Scala method which accepts multiple different objects, all of which extend the same trait

Question

I have a trait Document[T], and two case classes which extend that trait:

Memo(...) extends Document[Memo]
Fax(...) extends Document[Fax]

Each represents a different type of text file.

I want to clean these documents using some text tools, but creating a method that takes both types has vexed me.

val rawText: Seq[String]
// The text of all documents where one string in the sequence is the text of one document

val documentType = "Memo"
// Can also be "Fax"

val documentObjects = documentType match {
case "Memo" => rawText.map(_.makeMemoDocument) // results in Seq[Memo]
case "Fax" => rawText.map(_.makeFaxDocument) // results in Seq[Fax]
}

// Here lies my dilemma...
def processDocuments(docs: Seq[Document[T]]): Seq[Document[T]] = {...}

val processedDocs = processDocuments(documentObjects)

I want to define documentObjects in a way so that it can be easily accepted by processDocuments, i.e. processDocuments should accept either a Seq[Memo] or a Seq[Fax] as an argument.

I am creating case classes to be run through a Stanford CoreNLP pipeline, and I want to be able to support adding multiple case classes extending the Document[T] trait in the future (e.g. later on, add support for Pamphlet(...) extends Document[Pamphlet]).

Any help would be appreciated. If you require more information, I'd be happy to provide it.

Are your sure you need F-Bounded Polymorphism, I mean, do you really need your document to extend itself, like in class Memo(...) extends Document[Memo]? - Anyways, if yes, you can write your method like def processDocuments[T <: Document[T]](docs: Seq[T]): Seq[T] — Luis Miguel Mejía Suárez
– Luis Miguel Mejía Suárez, Commented May 17, 2019 at 19:20

Bogdan Vakulenko · Accepted Answer · 2019-05-17 19:55:17Z

1

val documentObjects:Document[_] = documentType match {
   case "Memo" => rawText.map(_.makeMemoDocument) // results in Seq[Memo]
   case "Fax" => rawText.map(_.makeFaxDocument) // results in Seq[Fax]
}

def processDocuments[T](docs: Seq[Document[T]]): Seq[Document[T]] = ???
val processedDocs = processDocuments(Seq(documentObjects))

or remove type declaration on documentObjects and use

def processDocuments(docs: Seq[Document[_]]): Seq[Document[_]] = ???
//or
def processDocuments[T <: Document[_]](docs: Seq[T]): Seq[T]= ???

edited May 17, 2019 at 19:55

answered May 17, 2019 at 19:43

Bogdan Vakulenko

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