[error] test.scala:31: ambiguous implicit values:
[error] both method taggedQueryParamDecoder in trait ExternalInstances0 of type [A, T](implicit evidence$2: org.http4s.QueryParamDecoder[A])org.http4s.QueryParamDecoder[scalaz.@@[A,T]]
[error] and method iiQueryParamDecoder in trait ExternalInstances1 of type [B](implicit ii: foo.InvariantInstances[B])org.http4s.QueryParamDecoder[B]
[error] match expected type org.http4s.QueryParamDecoder[scalaz.@@[String,foo.tags.Social]]
[error] implicitly[QueryParamDecoder[String @@ Social]]
[error] ^
I import instances._; instances extends ExternalInstances1, and ExternalInstances1 extends ExternalInstances0. Because of this inheritance I would expect ExternalInstances1's members to win out over ExternalInstances0's, rather than yielding an ambiguity.
Why is this happening, and how can I fix it? Thanks.
The source is at http://scastie.org/12233, reproduced below:
/***
scalaVersion := "2.11.7"
libraryDependencies += "org.http4s" %% "http4s-core" % "0.10.0"
resolvers ++= Seq(
"tpolecat" at "http://dl.bintray.com/tpolecat/maven",
"Scalaz Bintray Repo" at "http://dl.bintray.com/scalaz/releases",
Resolver.sonatypeRepo("releases")
)
libraryDependencies += "org.tpolecat" %% "doobie-core" % "0.2.2"
libraryDependencies += "com.github.alexarchambault" %% "argonaut-shapeless_6.1" % "0.3.1"
*/
import java.time.LocalDate
import argonaut._, Argonaut._
import doobie.imports._
import doobie.util.composite.Composite
import tags._
import instances._
import org.http4s._
import scala.reflect.runtime.universe.TypeTag
import scalaz._
object Main extends App {
implicitly[QueryParamDecoder[String @@ Social]]
println("ok")
}
object tags {
trait Social; val Social = Tag.of[Social]
}
object instances extends ExternalInstances1 {
implicit val ssn: InvariantInstances[String @@ Social] =
InvariantInstances { (raw: String) ⇒
val digitsOnly = raw.filter(_.isDigit)
require(digitsOnly.length == 9)
Social(digitsOnly)
}(Social.unwrap)
}
trait ExternalInstances1 extends ExternalInstances0 {
implicit def iiCodecJson[B](implicit ii: InvariantInstances[B]): CodecJson[B] = ii.codecJson
implicit def iiEncodeJson[B](implicit ii: InvariantInstances[B]): EncodeJson[B] = ii.encodeJson
implicit def iiDecodeJson[B](implicit ii: InvariantInstances[B]): DecodeJson[B] = ii.decodeJson
implicit def iiMeta[B](implicit ii: InvariantInstances[B]): Meta[B] = ii.meta
implicit def iiQueryParamEncoder[B](implicit ii: InvariantInstances[B]): QueryParamEncoder[B] = ii.queryParamEncoder
implicit def iiQueryParamDecoder[B](implicit ii: InvariantInstances[B]): QueryParamDecoder[B] = ii.queryParamDecoder
}
trait ExternalInstances0 {
implicit def taggedEncodeJson[A, T](implicit A: EncodeJson[A]): EncodeJson[A @@ T] =
A.contramap(Tag.of[T].unwrap)
implicit def taggedDecodeJson[A, T](implicit A: DecodeJson[A]): DecodeJson[A @@ T] =
A.map(Tag.of[T].apply)
implicit def taggedComposite[A: Composite, T]: Composite[A @@ T] =
Composite[A].xmap(a => Tag[A, T](a), Tag.unwrap(_))
implicit def taggedQueryParamDecoder[A: QueryParamDecoder, T]: QueryParamDecoder[A @@ T] =
QueryParamDecoder.decodeBy(Tag.of[T](_: A))
}
trait InvariantInstances[B] {
def codecJson: CodecJson[B]
def decodeJson: DecodeJson[B]
def encodeJson: EncodeJson[B]
def meta: Meta[B]
def queryParamEncoder: QueryParamEncoder[B]
def queryParamDecoder: QueryParamDecoder[B]
}
object InvariantInstances {
def apply[A: EncodeJson: DecodeJson: Meta: QueryParamDecoder: QueryParamEncoder, B: TypeTag](f: A ⇒ B)(g: B ⇒ A) =
new InvariantInstances[B] {
def codecJson: CodecJson[B] =
CodecJson.derived[B](encodeJson, decodeJson)
def decodeJson: DecodeJson[B] =
implicitly[DecodeJson[A]].map(f)
def encodeJson: EncodeJson[B] =
implicitly[EncodeJson[A]].contramap(g)
def meta: Meta[B] =
implicitly[Meta[A]].xmap(f, g)
def queryParamDecoder: QueryParamDecoder[B] =
CovariantInstances.queryParamDecoder(f)
def queryParamEncoder: QueryParamEncoder[B] =
ContravariantInstances.queryParamEncoder(g)
}
}
object CovariantInstances {
def queryParamDecoder[A, B](f: A ⇒ B)(implicit A: QueryParamDecoder[A],
B: reflect.runtime.universe.TypeTag[B]): QueryParamDecoder[B] =
new QueryParamDecoder[B] {
import scalaz.Validation.FlatMap._ // suppress deprecation warning
def decode(value: QueryParameterValue): ValidationNel[ParseFailure, B] =
A.decode(value).flatMap(
a ⇒ Validation.fromTryCatchNonFatal(f(a)).leftMap(t =>
ParseFailure(s"Query decoding ${B.tpe.typeSymbol} failed", t.getMessage)
).toValidationNel
)
}
}
object ContravariantInstances {
def queryParamEncoder[A, B](g: B ⇒ A)(implicit A: QueryParamEncoder[A]): QueryParamEncoder[B] =
new QueryParamEncoder[B] {
def encode(value: B): QueryParameterValue = A.encode(g(value))
}
}
implicitsofExternalInstances0. – Demos