Return NotFound if http4s response stream is empty, wrap it in a different status otherwise

http4s-onEmptyOrNonEmpty.scala

/*
Copyright 2021 Jakub Kozłowski

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

    http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.
*/

import cats.effect.MonadThrow
import cats.implicits._
import fs2.Pull
import fs2.Stream

//Example usage: onEmptyOrNonEmpty(stream)(Ok(_))(NotFound()).flatten
def onEmptyOrNonEmpty[F[_]: MonadThrow, A, B](
  stream: Stream[F, A]
)(
  onNonEmpty: Stream[F, A] => B
)(
  onEmpty: B
)(
  implicit SC: fs2.Compiler[F, F]
): F[B] = stream
  .pull
  .peek1
  .flatMap {
    _.traverse_ { case (h, t) =>
      Pull.output1(onNonEmpty((Stream.emit(h) ++ t)))
    }
  }
  .stream
  .compile
  .last
  .map(_.getOrElse(onEmpty))

Might take some time :D

I think if we didn't have a compile that essentially runs before the response body, we wouldn't need to touch scopes, yeah. B can't consume the stream internally now, because it's no longer in F[B] (of course you can pass an effect, but the result will be F[F[...]] so it's a separate "compilation" and needs scope extension)

@kubukoz I'm sure people will copy this, might make sense to throw a license file up, so them doing so is alright.

Good idea, thanks :)