Help with recursive "MyType" in 2.10

[Naftoli Gugenheim]

Hi, I'm trying to get lift-squeryl-record to compile on 2.10. It compiles on 2.9.x. Here's the relevant code:

trait Record[MyType <: Record[MyType]] extends FieldContainer {

  self: MyType =>

...

  def meta: MetaRecord[MyType]

...

  def fieldByName(fieldName: String): Box[Field[_, MyType]] = meta.fieldByName(fieldName, this)

...

}

trait MetaRecord[BaseRecord <: Record[BaseRecord]] {

  self: BaseRecord =>

...

  def fieldByName(fieldName: String, inst: BaseRecord): Box[Field[_, BaseRecord]] = {

    Box(fieldMap.get(fieldName).map(_.field(inst)))

...

  }

trait OwnedField[OwnerType <: Record[OwnerType]] extends BaseField {

...

}

trait BaseField extends FieldIdentifier with util.BaseField {

...

}

trait TypedField[ThisType] extends BaseField {

...

}

trait Field[ThisType, OwnerType <: Record[OwnerType]] extends OwnedField[OwnerType] with TypedField[ThisType] {

...

}

class RecordMetaDataFactory extends FieldMetaDataFactory {

...

  private var metaRecordsByClass: Map[Class[_], MetaRecord[_]] = Map.empty

...

  private def findMetaField(clasz: Class[_], name: String): BaseField = {

    def fieldFrom(mr: MetaRecord[_]): BaseField =

      mr.asInstanceOf[Record[_]].fieldByName(name) match {

        case Full(f: BaseField) => f

        case Full(_) => org.squeryl.internals.Utils.throwError("field " + name + " in Record metadata for " + clasz + " is not a TypedField")

        case _ => org.squeryl.internals.Utils.throwError("failed to find field " + name + " in Record metadata for " + clasz)

      }

    metaRecordsByClass get clasz match {

      case Some(mr) => fieldFrom(mr)

      case None =>

        try {

          val rec = clasz.newInstance.asInstanceOf[Record[_]]

          val mr = rec.meta

          metaRecordsByClass = metaRecordsByClass updated (clasz, mr)

          fieldFrom(mr)

        } catch {

          case ex => org.squeryl.internals.Utils.throwError("failed to find MetaRecord for " + clasz + " due to exception " + ex.toString)

        }

    }

  }

}

At this point I get errors inside findMetaField such as:

[error] /home/naftoli/dev/liftweb/framework/persistence/squeryl-record/src/main/scala/net/liftweb/squerylrecord/RecordMetaDataFactory.scala:39: type arguments [?,_$5] do not conform to trait Field's type parameter bounds [ThisType,OwnerType <: net.liftweb.record.Record[OwnerType]]

[error]         case Full(f: BaseField) => f

[error]                  ^

[error] /home/naftoli/dev/liftweb/framework/persistence/squeryl-record/src/main/scala/net/liftweb/squerylrecord/RecordMetaDataFactory.scala:38: type arguments [?,_$5] do not conform to trait Field's type parameter bounds [ThisType,OwnerType <: net.liftweb.record.Record[OwnerType]]

[error]       mr.asInstanceOf[Record[_]].fieldByName(name) match {

[error]                                             ^

I tried a lot of massaging and only managed to exchange the errors for some others. For instance using T forSome { type T <: Record[T] } didn't seem to work.

Any ideas?

[Aleksey Nikiforov]

From a quick glance it looks like the problem is creating a concrete type from a recursive type. If that is the case, then try the following hack. It is not pretty, but it should get the job done:

val Recursor: { type Recursive <: Record[Recursive] } = null
type Recursive = Recursor.Recursive

Having to rely on these hacks led me to abandon recursive types in the first place. Please let me know if you find a more elegant solution.

[Chris Marshall]

You can also use existentials:

MyType[T] forSome { type T <: MyType[T] }

Chris

[nafg]

Thanks, that worked!

It's not my code here, but in general how do you encode MyType without them? Abstract type members cannot be used in the self type, I don't think.