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scalascalacheck

Generate Option[T] in ScalaCheck

I am trying to generate optional parameters in ScalaCheck, without success.

There seems to be no direct mechanism for this. Gen.containerOf[Option, Thing](thingGenerator) fails because it cannot find an implicit Buildable[Thing, Option].

I tried 

for {
  thing <- Gen.listOfN[Thing](1, thingGenerator)
} yield thing.headOption

But this doesn't work because listOfN produces a list that is always of length N. As a result I always get a Some[Thing]. Similarly, listOf1 does not work, because (a) it doesn't produce empty lists, but also (b) it is inefficient because I can't set a max limit on the number of elements.

How can I generate Option[Thing] that includes Nones?

EDIT: I have found a solution, but it is not succinct. Is there a better way than this?

for {
  thing <- for {
    qty <- Gen.choose(0,1)
    things <- Gen.listOfN[Thing](qty, thingGenerator)
  } yield things.headOption
} yield thing

EDIT 2: I generalised this to 

def optional[T](g: Gen[T]) = 
  for (qty <- Gen.choose(0, 1); xs <- Gen.listOfN[T](qty, g)) yield xs.headOption

So I don't have to write it more than once. But surely this is in the library already and I just missed it?

Solution

  • Now you can just use:

    Gen.option(yourGen)