FreekTest.scala

object FreekQuickSearch {

  import cats.free.Free
  import cats.data.Xor
  import freek._

  final case class Wrong(message: String, cause: Option[Throwable] = None)
  type Maybe[A] = Xor[Wrong, A]

  ///
  /// Domain: a parser, a log, a backend processing query
  ///

  final case class Result(value: String)

  // log
  sealed trait Log[A]
  object Log {
    final case class DebugMsg(msg: String) extends Log[Unit]
  }

  /// parser
  sealed trait Parser[A]
  object Parser {
    final case class Parse(input: String) extends Parser[Maybe[Query]]
  }

  /// backend processing
  sealed trait Backend[A]
  object Backend {
    final case class Search(query: Query) extends Backend[Maybe[Seq[Result]]]
  }

  type PRG = Parser :|: Log :|: Backend :|: FXNil
  val PRG = Program[PRG]
  type O = Maybe :&: Bulb


  /// parse user input, log query, process query
  def search(input: String) = {
    for {
      query   <- Parser.Parse(input).freeko[PRG, O]     // error 1
      _       <- Log.DebugMsg(s"User query for '${input}', parsed as user query: '${query.userToken}' on objects: " +
                              s"'${query.objectClass.mkString(", ")}' and attributes '${query.attributes.mkString(", ")}'"
                 ).freeko[PRG, O]
      results <- Backend.Search(query).freeko[PRG, O]  // error 2
    } yield {
      results
    }
  }
}

/*
1:
type mismatch;  found   : 
  com.normation.rudder.services.quicksearch.Query => freek.OnionT[cats.free.Free,[t(in type Cop)]freek.
  In3[com.normation.rudder.services.quicksearch.FreekQuickSearch.Parser,com.normation.
  rudder.services.quicksearch.FreekQuickSearch.Log,com.normation.rudder.services.quicksearch.FreekQuickSearch.
  Backend,t(in type Cop)],freek.:&:[[A]cats.data.Xor[com.normation.rudder.services.quicksearch.FreekQuickSearch.
  Wrong,A],freek.Bulb], 
  Nothing]  
required: 
  com.normation.rudder.services.quicksearch.Query => freek.OnionT[cats.free.Free,[t(in type Cop)]freek.
  In3[com.normation.rudder.services.quicksearch.FreekQuickSearch.Parser,com.normation.
  rudder.services.quicksearch.FreekQuickSearch.Log,com.normation.rudder.services.quicksearch.FreekQuickSearch.
  Backend,t(in type Cop)],freek.:&:[[A]cats.data.Xor[com.normation.rudder.services.quicksearch.FreekQuickSearch.
  Wrong,A],freek.Bulb],
  B]	
  

2:
could not find implicit value for parameter lifter2: 
  freek.Lifter2.Aux[com.normation.rudder.services.quicksearch.FreekQuickSearch.Maybe[Seq[com.normation.rudder.services.
  quicksearch.FreekQuickSearch.Result]],com.normation.rudder.services.quicksearch.FreekQuickSearch.O,A]
not enough arguments for method freeko: (implicit ga: freek.HKK.Aux[com.normation.rudder.services.quicksearch.
  FreekQuickSearch.Maybe[Seq[com.normation.rudder.services.quicksearch.FreekQuickSearch.Result]],com.normation.rudder.
  services.quicksearch.FreekQuickSearch.Result], implicit subfx: freek.SubFX[[β]freek.In1[com.normation.rudder.services.
  quicksearch.FreekQuickSearch.Backend,β],com.normation.rudder.services.quicksearch.
  FreekQuickSearch.PRG], implicit lifter2: freek.Lifter2.Aux[com.normation.rudder.services.quicksearch.FreekQuickSearch.
  Maybe[Seq[com.normation.rudder.services.quicksearch.FreekQuickSearch.Result]],com.normation.rudder.services.quicksearch.
  FreekQuickSearch.O,com.normation.rudder.services.quicksearch.FreekQuickSearch.Result], implicit pointer: freek.
  Pointer[com.normation.rudder.services.quicksearch.FreekQuickSearch.O], implicit mapper: freek.Mapper[com.normation.rudder.
  services.quicksearch.FreekQuickSearch.O], implicit binder: freek.Binder[com.normation.rudder.services.quicksearch.
  FreekQuickSearch.O], implicit traverser: freek.Traverser[com.normation.rudder.services.quicksearch.FreekQuickSearch.O])freek.
  OnionT[cats.free.Free,subfx.Cop,com.normation.rudder.services.quicksearch.FreekQuickSearch.O,com.normation.rudder.services.
  quicksearch.FreekQuickSearch.Result]. Unspecified value parameters lifter2, pointer, mapper...
*/

@mandubian
The last one seems to be the only one giving a Vector[Result] - which is the expected type.
The other two are returning a Xor[..., Vector[Result]], certainly because they only go one frame deep in the stack (but we would like them to go two frames deep, or perhaps "keep only the last K frames". You don't have something like that in Freek, it seems I always need it :/