Can a Scala "extractor" use generics on unapply? - scala

Question

Can't I use a generic on the unapply method of an extractor along with an implicit "converter" to support a pattern match specific to the parameterised type?
I'd like to do this (Note the use of [T] on the unapply line),
trait StringDecoder[A] {
def fromString(string: String): Option[A]
}
object ExampleExtractor {
def unapply[T](a: String)(implicit evidence: StringDecoder[T]): Option[T] = {
evidence.fromString(a)
}
}
object Example extends App {
implicit val stringDecoder = new StringDecoder[String] {
def fromString(string: String): Option[String] = Some(string)
}
implicit val intDecoder = new StringDecoder[Int] {
def fromString(string: String): Option[Int] = Some(string.charAt(0).toInt)
}
val result = "hello" match {
case ExampleExtractor[String](x) => x // <- type hint barfs
}
println(result)
}
But I get the following compilation error
Error: (25, 10) not found: type ExampleExtractor
case ExampleExtractor[String] (x) => x
^
It works fine if I have only one implicit val in scope and drop the type hint (see below), but that defeats the object.
object Example extends App {
implicit val intDecoder = new StringDecoder[Int] {
def fromString(string: String): Option[Int] = Some(string.charAt(0).toInt)
}
val result = "hello" match {
case ExampleExtractor(x) => x
}
println(result)
}

Answer 1

A variant of your typed string decoder looks promising:
trait StringDecoder[A] {
def fromString(s: String): Option[A]
}
class ExampleExtractor[T](ev: StringDecoder[T]) {
def unapply(s: String) = ev.fromString(s)
}
object ExampleExtractor {
def apply[A](implicit ev: StringDecoder[A]) = new ExampleExtractor(ev)
}
then
implicit val intDecoder = new StringDecoder[Int] {
def fromString(s: String) = scala.util.Try {
Integer.parseInt(s)
}.toOption
}
val asInt = ExampleExtractor[Int]
val asInt(Nb) = "1111"
seems to produce what you're asking for. One problem remains: it seems that trying to
val ExampleExtractor[Int](nB) = "1111"
results in a compiler crash (at least inside my 2.10.3 SBT Scala console).