Hello! I have a REST API that needs to calls: one to `/playlists` to get the list of playlist, and...

# arrow

[kotlinforandroid]

Hello!

I have a REST API that needs to calls: one to /playlists to get the list of playlist, and then for each playlist I have to call /playlist/<id> to get more information about the playlists themselves.

All API calls are suspending. And I always return Either<Error, T> . Can I somehow leverage Arrow to do n requests in parallel? A higher-level abstraction over parZip basically. I want to query /playlist/<id> with 5 simultaneous requests and do it for all playlists returned from /playlists.

[dave08]

I was trying to find an idea for this (if I understand your question), over here: https://kotlinlang.slack.com/archives/C5UPMM0A0/p1699955211044099

[simon.vergauwen]

Hey @kotlinforandroid,

If I understood your problem correctly, then this should work. The alternative for parZip but for Iterable<A> is parMap, analogue to map from Kotlin Std.

data class Playlist(val id: Long, val ids: List<Long>)
object GetPlaylistResult

suspend fun getPlaylists(): List<Playlist> = TODO()

suspend fun getPlaylist(id: Long): GetPlaylistResult = TODO()

fun main(): Unit = runBlocking {
  getPlaylists().parMap(concurrency = 5) { playlist ->
    getPlaylist(playlist.id)
  } // List<GetPlaylistResult>
}

[dave08]

In the question, he says all the requests return an Either , would parMap short-circuit on error of either getPlaylists or one of the getPlaylist calls?

[kotlinforandroid]

Yes, my API is like this:

interface API {
    fun getPlaylists(): Either<NotAuthenticated, List<Playlist>>
    fun getPlaylistDetails(playlistId: PlaylistId): Either<NoPlaylistFound, PlaylistDetails>
}

I think maybe parMapOrAccumulate might be the function I am looking for.

[simon.vergauwen]

Oh, I'm sorry.

[simon.vergauwen]

fun main(): Unit = runBlocking {
  either {
    getPlaylists().parMap(concurrency = 5) { playlist ->
      getPlaylist(playlist.id).bind()
        // GetPlaylistResult
    } // List<GetPlaylistResult>
  } // Either<E, List<GetPlaylistResult>>
}

[simon.vergauwen]

The difference between parMapOrAccumulate is that parMap will stop with the first Either.Left it encounters.

Whilst parMapOrAccumulate guarantees to finishes all request (unless Throwable is encountered), and returns all Either.Left as a NonEmptyList<A> : Iterable<A>.

[dave08]

Not:

fun main(): Unit = runBlocking {
  either { ///      v
    getPlaylists().bind().parMap(concurrency = 5) { playlist ->
      getPlaylist(playlist.id).bind()
        // GetPlaylistResult
    } // List<GetPlaylistResult>
  } // Either<E, List<GetPlaylistResult>>
}

[dave08]

?

[simon.vergauwen]

Yes, you're right @dave08!

[simon.vergauwen]

Sorry, just writing pseudo code in chat is not great 😅 We need an integrated IntelliJ in Slack to write & typecheck snippets 🤣