I am still trying to get my head around Shapeless (and, to a lesser extent, Scala!) and I have been writing some simple code to generate random instance data for case classes - predominantly based on the guides here: http://enear.github.io/2016/09/27/bits-of-shapeless-2/ (the example covers a JSON Writer implementation)
I have created a Generator[A]
trait and created implicit implementations for simple types, and as per the example in the above link, I have also created implicit implementations to handle HList, HNil, Coproduct and CNil:
implicit def hnilGenerator = new Generator[HNil] {
override def generate(a: HNil) = HNil
}
implicit def hconsGenerator[H, T <: HList](implicit headGen: Generator[H], tailGen: Generator[T]) =
new Generator[H :: T] {
override def generate(a: H :: T) = headGen.generate(a.head) :: tailGen.generate(a.tail)
}
implicit def cnilGenerator: Generator[CNil] =
new Generator[CNil] {
override def generate(a: CNil): CNil = throw new RuntimeException("Invalid candidate configuration")
}
implicit def cconsGenerator[H, T <: Coproduct] =
new Generator[H :+: T] {
override def generate(a: H :+: T) = throw new RuntimeException("Invalid candidate configuration")
}
I can now use this code to generate a random instance based on a case class or a sealed trait:
it("should work with a case class to hlist") {
case class Test(x: IntGene, y: DoubleGene, z: BooleanGene)
val c = Generic[Test].to(Test(IntGene(), DoubleGene(), BooleanGene()))
generate(c)
}
it("what happens with sealed traits") {
sealed trait Shape
case class Square(width: Int, height: Int) extends Shape
case class Circle(radius: Int) extends Shape
val c = Generic[Shape].to(Circle(1))
generate(c)
}
Both of the above work no problem, however, if I try to make this a generic (as in parameter types) I get compilation errors not being able to find the necessary implicts:
it("should handle generics") {
case class GenericTest[A: Generic](x: A) {
def convert() = {
val c = Generic[A].to(x)
generate(c)
}
}
}
So from my understanding, because I have used the Generic
context bound A
, the compiler knows that is going to be available, so c
must be some possible return from the call to(x)
- Am I missing something in the implementation to handle that return type from the Generic
shapeless call? Or have I wildly misunderstood something?
I am hoping this is possible and I have just missed something - is it that the compiler doesn't know what will be passed in (Im assuming not), or is there another possible type that needs to be handled implicitly from that to(x)
call?
EDIT
Compile error added below - I'm really just trying to understand: Is it that there is some return case from the to(x)
shapeless call that I have not catered for, or is it because the compiler doesn't have any idea what will be passed in and there are some types not catered for (e.g. I haven't added a Date generator implicit - and a case class could potentially have any type included? I was hoping that was not the case, and as the compiler knows nothing is actually being passed to the class/method it knows there are no issues?)
GeneratorSpec.scala:44: could not find implicit value for parameter gen: io.github.robhinds.genotype.Generator[GenericTest.this.evidence$1.Repr]
generate(c)
And my generate method is just a simple helper method that gets given the implicit Generator
:
def generate[A](a: A)(implicit gen: Generator[A]) = gen.generate(a)
shapeless.TypeClassCompanion
, which is meant to deal with typeclasses for ADT? – Cyrille Corpetgenerate()
? – marios