How to properly map a tuple function across a Seq[A]? - scala

Question

I have a function that is attempting to build a JSON object containing a representation of multiple tuples that are stored in a Seq[A], where A is a (Loop, Option[User]). My code looks like so:
def loops = Action {
val page : Int = 1
val orderBy : Int = 1
val filter : String = ""
val jsonifyLoops : (Loop, Option[User]) => Map[String, String] = {
case (loop,user) =>
Map(
"name" -> loop.name,
"created_at" -> loop.createdAt.map(dateFormat.format).getOrElse(""),
"deleted_at" -> loop.deletedAt.map(dateFormat.format).getOrElse(""),
"user_name" -> user.map(_.name).getOrElse("")
)
}
Ok(toJson(Map(
"loops" -> toJson(
Loop.list( page = page, orderBy = orderBy, filter = ("%"+filter+"%") )
.items.map( jsonifyLoops )
)
)
))
}
Loops.list produces a Page[A], from the helper class below:
case class Page[A](items: Seq[A], page: Int, offset: Long, total: Long) {
lazy val prev = Option(page - 1).filter(_ >= 0)
lazy val next = Option(page + 1).filter(_ => (offset + items.size) < total)
}
Thus, Loops.list(...).items should get me a Seq[(Loop, Option[User])], onto which I should be able to apply a map function. I've defined my jsonifyLoops function to have what I think is the appropriate prototype, but I must be doing something wrong, because the compiler throws me the following error:
[error] [...] Application.scala:42: type mismatch;
[error] found : (models.Loop, Option[models.User]) => Map[String,String]
[error] required: (models.Loop, Option[models.User]) => ?
[error] .items.map( jsonifyLoops )
[error] ^
What am I doing wrong?

Answer 1

Your function jsonifyLoops takes two arguments: a Loop and an Option[User]. However, the members of items are tuples of the type (Loop, Option[User]), and thus items.map requires as an argument a function of one argument accepting that tuple. So, you need to convert jsonifyLoops from a binary function to a unary function that takes a pair of arguments; Function2#tupled will do this for you:
scala> :t jsonifyLoops
(Loop, Option[User]) => Map[String,String]
scala> :t jsonifyLoops.tupled
((Loop, Option[User])) => Map[String,String]
You'd use it like this:
Loop.list(page = page, orderBy = orderBy, filter = ("%"+filter+"%"))
.items.map(jsonifyLoops.tupled)

Answer 2

You need to add a default case to your pattern matching within jasonifyLoops.
In absence of a default case, if your case statement fails you return a Unit.
So something like this should work:
val jsonifyLoops : (Loop, Option[User]) => Map[String, String] = {
case (loop,user) =>
Map(
"name" -> loop.name,
"created_at" -> loop.createdAt.map(dateFormat.format).getOrElse(""),
"deleted_at" -> loop.deletedAt.map(dateFormat.format).getOrElse(""),
"user_name" -> user.map(_.name).getOrElse("")
)
case _ => Map[String, String]()
}
This just says that if the input does not match, return an empty Map. However, you should replace this with whatever handling you want to do for the default case.