Question

I am using scalacheck and am in the middle of a generic-programming soup right now. The official guide shows this example:

def matrix[T](g: Gen[T]): Gen[Seq[Seq[T]]] = Gen.sized { size =>
  val side = scala.math.sqrt(size).asInstanceOf[Int]
  Gen.listOfN(side, Gen.listOfN(side, g))
}

Meanwhile, for my test I require a matrix of type Array[Array[T]]. I tried with the following function:

def matrix[T](g: Gen[T]): Gen[Array[Array[T]]] = Gen.sized { size =>
  val side = scala.math.sqrt(size).asInstanceOf[Int]
  val g1 = Gen.containerOfN[Array, T](side, g)
  Gen.containerOfN[Array, Array[T]](side, g1)
}

Here, I run into trouble. The compiler says:

Multiple markers at this line
– not enough arguments for method containerOfN: (implicit b: org.scalacheck.util.Buildable[T,Array])org.scalacheck.Gen[Array[T]].
Unspecified value parameter b.
– could not find implicit value for parameter b: org.scalacheck.util.Buildable[T,Array]
– could not find implicit value for parameter b: org.scalacheck.util.Buildable[T,Array]
– not enough arguments for method containerOfN: (implicit b: org.scalacheck.util.Buildable[T,Array])org.scalacheck.Gen[Array[T]].
Unspecified value parameter b.

I understand that stuff like this is usually remedied by adding implicit parameters to the function, however, i havent made this work yet.

I usually encounter this error when building generic arrays, as an example:

  def build[T](n:Int)(implicit m:ClassManifest[T]) = Array.ofDim[T](n)

but, I am afraid I don’t fully understand what is happening or why this is needed.

Can someone explain how to make the correct matrix-function along with an example of usage in scalacheck? A thorough explanation of the details about building sequences with implicit class manifests would be very welcome!

edit

  import org.scalacheck.util.Buildable._
  def matrix[T](g: Gen[T])(implicit b: Buildable[T, Array]): Gen[Array[Array[T]]] = Gen.sized { size =>

    val side = scala.math.sqrt(size).asInstanceOf[Int]
    val g1 = Gen.containerOfN[Array, T](side, g)
    Gen.containerOfN[Array, Array[T]](side, g1)
  }

Still doesn’t work. Need implicit for Buildable[Array[T],Array]… Don’t know how to get this because I can only add 1 implicit argument :/

Answer 1

You’re almost there. The important part of the error is could not find implicit value for parameter b: org.scalacheck.util.Buildable[T,Array]

Looking at the method definition of containerOfN

  def containerOfN[C[_],T](n: Int, g: Gen[T])(implicit b: Buildable[T,C]): Gen[C[T]] = ...

So, there’s your missing argument. You need an implicit argument of type Buildable[T,Array]. Following through to where Buildable is defined in the scalacheck sources, I found that there was an object (org.scalacheck.util.Buildable) that provides implicits that provide Buildables for the common collection types which includes Array. So all you need to bring that into scope. You can do this with:

  import org.scalacheck.util.Buildable._
  def matrix[T](g: Gen[T]): Gen[Array[Array[T]]] = Gen.sized { size =>
    val bT = implicitly[Buildable[T, Array]]
    val bArrayT = implicitly[Buildable[Array[T], Array]]

    val side = scala.math.sqrt(size).asInstanceOf[Int]
    val g1 = Gen.containerOfN[Array, T](side, g)
    Gen.containerOfN[Array, Array[T]](side, g1)
  }

Or

  import org.scalacheck.util.Buildable._
  def matrix[T](g: Gen[T])(implicit bT: Buildable[T, Array], bArrayT: Buildable[Array[T], Array]): Gen[Array[Array[T]]] = Gen.sized { size =>
    ...
  }

The particular implicit you need in org.scalacheck.util.Buildable is:

implicit def buildableArray[T](implicit cm: ClassManifest[T]) = 
  new Buildable[T,Array] {
    def builder = mutable.ArrayBuilder.make[T]
  }