Use ScalaCheck generator inside another generator

Question

I have a generator that creates a very compelx object. I cannot create this object through something like

val myGen = for{ a <- Gen.choose(-10,10) ... } yield new MyClass(a,b,c,...)

I tried an approach of creating a custom generator like this

val myComplexGen :Gen[ComplexObject] = { ... val myTempVariable = Gen.choose(-10,10) val otherTempVal = Gen.choose(100,2000) new MyComplexObject(myTempVariable,otherTempVal,...) }

and then

test("myTest") { forAll(myComplexGen){ complexObj => ... // Here, complexObj.myTempVariable is always the same through all the iterations } }

While this works, the values generated are always the same. The inner Gen.choose yield always the same value.

Is there any way I can write a custom Gen with its own logic, and use inner Gen.choose inside, that would be random ?

There's good documentation of using for-comprehension to create a case class github.com/rickynils/scalacheck/blob/master/doc/… You only need to use new is the only difference. — ashawley
– ashawley, Commented Apr 16, 2019 at 17:43
The thing is, my complexObject is not a case class, it needs to be built with a builder, and other setters. It cannot be instanciated in a oneliner. — Arthur Attout
– Arthur Attout, Commented Apr 16, 2019 at 18:39
Oh, ok. You can use curly braces after yield to do the building you need for MyComplexObject. — ashawley
– ashawley, Commented Apr 17, 2019 at 1:22

Arthur Attout · Accepted Answer · 2019-04-16 08:57:29Z

I've been able to workaround the problem. The solution is definitely not elegant but that's the only way I could work it out.

I have transformed myComplexGen into a def, and called it inside another gen with dummy variables

def myComplexGen :ComplexObject = { ... val myTempVariable = Gen.choose(-10,10) val otherTempVal = Gen.choose(100,2000) new MyComplexObject(myTempVariable,otherTempVal,...) }

val realComplexGen :Gen[ComplexObject] = for { i <- Gen.choose(0,10) // Not actually used, but for cannot be empty } yield myComplexGen()

Now I can use realComplexGenin a forAll and the object is really random.

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