[ Acquiring 2 implicits for scalacheck function ]
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 Buildable
s 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]
}