Move, ST <: State[MT]] (implicit val toSM : State[MT] => ST) extends Algorithm {
override type M = MT // finalize types, no further subtyping allowed
override type S = ST // finalize types, no further subtyping allowed
def score(s : S) : Int = s.moves.size
override def bestMove(s : S) : M = {
val groups = s.moves.groupBy( m => score(toSM ( s.successor(m)) ) )
val max = groups.keys.max
groups(max).head
}
}
implicit def toMyState(state : State[MyMove]) : MyState = state.asInstanceOf[MyState]
object ConcreteAlgorithm extends MyAlgorithm[MyMove,MyState]
object Main extends App {
val s = new MyState(Map())
val m = ConcreteAlgorithm.bestMove(s)
println(m)
}
There are two drawbacks in this solution
- using implicit conversion with asInstanceOf
- tying types
You may extinguish first as the cost of further type tying.
Solution 1
Let use Algorithm as sole type parameterization source and rewrite type structure accordingly
trait State[A <: Algorithm] { _:A#S =>
def moves : List[A#M]
def successor(m : A#M): A#S
}
trait Algorithm{
type M <: Move
type S <: State[this.type]
def bestMove(s : S) : M
}
In that case your MyAlgorithm may be used without rewriting
trait MyAlgorithm extends Algorithm {
def score(s : S) : Int = s.moves.size
override def bestMove(s : S) : M = {
val groups = s.moves.groupBy(m => score(s.successor(m)))
val max = groups.keys.max
groups(max).head
}
}
Using it:
class MyState(val s : Map[MyMove,Int]) extends State[ConcreteAlgorithm.type] {
def moves = MyMove(1) :: MyMove(2) :: Nil
def successor(p : MyMove) = new MyState(s.updated(p,1))
}
object ConcreteAlgorithm extends MyAlgorithm {
override type M = MyMove
override type S = MyState
}
object Main extends App {
val s = new MyState(Map())
val m = ConcreteAlgorithm.bestMove(s)
println(m)
}
See more abstract and complicated usage example for this tecnique: Scala: Abstract types vs generics
Solution 2
There is also a simple solution to your question but I doubt it can solve your problem. You will eventually stuck upon type inconsistency once again in more complex use cases.
Just make MyState.successor return this.type
instead of State[M]
trait State[M <: Move] {
def moves : List[M]
def successor(m : M): this.type
}
final class MyState(val s : Map[MyMove,Int]) extends State[MyMove] {
def moves = MyMove(1) :: MyMove(2) :: Nil
def successor(p : MyMove) = (new MyState(s.updated(p,1))).asInstanceOf[this.type]
}
other things are unchanged
trait Algorithm{
type M <: Move
type S <: State[M]
def bestMove(s : S) : M
}
trait MyAlgorithm extends Algorithm {
def score(s : S) : Int = s.moves.size
override def bestMove(s : S) : M = {
val groups = s.moves.groupBy(m => score(s.successor(m)))
val max = groups.keys.max
groups(max).head
}
}
object ConcreteAlgorithm extends MyAlgorithm {
override type M = MyMove
override type S = MyState
}
object Main extends App {
val s = new MyState(Map())
val m = ConcreteAlgorithm.bestMove(s)
println(m)
}
Pay attention to final
modifier to MyState class. It ensures that conversion asInstanceOf[this.type] is correct one. Scala compiler may compute itself that final class keeps always this.type
but it still have some flaws.
Solution 3
There is no need to tie Algorithm with custom State. As long as Algorithm does not use specific State function it may be written simpler without type bounding exercises.
trait Algorithm{
type M <: Move
def bestMove(s : State[M]) : M
}
trait MyAlgorithm extends Algorithm {
def score(s : State[M]) : Int = s.moves.size
override def bestMove(s : State[M]) : M = {
val groups = s.moves.groupBy(m => score(s.successor(m)))
val max = groups.keys.max
groups(max).head
}
}
This simple example doesn't come to my mind quickly because I've assumed that binding to different states are obligatory. But sometimes only part of system really should be parameterized explicitly and your may avoid additional complexity with it
Conclusion
Problem discussed reflects bunch of problems that arises in my practice very often.
There are two competing purposes that should not exclude each other but do so in scala.
First means that you can build complex system, implement some basic realization and be able to replace its parts one by one to implement more complex realization.
Second allows your to define very abstract system, that may be used for different cases.
Scala developers had very challenging task for creating type system for a language that can be both functional and object oriented while being limited to jvm implementation core with huge defects like type erasure. Co/Contra-variance type annotation given to users are insufficient for expressing types relations in complex system
I have my hard times every time I encounter extensiblity-generality dilemma deciding which trade-off to accept.
I'd like not to use design pattern but to declare it in the target language. I hopes that scala will give me this ability someday.