@aeruhxi Assuming both

post

and

likesCount

give you back a

Deferred<Option>

you can use here

OptionT

since really your concerns in terms of types is that you have an Option nested inside

Deferred

and that is what

OptionT

is for. Whenever

Option

is found inside a different effect, its transformer can be applied to flatten all effects through the potential absence of values nested in the computation. If any of the options happens to be a

None

`,

OptionT

will shortcircuit yielding a

DeferredK(None)

in this case.

sealed class LikeStatus
data class LikeCount(val likes: Int) : LikeStatus
object UnknownLikes : LikeStatus

data class PostId(val value: String)
data class PosterId(val value: String)
data class PostInfo(postId: PostId, posterId: PosterId, likeStatus: LikeStatus)

fun fetchPostIds(query: Query, postId: PostId, userId: UserId): DeferredK<Option<PostInfo>> = 
  session.writeTransactionAsync {
    it.runAsync(query, parameters("postId", postId, "userId", userId, "timestamp", System.currentTimeMillis()))
      .thenCompose(StatementResultCursor::peekAsync)
      .thenApply { record ->
        Option.fromNullable(record).map { 
            PostInfo(
              PostId(it["post"]["id"].asString()), 
              PosterId(it["poster"]["id"].asString()),
              UnknownLikes
            )
        }
      }
  }.k()

fun fetchLikesCount(postId : PostId): DeferredK<Option<LikeStatus>> =
  session.readTransactionAsync { tx ->
    tx.runAsync(notificationQuery, parameters("postId", it.a))
      .thenCompose(StatementResultCursor::peekAsync)
      .thenApply { record ->
        Option.fromNullable(record).map { it["likesCount"].asInt() }
      }
  }.k()


fun fetchAllPostInfo(query: Query, postId: PostId, userId: UserId): DeferredK<Option<PostInfo>>
  OptionT.monad(DeferredK.monad()).binding {
    val persistedPost = OptionT(fetchPostIds(query, postId, userId)).bind()
    val likesCount = OptionT(fetchLikesCount(persistedPost.Id)).bind() //do you really need the post id from the persisted one or can we fetch in parallel?
  }.fix().value()

More info http://arrow-kt.io/docs/datatypes/optiont/

Hey, thanks for the detailed example. I will check more into transformers. Yes I do need the post id and we can't fetch in parallel.