How to read an element from a Scala HList?

Question

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?

Answer 1

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

Answer 2

Just one thing... if it is not too important than just stick to HList as the type . Do not alias it to MyRow unless necessary.

So.. you had

val row = a :: b :: c :: ... :: HNil

How about this ?

val yourX = row match { case a :: b :: c :: x ::: rest => x }

notice that ::: instead of :: at the end.

Or... how about this,

val yourX = row.tail.tail.tail.head

// this may change a little if you had,
def MyRow(a, b, c, ...): MyRow = a :: b :: c :: ... :: HNil

val row = MyRow(a, b, c, ...)

val yourX = row.asInstanceOf[HList].tail.tail.tail.head