Automatic generation of DSL given a case class

[gaeljw]

In some simple cases, it would be nice to support automatic generation of a DSL given a case class. Similar to what the Kotlin implementation offers with basedOnRequiredConstructorFields.

Expected usage would be something like following:

case class MyModel(a: String, b: Int, c: NestedModel)
case class NestedModel(x: Seq[Long])

val dsl = DslJsonBodyBuilder.fromType[MyModel]
// Gives a LambdaDslJsonBody => Unit
// Same as what would be given to:
newJsonObject { rootObj =>
  rootObj.stringType("a")
  rootObj.integerType("b")
  rootObj.newObject("c") { o =>
    o.newArray("x") { a =>
      a.longType()
    }
  }
}

We could also provide a helper in ScalaDsl (#355 ):

newJsonObject[MyModel]

The generated DSL would be using matching types, not values.

To implement that, we will need to use TypeTag but this won't be available in Scala 3 (there are other alternatives in Scala 3). This would mean a different codebase for Scala 2 & Scala 3 which is perfectly possible but I believe the sbt structure does not allow it for now.

[gaeljw]

Giving it a 2nd thought, maybe a simpler approach could be to expose a bodyForm(case class instance) method which would act similar to the existing body(...) but would take benefit of the JSON encoder provided by the user though it's library of choice (Circe, play JSON..) to generate a DSL given an actual JSON value.

This feels a bit like a hack though as we should not need a JSON library to generate a DSL from a case class and we would have to do the work once for each library.

On the other hand, it could be a simpler approach and it would reuse potentially custom encoders written by the user.

[jbwheatley]

is this not what we already provide with our body syntax?

A user can provide a case class and it will use e.g. a circe encoder to produce a DSL if using the circe import.

[gaeljw]

Unfortunately no because the body syntax already provided generates a DSL with "value matching" whereas I'm looking for "type matching".

Actually the body[A : PactBodyJsonEncoder] method doesn't bring much value IMHO as the same could be achieved with body(myJsonLib.generateJsonString(instance)). For instance: https://github.com/jbwheatley/pact4s/blob/main/play-json/src/main/scala/pact4s/playjson/implicits.scala#L29

There's actually a PactDslJsonBodyEncoder which looks more like what I'd want but there's no body method to use it right? And anyway, it still defines a DSL with "value matching" like https://github.com/jbwheatley/pact4s/blob/main/play-json/src/main/scala/pact4s/playjson/JsonConversion.scala#L30, here we call stringValue but I'd want stringType.