Question about how to combine `Either<>`s th...

# arrow

[streetsofboston]

Question about how to combine `Either<>`s that are wrapped in Rx `Single`s.
This is the example, and it seems to work, but I wonder if there is an easier/more-concise/better way of doing this

There are these two functions:

fun getAccountKeys(): Single<Either<List<AccountKey>>>

and

fun getAccount(key: AccountKey): Single<DataModelResult<Account>>

We’d like to create the following function, that will use the two above

fun getAccounts(): Single<DataModelResult<List<Account>>>

(DataModelResult<T> = Either<DataError, T>)

This is one implementation of `getAccounts()`:

val result: Single<DataModelResult<List<Account>>> =
    getAccountKeys()
        .flatMap { listOfKeys: DataModelResult<List<AccountKey>> ->

            val rxOfKeys: DataModelResult<Observable<AccountKey>> =
                listOfKeys.map { Observable.fromIterable(it) }

            val rxOfAccounts: DataModelResult<Observable<DataModelResult<Account>>> =
                rxOfKeys.map { rxKey -> rxKey.flatMapSingle(::getAccount) }

            val rxOfListOfAccounts: DataModelResult<Single<List<DataModelResult<Account>>>> =
                rxOfAccounts.map { it.toList() }

            val single: Single<DataModelResult<List<Account>>> = when (rxOfListOfAccounts) {
                is Either.Left -> Single.just(rxOfListOfAccounts)
                is Either.Right -> rxOfListOfAccounts.b.map { listOfAccounts: List<DataModelResult<Account>> ->
                    listOfAccounts.fold(emptyList<Account>().just(), ::concat)
                }
            }

            single
        }
return result

.. and this top-level concat function ..
fun <L, T> concat(result: Either<L, List<T>>, item: Either<L, T>): Either<L, List<T>> =
    result.flatMap { list -> item.map { list + it } }

Is this the way to go or is there a more Arrow/Functional way to go (we use Arrow version 0.9.0)?
Thank you!

[pakoito]

ye one sec

[pakoito]

getAccountKeys()
  .toObservable()
  .flatMap {
    it.fold({ Observable.empty() }, { Observable.fromIterable(it) })
  }.flatMap {
    getAccount(it).toObservable()
  }.toList()
  .toSingle()

[pakoito]

ezpzlemonsqueezy

[pakoito]

it’ll fail if Either is left or the List is empty tho

[pakoito]

if you have List<Either<E, List<A>>> then

list.map {
  it.fold({ Observable.empty() }, { Observable.fromIterable(it) })
}

and I believe there’s an operator to create an Observable from a list of Observables

[pakoito]

so tl;dr flatten the list to Observable, call getAccount on each, collect to Single afterwards

[pakoito]

use concatMap if you want to assure correct ordering

[streetsofboston]

Thank you.
But with your example, the result is a Single<List<DataModelResult<Account>>>,
We’d need a Single<DataModelResult<List<Account>>> instead (swapped the List and the DataModelResult 🙂 )

[streetsofboston]

Also, the final result no longer carries the DataError (if any was raised) from the getAccountKeys() call, since it was folded into a Observable.empty()….

[pakoito]

what is a DataModelResult?

[streetsofboston]

typealias DataModelResult<T> = Either<DataError, T>

[pakoito]

so the question is: do you want to short-circuit on error or not

[pakoito]

if any of the Single<Either<List<AccountKey>>> is Left, is the whole operation Left or not?

[pakoito]

getAccountKeys()
  .toObservable()
  .flatMap {
    it.fold({ 
      Observable.just(it.left()) 
    }, { 
      Observable.fromIterable(it) 
        .flatMap {
          getAccount(it).toObservable()
        }.toList()
        .map { it.right() }
    })
  }
  .toSingle()

[pakoito]

that’s the only change you need

[streetsofboston]

If one fails (getAccountKeys or any of the getAccount), the entire operation fails with the first encountered DataError

[pakoito]

yes

[pakoito]

check the latest

[pakoito]

is that it?