Parse List[String] into HList - scala

Question

I would like to write def parse[T <: HList](list: List[String]): Validation[T]. list could be List("fooid", "barid"), and T FooId :: BarId :: HNil, and a typeclass Parse[T] which implements String => Validation[FooId]. How would I write said parse, which parses the list into T? I'm not sure how to summon the implicit typeclasses for each of the elements of T.

Answer 1

We can adapt the code from shapeless.ops.traversable.FromTraversable.
I'm not sure what your Validation type is, but I used it as an alias for scalaz.ValidationNel[String, A] below (mostly so I could use the string syntax to easily give me some Parse instances).
import scalaz.{Validation => _, _}, Scalaz._
type Validation[A] = ValidationNel[String, A]
trait Parse[T] {
def apply(s: String): Validation[T]
}
object Parse {
def fromScalazParse[E <: Exception, T](f: String => scalaz.Validation[E, T]) =
new Parse[T] {
def apply(s: String): Validation[T] =
f(s).leftMap(_.getMessage).toValidationNel
}
implicit val booleanParse = fromScalazParse(_.parseBoolean)
implicit val intParse = fromScalazParse(_.parseInt)
implicit val doubleParse = fromScalazParse(_.parseDouble)
}
With the Parser type class sorted, we can now create a type class based on FromTraversable to parse a List[String] and give us a Validation[A :: B :: HNil] :
import shapeless._
import scala.collection.GenTraversable
trait FromTraversableParsed[Out <: HList] extends Serializable {
def apply(l: GenTraversable[String]) : Validation[Out]
}
object FromTraversableParsed {
def apply[Out <: HList](implicit from: FromTraversableParsed[Out]) = from
implicit val hnilFromTraversableParsed =
new FromTraversableParsed[HNil] {
def apply(l: GenTraversable[String]): Validation[HNil] =
if(l.isEmpty) HNil.successNel[String]
else "Traversable is not empty".failureNel[HNil]
}
implicit def hlistFromTraversableParsed[OutH, OutT <: HList](implicit
ftpT: FromTraversableParsed[OutT],
parseH: Parse[OutH]
): FromTraversableParsed[OutH :: OutT] =
new FromTraversableParsed[OutH :: OutT] {
def apply(l : GenTraversable[String]) : Validation[OutH :: OutT] =
if(l.isEmpty) "Empty traversable".failureNel[OutH :: OutT]
else (parseH(l.head) |#| ftpT(l.tail))(_ :: _)
}
}
We can add some syntax to make using FromTraversableParsed a little bit easier :
implicit class ParseStringListOps(val strings: List[String]) extends AnyVal {
def parse[L <: HList](implicit ftp: FromTraversableParsed[L]): Validation[L] =
ftp(strings)
}
Now we can do :
List("1", "true", "3.0").parse[Int :: Boolean :: Double :: HNil]
// Validation[Int :: Boolean :: Double :: HNil] = Success(1 :: true :: 3.0 :: HNil)