Creating an Arbitrary of a class

Question

I have a class that looks like this:

class EpidemySimulator extends Simulator {
   ...
   class Person{
   // Implementation
   }
}

I want to add an Arbitrary of the inner class Persons so that I can define some properties on the EpidemySimulator class. I thought the companion object can define the necessary implicit Generators needed in order to define properties which will generate objects of type Person.

object EpidemySimulator
{
    implicit def arbPerson(implicit sim: EpidemySimulator ) = 
    Arbitrary(for(n <- Gen.choose(1, roomRows * roomColumns))yield{ new sim.Person(n) })

    def propertyOne(sim: EpidemySimulator ) = forAll { person: sim.Person =>
         person.row > 0 && person.row < roomRows && person.col > 0 && person.col < roomColumns  
    }
}

My understanding was that if I provided the implicit function that creates Arbitrary[sim.Person] given a particular sim:EpidemySimulator ( which is provided implicitly ) then I should be able to create a property as in propertyOne. However I get a compiler error on the property which says

Multiple markers at this line
- identifier expected but ';' found.
- not enough arguments for method forAll: (implicit p: Boolean => org.scalacheck.Prop, implicit a1: org.scalacheck.Arbitrary[sim.Person], 
 implicit s1: org.scalacheck.Shrink[sim.Person], implicit pp1: sim.Person => org.scalacheck.Pretty)org.scalacheck.Prop. Unspecified value parameters 
 a1, s1, pp1.
- could not find implicit value for parameter a1: org.scalacheck.Arbitrary[sim.Person]
- type <error> is not a member of simulations.EpidemySimulator

Any ideas what I'm doing wrong. I want to be able to ultimately:

class EpidemySuite extends FunSuite with Checkers {
    test("EpidemySimulator fulfills property one"){
        val es = new EpidemySimulator
        check( EpidemySimulator.propertyOne(sim) )
    }
}

Answer 1

I think the issue is the following: you have provided an implicit to generate an arbitrary EpidemySimulator#Person given an EpidemySimulator.

implicit def arbPerson(implicit sim: EpidemySimulator ) = 
Arbitrary(for(n <- Gen.choose(1, roomRows * roomColumns))yield{ new sim.Person(n) })

But in the test for the EpidemySimulator, there is no implicit value of type EpidemySimulator available, so the above implicit for a person can not be used. Making the sim implicit or defining a local implicit of type epidemySimulator should fix the issue

def propertyOne(implicit sim: EpidemySimulator ) = forAll { person: sim.Person =>
     person.row > 0 && person.row < roomRows && person.col > 0 && person.col < roomColumns  
}

Given that you will probably want to test various properties of an EpidemySimulator, it is probably a good idea to define an implicit epidemySimulator val somewhere in your object.

Note that I only checked these with a scala console session like this:

import org.scalacheck._
import org.scalacheck.Prop.forAll

// define a class with an inner class
class A { class B }
// define an implicit that provides a A#B given an A
implicit def arbAB(implicit a:A) : Arbitrary[a.B] = Arbitrary[a.B] { new a.B }
// the non-working code
def test(a:A) = forAll { x:a.B => true }
<console>:25: error: could not find implicit value for parameter a1: org.scalacheck.Arbitrary[a.B]
   def test(a:A) = forAll { x:a.B => true }
                          ^ 
// making a an implicit works
def test(implicit a:A) = forAll { x:a.B => true }