There is very few readable documentation about HLists, and the answers I can find on SO come from outer space for a humble Scala beginner.
I encountered HLists because Slick can auto-generate some to represent database rows. They are slick.collection.heterogeneous.HList (not shapeless').
Example:
type MyRow = HCons[Int,HCons[String,HCons[Option[String],HCons[Int,HCons[String,HCons[Int,HCons[Int,HCons[Option[Int],HCons[Option[Float],HCons[Option[Float],HCons[Option[String],HCons[Option[String],HCons[Boolean,HCons[Option[String],HCons[Option[String],HCons[Option[String],HCons[Option[String],HCons[Option[String],HCons[Option[Int],HCons[Option[Float],HCons[Option[Float],HCons[Option[Float],HCons[Option[String],HCons[Option[String],HNil]]]]]]]]]]]]]]]]]]]]]]]]
def MyRow(a, b, c, ...): MyRow = a :: b :: c :: ... :: HNil
Now given one of these rows, I'd need to read one element, typed if possible. I just can't do that. I tried
row(4) // error
row._4 // error
row.toList // elements are inferred as Any
row match { case a :: b :: c :: x :: rest => x } // "Pattern type is incompatible. Expected MyRow."
row match { case MyRow(_,_,_,_,_,x,...) => x } // is not a case class like other rows
row match { HCons[Int,HCons[String,HCons[Option[String],HCons[Int,HCons[String, x]]]]] => x.head } // error
row.tail.tail.tail.tail.head // well, is that really the way??
Could somebody please explain how I can extract a specific value from that dinosaur?
I'd expect your row(0) lookup to work based on the HList API doc for apply. Here's an example I tried with Slick 3.1.1:
scala> import slick.collection.heterogeneous._
import slick.collection.heterogeneous._
scala> import slick.collection.heterogeneous.syntax._
import slick.collection.heterogeneous.syntax._
scala> type MyRow = Int :: String :: HNil
defined type alias MyRow
scala> val row: MyRow = 1 :: "a" :: HNil
row: MyRow = 1 :: a :: HNil
scala> row(0) + 99
res1: Int = 100
scala> val a: String = row(1)
a: String = a