Family polymorphism with ScalaTest

Question

I'm trying to achieve something with ScalaTest and based on C. Hosrtmann's "Scala for the impatient" I think this is related to "Family polymorphism". Apparently I don't understand something about Scala's type system, so I fail.

To cut the long story short, this is what I'm trying to do.

I have a base (production) trait:

trait MaxPQ[Key] {

  implicit protected val cmp: Ordering[_ >: Key]
  
  ...

  def insert(v: Key): Unit

  def max(): Key

  def delMax(): Key

  ...
}

then there is a number of implementations (that use either a backing tree or an array).

In tests I want to create an abstract structure which would allow to test any implementation for three Ordered Keys: Char, Int, Double.

First I've written two behaviors (for empty and non-empty priority queues). Here's a snippet:

trait MaxPQBehaviours {
  // underlying tests spec
  self: BaseSpec =>

  def nonEmptyMaxPQ[T <: Ordered[T], ImplLà <: MaxPQ[T]](instanceSupplier: () => ImplLà, sortedInput: List[T]): Unit = {

    ...

    behavior of "size"

    it should s"be equal to ${sortedInput.size}" in {
      val instance = instanceSupplier()
      instance.size() shouldEqual sortedInput.size
    }

    behavior of "max"

    it should s"return the expected $max" in {
      val instance = instanceSupplier()
      instance.max() shouldEqual max
    }
    ...

Finally to add a last layer of abstraction I'm adding a BaseMaxPQSpec that mixes-in the above MaxPQBehaviours and calls its behaviors for three abstract MaxPQ types. Here I'only provide example for Char:

trait BaseMaxPQSpec extends BaseSpec with MaxPQBehaviours {

  type CharMaXPQ <: MaxPQ[Char]
 
  def charMaxPQ: CharMaXPQ
  val sortedCharsList: List[Char] = List[Char]('C', 'b', 'd', 'a', 'z').sorted

  it should behave like nonEmptyMaxPQ(() => charMaxPQ, sortedCharsList)
}

And this is where compiler spits this at me:

[error] ~/Algorithms/Chapter 2 Sorting/algorithms2_1/src/test/scala/ca/vgorcinschi/algorithms2_4/BaseMaxPQSpec.scala:18:25: inferred type arguments [Char,BaseMaxPQSpec.this.CharMaXPQ] do not conform to method nonEmptyMaxPQ's type parameter bounds [T <: Ordered[T],ImplLà <: ca.vgorcinschi.algorithms2_4.MaxPQ[T]]
[error]   it should behave like nonEmptyMaxPQ(() => charMaxPQ, sortedCharsList)
[error]                         ^
[error] ~/Algorithms/Chapter 2 Sorting/algorithms2_1/src/test/scala/ca/vgorcinschi/algorithms2_4/BaseMaxPQSpec.scala:18:42: type mismatch;
[error]  found   : () => BaseMaxPQSpec.this.CharMaXPQ
[error]  required: () => ImplLà
[error]   it should behave like nonEmptyMaxPQ(() => charMaxPQ, sortedCharsList)
[error]                                          ^
[error] ~/Algorithms/Chapter 2 Sorting/algorithms2_1/src/test/scala/ca/vgorcinschi/algorithms2_4/BaseMaxPQSpec.scala:18:56: type mismatch;
[error]  found   : List[Char]
[error]  required: List[T]
[error]   it should behave like nonEmptyMaxPQ(() => charMaxPQ, sortedCharsList)
[error]                                                        ^
[error] three errors found
[error] (Test / compileIncremental) Compilation failed
[error] Total time: 12 s, completed Feb 15, 2020 6:21:35 PM

What is the correct way to set-up my testing framework? Please don't hesitate to ask for details, clarifications.

Mario Galic Mario Galic · Accepted Answer · 2020-02-16T00:13:06

Given

nonEmptyMaxPQ(() => charMaxPQ, sortedCharsList)

type inference deduces type T = Char because the type of charMaxPQ is MaxPQ[Char], thus

T <: Ordered[T]

becomes Char <: Ordered[Char]

which certainly is not true. Perhaps try specifying Ordering like so

def nonEmptyMaxPQ[T: Ordering, Impl <: MaxPQ[T]](instanceSupplier: () => Impl, sortedInput: List[T])

Note the difference between upper bound T <: Ordered[T] and context bound T: Ordering.

Family polymorphism with ScalaTest

1 Answers