How to create a random instance of a case class? - scala

Question

Suppose I've got a few case classes, e.g.:
case class C(c1: Int, c2: Double, c3: Option[String])
case class B(b: Int, cs: Seq[C])
case class A(a: String, bs: Seq[B])
Now I would like to generate a few instances of A with random values for tests.
I am looking for a generic way to do that. I can probably do it with runtime reflection but I prefer a compile-time solution.
def randomInstance[A](a: A): A = ???
How can I do it ? Can it be done with shapeless ?

Answer 1

The easiest way for you to do that would be using ScalaCheck. You do so by defining a Gen[A] for your instances:
import org.scalacheck.Gen
final case class C(c1: Int, c2: Double, c3: Option[String])
object C {
val cGen: Gen[C] = for {
c1 <- Gen.posNum[Int]
c2 <- Gen.posNum[Double]
c3 <- Gen.option(Gen.oneOf("foo", "bar", "hello"))
} yield C(c1, c2, c3)
}
And you consume it:
object F {
def main(args: Array[String]): Unit = {
val randomC: C = C.cGen.sample.get
}
}
On top of that, you can add scalacheck-shapeless which generates the Gen[A] for you, with completely random values (where you have no control over them).
You may also want to look into random-data-generator (thanks #Gabriele Petronella), which simplifies things even further. From the docs:
import com.danielasfregola.randomdatagenerator.RandomDataGenerator
object MyApp extends RandomDataGenerator {
case class Example(text: String, n: Int)
val example: Example = random[Example]
// Example(ਈ䈦㈾钜㔪旅ꪔ墛炝푰⡨䌆ᵅ퍧咪, 73967257)
}
This is also especially helpful in property based testing.

Answer 2

We've just moved away from scalacheck-shapeless and use Scala/Java reflection instead.
The main reasons are (1) scalacheck-shapeless uses Macros (slow compilation), (2) the API is a bit more verbose than my liking, and (3) the generated values are way too wild (e.g. generating strings with Japanese characters).
However, setting it up is a bit more involved. Here is a full working code that you can copy into your codebase:
import scala.reflect.api
import scala.reflect.api.{TypeCreator, Universe}
import scala.reflect.runtime.universe._
object Maker {
val mirror = runtimeMirror(getClass.getClassLoader)
var makerRunNumber = 1
def apply[T: TypeTag]: T = {
val method = typeOf[T].companion.decl(TermName("apply")).asMethod
val params = method.paramLists.head
val args = params.map { param =>
makerRunNumber += 1
param.info match {
case t if t <:< typeOf[Enumeration#Value] => chooseEnumValue(convert(t).asInstanceOf[TypeTag[_ <: Enumeration]])
case t if t =:= typeOf[Int] => makerRunNumber
case t if t =:= typeOf[Long] => makerRunNumber
case t if t =:= typeOf[Date] => new Date(Time.now.inMillis)
case t if t <:< typeOf[Option[_]] => None
case t if t =:= typeOf[String] && param.name.decodedName.toString.toLowerCase.contains("email") => s"random-$arbitrary#give.asia"
case t if t =:= typeOf[String] => s"arbitrary-$makerRunNumber"
case t if t =:= typeOf[Boolean] => false
case t if t <:< typeOf[Seq[_]] => List.empty
case t if t <:< typeOf[Map[_, _]] => Map.empty
// Add more special cases here.
case t if isCaseClass(t) => apply(convert(t))
case t => throw new Exception(s"Maker doesn't support generating $t")
}
}
val obj = mirror.reflectModule(typeOf[T].typeSymbol.companion.asModule).instance
mirror.reflect(obj).reflectMethod(method)(args:_*).asInstanceOf[T]
}
def chooseEnumValue[E <: Enumeration: TypeTag]: E#Value = {
val parentType = typeOf[E].asInstanceOf[TypeRef].pre
val valuesMethod = parentType.baseType(typeOf[Enumeration].typeSymbol).decl(TermName("values")).asMethod
val obj = mirror.reflectModule(parentType.termSymbol.asModule).instance
mirror.reflect(obj).reflectMethod(valuesMethod)().asInstanceOf[E#ValueSet].head
}
def convert(tpe: Type): TypeTag[_] = {
TypeTag.apply(
runtimeMirror(getClass.getClassLoader),
new TypeCreator {
override def apply[U <: Universe with Singleton](m: api.Mirror[U]) = {
tpe.asInstanceOf[U # Type]
}
}
)
}
def isCaseClass(t: Type) = {
t.companion.decls.exists(_.name.decodedName.toString == "apply") &&
t.decls.exists(_.name.decodedName.toString == "copy")
}
}
And, when you want to use it, you can call:
val user = Maker[User]
val user2 = Maker[User].copy(email = "someemail#email.com")
The code above generates arbitrary and unique values. They aren't exactly randomised. It's best for using in tests.
Read our full blog post here: https://give.engineering/2018/08/24/instantiate-case-class-with-arbitrary-value.html

Answer 3

We've started using Magnolia, which provides a faster type class derivation compared to shapeless for derivation of Arbitrary instances.
Here is the library to use, and here is an example (docs):
case class Inner(int: Int, str: String)
case class Outer(inner: Inner)
// ScalaCheck Arbitrary
import magnolify.scalacheck.auto._
import org.scalacheck._ // implicit instances for Arbitrary[Int], etc.
val arb: Arbitrary[Outer] = implicitly[Arbitrary[Outer]]
arb.arbitrary.sample
// = Some(Outer(Inter(12345, abcde)))