Scala chaining implicits conversions for Option[T]

Question

I try to make implicit conversions chain that from Symbol -> A -> Option[A]. But fail to make it working with generic Option conversion, for example:

implicit def toInt(n: Symbol): Int = n.toString.length
implicit def symbolToString(n: Symbol): String = n.toString
implicit def toOptStr[T](b: T)(implicit fn: T ⇒ String): Option[String] = Option(b)
implicit def toOptInt[T](b: T)(implicit fn: T ⇒ Int): Option[Int] = Option(b)

And it works fine:

val c: Option[Int] = 'a25768xffff // returns Some(12)
val d: Option[String] = 'a2699 // returns Some('a2699)

But I have to explicitly define toOptStr[T](b: T): Option[String] and toOptInt[T](b: T)(implicit fn: T ⇒ Int): Option[Int]

What I want to achieve instead is to have only one generic toOptT conversion, which can convert to Option[T] provided there is implicit conversion from T=>V, something like following:

implicit def toInt(n: Symbol): Int = n.toString.length
implicit def symbolToString(n: Symbol): String = n.toString
implicit def toOptT[T,V](b: T)(implicit fn: T ⇒ V): Option[V] = Option(fn(b))

val c: Option[Int] = 'a25768xffff 
val d: Option[String] = 'a2699

Unfortunately, 2 last lines give compilation error:

Error:(34, 93) type mismatch;
   found   : Symbol
   required: Option[Int]
Error:(35, 96) type mismatch;
   found   : Symbol
   required: Option[String]

Any help is very much appreciated.

Tried it on Scala 12.2.5. There is somewhat related question: Chain implicit conversion of collection, chaining implicits details FAQ: https://docs.scala-lang.org/tutorials/FAQ/chaining-implicits.html

V should another implicit in scope and the return type of fn should match with V. so if we can see whats that V implicit is and what fn function is then we can provide you with stabilized answer ;) — Ramesh Maharjan, Apr 25 '18 at 13:43
@RameshMaharjan I have updated a question to make it a bit more clearer. Essentially, there are 2 implicit conversions in scope: Symbol -> Int and Symbol -> String. And we want 3 to make chain Symbol -> T -> Option[T] where T can be any T which have conversion from Symbol to T in scope (like Int and String in the example). So that we should be able to do: val c: Option[Int] = 'anysymbol — D. Kuzniatsou, Apr 25 '18 at 13:57

score 0 · Answer 1 · answered Apr 25 '18 at 15:01

Couldn't make it work on 2.12.5.

Here is one possible workaround:

implicit val toInt: Symbol => Int = _.toString.length
implicit val symbolToString: Symbol => String = _.toString
implicit class ToOptOps[S](s: S) {
  def toOpt[T](implicit c: S => T): Option[T] = Option(c(s))
}

val c = 'a25768xffff.toOpt[Int]
val d = 'a2699.toOpt[String]

I'd advice you to use only typeclasses in combination with implicit classes to "pimp-the-interface" (adding new methods, as toOpt in this case). Here, one should actually replace the implicit c: S => T by a proper typeclass, to avoid unwanted collisions. The combination of type classes with "pimping" has proven to be robust. In contrast to that, implicit conversions seem just evil.

thanks for you answer. I agree it is more safe approach and nicer solution in general. Just wondering is there a way to make such conversion working if needed and if no why it does not work. — D. Kuzniatsou, Apr 26 '18 at 08:22

Scala chaining implicits conversions for Option[T]

1 Answers1