Mapping over HLIST with poly from different input type to same return type does not compile

[drexin]

Hi,

writing a poly function like this:

object testfun extends Poly1 {
  implicit val atString = at[String](identity)
  implicit val atInt = at[Int](_.toString)
  implicit def default[A] = at[A](identity)
}

and trying to map over an HList leads to following error:

val hlist = 1 :: "foo" :: 1.5 :: HNil

hlist map testfun

[error] /Users/dario.rexin/projects/shapeless-foo/src/main/scala/Main.scala:15: could not find implicit value for parameter mapper: shapeless.ops.hlist.Mapper[main.scala.Test.testfun.type,shapeless.::[Int,shapeless.::[String,shapeless.::[Double,shapeless.HNil]]]]
[error]   hlist map testfun

[milessabin]

Sorry it's taken me so long to get back to you on this. What you're seeing is the result of quite ordinary implicit ambiguity due to the overlap of the result types between your default case and the other two cases. The fix for this is the same as for any other case of implicit ambiguity of this sort: move the offending implicit definition to a low priority super-trait,

trait lpTestfun extends Poly1 {
  implicit def default[A] = at[A](identity)
}

object testfun extends lpTestFun {
  implicit val atString = at[String](identity)
  implicit val atInt = at[Int](_.toString)
}

Now mapping this over an HList will do exactly what you want,

scala> hlist map testfun
res0: testfun.atInt.Result :: testfun.atString.Result :: Double :: HNil =
  1 :: foo :: 1.5 :: HNil

[drexin]

Ah, of course. Thank you!