Mike Slinn

Scala Type Parameters, Implicit Manifests and Salat

Published 2012-08-02.
Time to read: 1 minutes.

This page is part of the posts collection, categorized under Scala.

Type parameters provide types for constructing any arguments that are implicit manifests.

This helps when working with Salat. In the following code, dbo is a com.mongodb.DBObject, perhaps retrieved as the result of a find(). Salat uses _typeHint to store the fully qualified name of the persisted class; this property is returned as one of the properties of dbo. The call to Salat's grater() method converts dbo into the desired type. The type is constrained using a lower bound to be a subclass of PubSubAction.

def findWithSeqNum[T <: PubSubAction](seqNum: Long)
    (implicit manifest: Manifest[T]): Option[T] = {
  val klass = manifest.erasure.asInstanceOf[Class[T]]
  val item = myCollection.findOne(
    MongoDBObject("seqNum" -> seqNum, "_typeHint" -> klass.getName))
  item match {
    case Some(dbo: DBObject) =>
      Some(grater(ctx, Manifest.classType(klass)).asObject(dbo))
case None => None } }

When called as follows, the type parameter need not be provided because there is an explicit manifest argument:


When called as follows, the type parameter provides the value for constructing the implicit manifest argument:


Of course, you can also explicitly define an implicit manifest at any scope that you desire:

implicit val manifest: ClassManifest[Blah]

If the type of the manifest is dynamic (only known at runtime), you can define manifest this way:

val mtype = "com.micronautics.Blah"
implicit val manifest = Manifest.classType(Class.forName(mtype))

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