Generate Option[T] in ScalaCheck

22
votes

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?

2
scalascalacheck
About EDIT 2. I just found out that Gen#option[T] was introduced with version 1.11.5 of scalacheck - see here. The method will return either an instance of Some[T] or None with equal probability. – erasing

2 Answers

30
votes

Now you can just use:

Gen.option(yourGen)
9
votes

You can use pick to randomly choose between a Some and a None generator:

val someThing = thingGenerator.map( Some.apply )
val noThing = Gen.value( None:Option[Thing] )
val optThing = Gen.oneOf( someThing, noThing )