Implicit Generic.Aux missing on conversion from Shapeless HList to case class

Question

I just recently started learning scala and today I decided I wanted to write a CSV parser that would load nicely into case classes but store the data in rows (lists) of Shapeless's HList object so that I could get some exposure to type-level programming.

Here's what I have so far:

// LoadsCsv.scala

import shapeless._
import scala.collection.mutable

trait LoadsCsv[A, T <: HList] {

    val rows: mutable.MutableList[T] = new mutable.MutableList[T]

    def convert(t: T)(implicit gen: Generic.Aux[A, T]): A = gen.from(t)

    def get(index: Int): A = {
        convert(rows(index))
    }

    def load(file: String): Unit = {
        val lines = io.Source.fromFile(file).getLines()
        lines.foreach(line => rows += parse(line.split(",")))
    }

    def parse(line: Array[String]): T

}

And the object that's loading the data set:

// TennisData.scala

import shapeless._

case class TennisData(weather:String, low:Int, high:Int, windy:Boolean, play:Boolean)

object TennisData extends LoadsCsv[TennisData, String :: Int :: Int :: Boolean :: Boolean :: HNil] {

    load("tennis.csv")

    override def parse(line: Array[String]) = {
        line(0) :: line(1).toInt :: line(2).toInt :: line(3).toBoolean :: line(4).toBoolean :: HNil
    }

}

Things seem to be working alright until I added the get() with the conversion from the HList to the case class where I now get a compilation error. Why isn't the implicit getting loaded and what can I do to fix it or otherwise convert from the HList to the case class?

Error:(14, 17) could not find implicit value for parameter gen: shapeless.Generic.Aux[A,T]
        return convert(rows(index))
                      ^

I've been reading the shapeless documentation and it mentions that this area had been in flux between version 1 and 2, but I believe things should be working on my version of shapeless and scala so I suspect I've just done something incorrectly.

https://github.com/milessabin/shapeless/wiki/Migration-guide:-shapeless-1.2.4-to-2.0.0#iso-is-now-generic

For reference, I'm running scala 2.11.6 and shapeless 2.2.2

Answer 1

You're very close. The problem is that Scala isn't going to propagate implicit requirements up the call chain automatically for you. If you need a Generic[A, T] instance to call convert, then you'll have to make sure that one's in scope every time you call convert convert. If A and T are fixed (and are actually an case class-HList pair), Shapeless will generate one for you. In your get method, however, the compiler still knows nothing about A and T except that T is an HList, so you need to require the instance again in order to call convert:

def get(index: Int)(implicit gen: Generic.Aux[A, T]): A = convert(rows(index))

Everything should work just fine after this change.

Note that you could also require the instance at the trait level by adding an (abstract) method like the following:

implicit def genA: Generic.Aux[A, T]

Then any class implementing LoadsCsv could have an implicit val genA parameter (or could supply the instance in some other way).