Gen of and harcoded collection one by one

[Marc-André Laverdière]

Hello everybody.

I really like ScalaCheck's power, and I've been able to do a lot with it so far. But there is always one use case that keeps tripping me up.

I have this collection of items and I need a Gen that will return one entry at a time, yet ensure that the whole collection is eventually returned.
This is useful when I need to generate associations.

The closest I got is Gen.zip(Gen.oneOf(collection), ...) but this doesn't work - I need a suchThat condition down the road and that gives me the dreaded
"Gave up after 0 successful property evaluations. 101 evaluations were discarded."

So how can I do this?

[Marc-André Laverdière]

[etorreborre]

I think that there should be a way to do this better but here is how to make it work with a bit of mutation:

implicit def ArbitraryList: Arbitrary[Int] =
  Arbitrary(distinct((1 to 5).toList))

def distinct[T](list: List[T]): Gen[T] = {
  var seen = List[T]()
  Gen.delay {
    if (seen.size == list.size)
      seen = List()

    Gen.oneOf(list.filterNot(seen.contains)).sample match {
      case Some(x) =>
        seen = x +: seen
        Gen.const(x)
      case None =>
        Gen.oneOf(list)
    }
  }
}

If I print out the generated elements I get:

1
2
3
4
5 

2
1
4
5
3

5
1
4
2
3 

4
1
2
3
5

3
5
1
2
4

Eric.