Strange compiler error on mixin algebras

[gvolpe]

I'm just trying out this plugin in Redis4Cats and stumbled upon the following issue:

[info] Compiling 16 Scala sources to /workspace/oss/redis4cats/modules/effects/target/scala-2.13/classes ...
[error] /workspace/oss/redis4cats/modules/effects/src/main/scala/dev/profunktor/redis4cats/algebra/sortedsets.scala:24:35: type mismatch;
[error]  found   : <notype>
[error]  required: F[List[V]]
[error] trait SortedSetGetter[F[_], K, V] {
[error]                                   ^
[info] <notype> <: F[List[V]]?
[info] false
[info] <notype> <: F[List[dev.profunktor.redis4cats.effects.ScoreWithValue[V]]]?
[info] false
[info] <notype> <: F[List[V]]?
[info] false
[info] <notype> <: F[List[dev.profunktor.redis4cats.effects.ScoreWithValue[V]]]?
[info] false
[error] one error found
[error] (redis4cats-effects / Compile / compileIncremental) Compilation failed
[error] Total time: 5 s, completed Dec 31, 2019 3:32:32 PM

This happens with the following algebra, which is a mixin:

trait SortedSetCommands[F[_], K, V] extends SortedSetGetter[F, K, V] with SortedSetSetter[F, K, V]

trait SortedSetGetter[F[_], K, V] {
  def zCard(key: K): F[Option[Long]]
  def zCount(key: K, range: ZRange[V])(implicit ev: Numeric[V]): F[Option[Long]]
  def zLexCount(key: K, range: ZRange[V]): F[Option[Long]]
  def zRange(key: K, start: Long, stop: Long): F[List[V]]
  def zRangeByLex(key: K, range: ZRange[V], limit: Option[RangeLimit]): F[List[V]]
  def zRangeByScore[T: Numeric](key: K, range: ZRange[T], limit: Option[RangeLimit]): F[List[V]]
  def zRangeByScoreWithScores[T: Numeric](key: K,
                                          range: ZRange[T],
                                          limit: Option[RangeLimit]): F[List[ScoreWithValue[V]]]
  def zRangeWithScores(key: K, start: Long, stop: Long): F[List[ScoreWithValue[V]]]
  def zRank(key: K, value: V): F[Option[Long]]
  def zRevRange(key: K, start: Long, stop: Long): F[List[V]]
  def zRevRangeByLex(key: K, range: ZRange[V], limit: Option[RangeLimit]): F[List[V]]
  def zRevRangeByScore[T: Numeric](key: K, range: ZRange[T], limit: Option[RangeLimit]): F[List[V]]
  def zRevRangeByScoreWithScores[T: Numeric](key: K,
                                             range: ZRange[T],
                                             limit: Option[RangeLimit]): F[List[ScoreWithValue[V]]]
  def zRevRangeWithScores(key: K, start: Long, stop: Long): F[List[ScoreWithValue[V]]]
  def zRevRank(key: K, value: V): F[Option[Long]]
  def zScore(key: K, value: V): F[Option[Double]]
}

trait SortedSetSetter[F[_], K, V] {
  def zAdd(key: K, args: Option[ZAddArgs], values: ScoreWithValue[V]*): F[Unit]
  def zAddIncr(key: K, args: Option[ZAddArgs], value: ScoreWithValue[V]): F[Unit]
  def zIncrBy(key: K, member: V, amount: Double): F[Unit]
  def zInterStore(destination: K, args: Option[ZStoreArgs], keys: K*): F[Unit]
  def zRem(key: K, values: V*): F[Unit]
  def zRemRangeByLex(key: K, range: ZRange[V]): F[Unit]
  def zRemRangeByRank(key: K, start: Long, stop: Long): F[Unit]
  def zRemRangeByScore(key: K, range: ZRange[V])(implicit ev: Numeric[V]): F[Unit]
  def zUnionStore(destination: K, args: Option[ZStoreArgs], keys: K*): F[Unit]
}

There are many other algebras like this but I only get a compiler error with this one. Maybe because the compiler didn't reach out to the other algebras I guess?

Here's the corresponding PR for more details: https://github.com/profunktor/redis4cats/pull/205

---

EDIT: It happens the same if I make it a single algebra trait SortedSetCommands[F[_], K, V] without mixing any other trait but preserving all the methods.

[augustjune]

Cool, thanks a lot for trying the plugin out and raising the issue. I will take a closer look at this later (probably tomorrow) and let you know what I found.

[gvolpe]

No rush @augustjune, thanks for making it and happy new year :)

[augustjune]

The issue is actually caused by incorrect handling of trait methods declared with context bounds. PR link to redis4cats was very helpful. Thanks for improving the state of the project :)