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


i started learning scala , today decided wanted write csv parser load nicely case classes store data in rows (lists) of shapeless's hlist object exposure type-level programming.

here's have 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]): = gen.from(t)      def get(index: int): = {         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 object that's loading 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 working alright until added get() conversion hlist case class compilation error. why isn't implicit getting loaded , can fix or otherwise convert hlist case class?

error:(14, 17) not find implicit value parameter gen: shapeless.generic.aux[a,t]         return convert(rows(index))                       ^

i've been reading shapeless documentation , mentions area had been in flux between version 1 , 2, believe things should working on version of shapeless , scala suspect i've done 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 , shapeless 2.2.2

you're close. problem scala isn't going propagate implicit requirements call chain automatically you. if need generic[a, t] instance call convert, you'll have make sure one's in scope every time call convert convert. if a , t fixed (and case class-hlist pair), shapeless generate 1 you. in get method, however, compiler still knows nothing a , t except t hlist, need require instance again in order call convert:

def get(index: int)(implicit gen: generic.aux[a, t]): = convert(rows(index))

everything should work fine after change.

note require instance @ trait level adding (abstract) method following:

implicit def gena: generic.aux[a, t]

then class implementing loadscsv have implicit val gena parameter (or supply instance in other way).


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