So I'm trying to work through Norvig & Russell's "Artificial Intelligence, A Modern Approach" as a way to learn Scala. I have a pretty good grasp on the language basics at this point, but I still find myself often "fighting" the type system.
Long story short, breadth-first and depth-first search algorithms are the same aside from the mechanics of pushing/popping to their underlying collection. Depth-first would prepend new possibilities and use a Stack, while Breadth-first would append and use a Queue.
To keep my algorithm the same, I created a typeclass called "GiveGrab" (I know, horrible name) with the intention of pimping ... err ... enriching collections with these "default" push (give) and pop-like (grab) operations.For example, grab would result in a call to .dequeue() for queues, and .pop() for stacks.
Here's (a somewhat abbreviated version of) the code:
object Example extends App {
trait GiveGrab[A, M[A]] {
def give(x: A*): M[A]
def grab(): A
}
implicit class GiveGrabQueue[T](q: Queue[T]) extends GiveGrab[T,Queue[T]] {
override def give(x: T*) = q ++= x
override def grab() = q.dequeue()
}
class TestClass[T, X <% GiveGrab[T, Queue[T]]](var storage: X) {}
val test = new TestClass[Int, Queue[Int]](new Queue[Int]())
}
When trying to compile this, I get the following errors:
Error:(18, 39) scala.collection.mutable.Queue[T] takes no type parameters, expected: one
class TestClass[T, X <% GiveGrab[T, Queue[T]]](var storage: X) {}
^
Error:(13, 67) scala.collection.mutable.Queue[T] takes no type parameters, expected: one
implicit class GiveGrabQueue[T](q: Queue[T]) extends GiveGrab[T,Queue[T]] {
^
That said, it took me a lot of trial and error to even get to this point. I'm not sure if my trait is really supposed to be typed
trait GiveGrab[A, M[A]]
or
trait GiveGrab[A, M[_]]
or
trait GiveGrab[A, M]
The error "takes no type parameters, expected: one" doesn't make a whole lot of sense to me at this point, and there's only a handful of other posts about that message (some related to dependent types, and some related to the Play framework).
Somewhat related: is there a good article for understanding Scala type signatures? I read through Programming in Scala 2nd Ed, but it didn't really touch on this sort of type gymnastics (either that, or I just missed it.)
Edit: Typos