Support for auto-pagination

[BenFradet]

like in octokit:

• https://github.com/octokit/octokit.rb#pagination
• https://github.com/octokit/octokit.rb#hypermedia-agent

[BenFradet]

I have something like that at the moment:

  def autoPaginate[T](
    call: Pagination => IO[Either[GHException, GHResult[List[T]]]]
  ): IO[Either[GHException, List[T]]] = (for {
    firstPage <- EitherT(call(Pagination(1, 100)))
    pages = (utils.getNrPages(firstPage.headers) match {
      case Some(n) if n >= 2 => (2 to n).toList
      case _ => Nil
    }).map(Pagination(_, 100))
    restPages <- EitherT(pages.traverse(call(_)).map(_.sequence))
  } yield firstPage.result ++ restPages.map(_.result).flatten).value

  final case class Relation(name: String, url: String)
  private val relPattern = """<(.*?)>; rel="(\w+)"""".r
  def getNrPages(headers: Map[String, Seq[String]]): Option[Int] = for {
    links <- headers.map { case (k, v) => k.toLowerCase -> v }.get("link")
    h <- links.headOption
    relations = h.split(", ").flatMap {
      case relPattern(url, name) => Some(Relation(name, url))
      case _ => None
    }
    lastRelation <- relations.find(_.name == "last")
    uri <- Uri.fromString(lastRelation.url).toOption
    lastPage <- uri.params.get("page")
    nrPages <- Try(lastPage.toInt).toOption
  } yield nrPages

[dcsobral]

Whilst still being limited by not being able to use the URLs directly, as Github recommends, I came up with an alternative solution that does not rely on the number of pages changing:

  def autoPage[F[_]: Sync, T](first: Pagination)
                             (call: Pagination => F[Either[GHException, GHResult[List[T]]]]): Stream[F, T] = {
    val chunker = call.andThen(_.rethrow.map(res => nextPage(getRelations(res.headers)).map(Chunk.seq(res.result) -> _)))
    Stream.unfoldChunkEval(first)(chunker)
  }

  def nextPage(relations: Map[String, (Int, Int)]): Option[Pagination] = {
    relations.get("next").map((Pagination.apply _).tupled)
  }

  def getRelations(headers: Map[String, String]): Map[String, (Int, Int)] = {
    val relations = for {
      header <- headers.map { case (k, v) => k.toLowerCase -> v }.collect { case ("link", header) => header }
      link(url, relation) <- link.findAllMatchIn(header)
      uri <- Uri.fromString(url).toSeq
      page <- uri.params.get("page")
      pageNum <- Try(page.toInt).toOption
      perPage <- uri.params.get("per_page")
      perPageNum <- Try(perPage.toInt).toOption
    } yield (relation, (pageNum, perPageNum))
    relations.toMap
  }

  val link: Regex = """<(.*?)>; rel="(\w+)"""".r

It can be easily adapted to "move" in either direction, and relies on fs2 instead of loading it all to memory.

As a side note, v4 API (graphql) does not support page numbers. Navigation is done by getting N items before the first one or after the last one in the current window. These "positions" are returned as "cursors", which are opaque types.

The code above can be adapted to v4's system.

[dcsobral]

My autoPage function had a bug: it missed the last page, if there was more than one. Small change in implementation:

  def autoPage[F[_]: Sync, T](
      first: Pagination)(
      call: Pagination => F[Either[GHException, GHResult[List[T]]]])
      : Stream[F, T] = {
    val chunker: Option[Pagination] => F[Option[(Chunk[T], Option[Pagination])]] = {
      case Some(pagination) =>
        call(pagination).rethrow.map(res => Option(Chunk.seq(res.result) -> nextPage(getRelations(res.headers))))
      case None =>
        Sync[F].pure(None)
    }
    Stream.unfoldChunkEval(Option(first))(chunker)
  }

[dev590t]

I see many solution. @BenFradet @dcsobral It is possible to make a PR to integrate one into the project?

[BenFradet]

@zhenleibb yes feel free to open one 👍

[omerlh]

Updated the code @dcsobral wrote to match current code in this repo - I am not that good with cats, so I had to replace F with IO:

def autoPage[T](
                               first: Pagination)(
                               call: Pagination => IO[GHResponse[List[T]]])
  : Stream[IO, T] = {
    val chunker: Option[Pagination] => IO[Option[(Chunk[T], Option[Pagination])]] = {
      case Some(pagination) =>
        call(pagination)
          .flatMap{ res =>
            IO.fromEither(res.result).map{ items =>
              Option(Chunk.seq(items) -> nextPage(getRelations(res.headers)))
            }

          }
      case None =>
        IO.pure(None)
    }
    Stream.unfoldChunkEval(Option(first))(chunker)
  }

[dcsobral]

Did it migrate to Cats 3? Hardcoding IO is the wrong thing for Cats 2.

[theqp]

A simple solution which depends on only Monad and MonadError

def listAllOrgRepos[F[_]](repos: Repositories[F], orgName: String)(implicit
    monadError: MonadError[F, Throwable]
): F[List[Repository]] = mergeGhPages(page =>
  repos
    .listOrgRepos(orgName, pagination = Some(Pagination(page, 100)))
)

def mergeGhPages[F[_], T](getPage: Int => F[GHResponse[List[T]]])(implicit
    monadError: MonadError[F, Throwable]
) =
  mergePages(
    getPage.andThen(response =>
      response
        .map(_.result)
        .flatMap(monadError.fromEither)
    )
  )

def mergePages[F[_]: Monad, T](getPage: Int => F[List[T]]): F[List[T]] =
  Monad[F].tailRecM((1, Nil: List[T]))({ case (page: Int, acc: List[T]) =>
    getPage(page).map({
      case Nil => Right(acc)
      case ts  => Left((page + 1, acc.prependedAll(ts)))
    })
  })