Broken code generation for empty enums

[cornerman]

We are working with a hasura-generated schema, which somehow contains empty enums like this one:

enum my_empty_enum {}

But when compiling the generated code, we get this compile error:

[error]      type mismatch;
[error]        scala.Unit|caliban.client.ArgEncoder[example.hasura.unauthorized.graphql.client.HasuraUnauthorized.my_empty_enum]
[error]      L523:     implicit val encoder: ArgEncoder[my_empty_enum]    = {}

I am not sure what an empty enum is supposed to mean. It is only empty when accessing hasura unauthorized, but has values when accessing it authorized.

Should this be supported or is that an invalid schema?

[ghostdogpr]

See my comment in the other issue, that's an invalid schema. We won't be able to generate a proper query for it.

[cornerman]

Thank you! Does it make sense to fail early while parsing the schema? That way you get a proper error message. It is already the case when you try the generate code for enum my_empty_enum.

[ghostdogpr]

Let's catch that case and fail with a nice error message rather than generating code that doesn't compile.

[jyoo980]

I can take this issue on. From a preliminary investigation, I can see that there does appear to be some early validation for non-empty enum values in Validator.scala

caliban/core/src/main/scala/caliban/validation/Validator.scala

Lines 739 to 747 in a1673fe

	private[caliban] def validateEnum(t: __Type): IO[ValidationError, Unit] =
	t.enumValues(__DeprecatedArgs(Some(true))) match {
	case Some(_ :: _) => IO.unit
	case _ =>
	failValidation(
	s"Enum ${t.name.getOrElse("")} doesn't contain any values",
	"An Enum type must define one or more unique enum values."
	)
	}

So I'm wondering why this doesn't appear to be catching the case where there's an empty enum. That said, I'm not super familiar with the system.

It looks like the place to catch it would likely be in SchemaWriter.scala, is that right?

[ghostdogpr]

@jyoo980 that would be in ClientWriter.scala

[jyoo980]

Got it, I see this function

caliban/tools/src/main/scala/caliban/tools/ClientWriter.scala

Lines 664 to 668 in a1673fe

	def writeEnum(
	typedef: EnumTypeDefinition,
	extensibleEnums: Boolean
	): String = {

A pretty hacky way of catching an empty enum would be to add something like

require(typedef.enumValuesDefinition.nonEmpty, s"Code generation for empty enum '${typedef.name}' failed")

to line 668, but that doesn't seem quite right, do you have any recommendations? It's pretty difficult to see failure cases for this specific object since I think it's predicated on the assumption that most invariants are checked beforehand.

That said, there does seem to be some precedent for this kind of early escape hatch; line 580 throws an exception when codegen is attempted for an object with less than one field:

caliban/tools/src/main/scala/caliban/tools/ClientWriter.scala

Line 589 in a1673fe

case Nil => throw new Exception("Invalid GraphQL Schema: an object must have at least one field")

Do you have a preference?

[ghostdogpr]

I think it's fine to fail like on line 589 👍
We want to fail the code generation instantly with a useful message rather than generate code that doesn't compile.

[jyoo980]

I'd be happy to add some tests. Do you have any pointers?

Mixing exceptions with functional effects makes thing kinda hairy; I don't think there's a super nice way of testing this as it is right now.

[ghostdogpr]

In ClientWriterSpec, you can wrap the call to ClientWriter.write into a Task so that the exception is caught as a Failure rather than Die. Then with ZIO Test, you can do something like assertM(myEffect.run)(fails(someAssertionAboutTheFailure)). Have a look at check in ValidationSpec for an example.

[jyoo980]

Interesting, I have something like

      testM("enum with no body") {
        val schema =
          """
             enum Origin { }
            """.stripMargin
        assertM(ZIO.effect(gen(schema)).run)(fails(isSubtype[Exception](anything)))
      },

Which yields

 Success(zio.ZIO$FlatMap@468b5697) did not satisfy fails(isSubtype(Exception))

Strange, since the gen(scheme) should be throwing an exception inside ZIO.effect, which should cause it to be a Failure

[ghostdogpr]

gen returns a Task, so wrapping it into a Task will execute nothing (you'll have a Task[Task[A]]). You need to wrap ClientWriter.write and flatmap on it so that gen still return a Task.