calling reduce on list of akka http routes yields compilation error (no implicit value for parameter join)

Question

Given the following routes

val route1: PathMatcher[Unit] = PathMatcher("app")
val route2: PathMatcher1[String] = PathMatchers.Segment
val route3: PathMatcher[Unit] = PathMatcher("lastSegment")

I can easily define

val resultingRoute: PathMatcher[Tuple1[String]] = route1 / route2 / route3

getting the expected type (PathMatcher[Tuple[String]]).

But creating the route programmatically like in

val routeDef = List(route1, route2, route3)
val resultingRoute = routeDef.reduce((a,b) => a / b)

will not compile, giving me

could not find implicit value for parameter join: akka.http.scaladsl.server.util.TupleOps.Join[_1,_1]

Furthermore, the inferred type of resultingRoute is

PathMatcher[_ >: Unit with Tuple1[String] with join.Out]

I'd really appreciate any hint giving me some indication of what I'm doing wrong here or how this could be resolved.

For completeness, here's my imports:

import akka.http.scaladsl.server.Directives._
import akka.http.scaladsl.server.{PathMatcher, _}

Thanks so much!

Answer 1

Your problem is that your routeDef list is actually heterogeneous, the compiler infers its type to be List[PathMatcher[_ >: Tuple1[String] with Unit]].

Given that, the (a: PathMatcher[L])./(b: PathMatcher[R]) method needs implicitly a TupleOps.Join[L, R]: akka.http.scaladsl.server.PathMatcher. It cannot be inferred from PathMatchers types in your routeDef list.

---

If you are willing to use shapeless you can then work on heterogeneous lists (called HLists in this context) easily:

import shapeless._

val routeDef = route1 :: route2 :: route3 :: HNil

object join extends Poly {
  implicit def casePathMatcher[A, B](
    implicit t: akka.http.scaladsl.server.util.TupleOps.Join[A,B]
  ) = use((a: PathMatcher[A], b: PathMatcher[B]) => a/b)
}

val resultingRoute: PathMatcher[Tuple1[String]] = routeDef.reduceLeft(join)