[BUG] All optional query parameters are required about tapir HOT 5 CLOSED

This happens because you are defining custom codecs from String to your type (Option[List[CarName]] in your case, but from tapir's point of view this is just a type T):

  implicit val carNameCodec: Codec[String, Option[List[CarName]], TextPlain] =
    Codec.string.mapDecode(h => decode[CarName](h, CarName))(encode[CarName](_, _.name))

So we have a String <-> T conversion, where the base type is required (a String). If you then create a query[T] input, tapir it's trying to construct a List[String] <-> T codec, and to do that, it's using the following chain: List[String] -> get the only element (or fail) -> T.

I think defining your codec as String <-> List[CarName], and then creating the input as an optional one: query[Option[List[CarName]]] should work.

from tapir.

This seems to work:

package sttp.tapir.examples

import org.apache.pekko.actor.ActorSystem
import org.apache.pekko.http.scaladsl.Http
import org.apache.pekko.http.scaladsl.server.Route
import sttp.tapir.*
import sttp.tapir.CodecFormat.TextPlain
import sttp.tapir.server.pekkohttp.PekkoHttpServerInterpreter

import scala.concurrent.duration.*
import scala.concurrent.{Await, Future}
import scala.io.StdIn

object HelloWorldPekkoServer extends App {
  implicit val actorSystem: ActorSystem = ActorSystem()
  import actorSystem.dispatcher

  case class CarName(name: String)

  def decode[T](s: String, fromString: String => T): DecodeResult[List[T]] = DecodeResult.Value(s.split(",", -1).toList.map(fromString))
  def encode[T](list: List[T], asString: T => String): String = list.map(asString).mkString(",")

  implicit val carNameCodec: Codec[String, List[CarName], TextPlain] =
    Codec.string.mapDecode(h => decode[CarName](h, CarName))(encode[CarName](_, _.name))

  val helloWorld: PublicEndpoint[Option[List[CarName]], Unit, String, Any] =
    endpoint.get.in("hello").in(query[Option[List[CarName]]]("name")).out(stringBody)

  val helloWorldRoute: Route =
    PekkoHttpServerInterpreter().toRoute(helloWorld.serverLogicSuccess(cars => Future.successful(s"Got: $cars!")))

  Await.result(Http().newServerAt("localhost", 8080).bindFlow(helloWorldRoute), 1.minute)

  StdIn.readLine()
}

Test:

[~]% curl "http://localhost:8080/hello?name=x"
Got: Some(List(CarName(x)))!%
[~]% curl "http://localhost:8080/hello?name=x,y"
Got: Some(List(CarName(x), CarName(y)))!%
[~]% curl "http://localhost:8080/hello"
Got: None!%

from tapir.

First - thank you for such a fast answer.
Second - works like a charm.
Third - I am aware of type erasure in JVM, but not understand how removing Option from a codec helps to make a parameter optional. It's counterintuitive. Still, I can live with that as long as it works.
Best regards,
PK

from tapir.

This doesn't reach type erasure, everything is done at compiler level (where we have full type info). There are no run-time checks if something is an instance of an option:

For a query[T] input we need a Codec[List[String], T] (the parameter might appear multiple times in the url). So the compiler tries to find the appropriate instance in the implicit scope.

In your case, if you have query[Option[List[CarName]]], the compiler will try to find Codec[List[String], Option[List[CarName]]]. So it tries to look that up - it will try several available options, among them this one, which is a rule on how to construct a Codec[List[T], Option[U]] given a Codec[T, U]. Substituting our types for T and U, we reduce the problem to looking up Codec[String, List[CarName]] - which you provide in your own code base. Problem solved!

Now in your original example, you provided a Codec[String, Option[List[CarName]]. So this can only be used, if we have the String to decode (tapir can't, and doesn't inspect the exact shape of the right-hand side - it can be whatever; here it's an option, but that's completely opaque to tapir). While constructing the Codec[List[String], Option[List[CarName]]] required for the query paramter, another rule was used, to get the single element from the list, and if it's not there - report a missing element.

from tapir.

Thank you very much for such a detailed and clear response. Now I understand the Option should be only in endpoint input declaration (and why input is List not single String)!

from tapir.