Hi I need some guidance on how to employ `Either`...

# arrow

[Tower Guidev2]

Hi

I need some guidance on how to employ Either while completing related operations
for example i am calling retrofit api calls then persisting the data into my local sqlite database
i am employing io.arrow-kt:arrow-core-retrofit for my retrofit calls as follows:-


/**
* <https://api.discogs.com/artists/45467>
*
*/
@GET("artists/{artist_id}")
suspend fun artist(@Header(HEADER_AUTHORISATION) accessToken: String = ACCESS_TOKEN, @Path("artist_id") artistId: Long): Either<CallError, ArtistResponse>


then in my worker:-

override suspend fun doActualWork(): Result {
return repository
.artist(artistId = 45467)
.fold(
ifLeft = { callError -> consumeCallError(callError) },
ifRight = {
val artist = ARTIST_MAPPER.map(it)
println(artist)

repository.database.artistDao().insertAsync(artist)
Result.success()
})
}

how can "wrap" or employ Either for the code i execute in my ifRight block above?
e.g. the database insert. As I am using Room and it does not accept Either type

[simon.vergauwen]

Hey @Tower Guidev2,

Can you share a complete example in a text snippet? It’ shard to follow the code here since I am not sure about the other signatures.

There are some good docs on Either, and Effect here.

https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core/-either/

https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core.continuations/-effect/

Hope that helps. If not I’d be happy to help you if you could provide a more complete example. I’m not sure I understood the problem correctly.

[Tower Guidev2]

@simon.vergauwen thanks for taking the time to look at my issue
its tricky to share my code
what i am trying to achieve is to be able to call a sequence of functions and have them fail fast

for example:-

fun manageMultipleCalls() {
     val stage1: Either<Bad1, Good1> = service.call()
     val stage2: Either<Bad2, Good2> = database.call(stage1.good1)
     val stage3: Either<Bad3, Good3> = somethingElse.call(stage2.good2)
}

when any stage throws an exception i need to log the exception details and stop processing further calls

do i have to add if/when statements after each stage to check isLeft/isRight?

is there a better approach?

[Tower Guidev2]

I have refactored my code to the following

override suspend fun doActualWork(): Result = repository
    .artist(artistId = 45467L)
    .fold(
        ifLeft = { callError -> consumeCallError(callError) },
        ifRight = { artistResponse ->
            when (val result = Either.catch { repository.database.artistDao().insertAsync(ARTIST_MAPPER.map(artistResponse)) }) {
                is Either.Left -> consumeDatabaseError(result.value)
                is Either.Right -> Result.success()
            }
        }
    )

which is fine in this case as i only have two calls to contend with, e.g. the repository.artist retrofit call and the database repository.database.artistDao().insertAsync() call

however if i had three calls to process in sequence its going to start to get "messy"

[simon.vergauwen]

what i am trying to achieve is to be able to call a sequence of functions and have them fail fast

when any stage throws an exception i need to log the exception details and stop processing further calls

Okay, that is a very common use-case which is normally handled like this.

either<Throwable, Result> {
  val x: Int = Either.catch {
      // throwing code returning Int
   }.tapLeft { throwable -> log(...) }
    .bind() // exits either if Left
   ...  
}

[Tower Guidev2]

thanks
can you achieve similar with effect {} ?
is one approach better than the other?

[simon.vergauwen]

This uses effect under the hood if you’re using latest Arrow version since either { } is effect { }.toEither().
Where toEither() is fold({ Either.Left(it) }, { Either.Right(it) }).

So one approach is not better as the other, it depends on your use-case 🙂

[Tower Guidev2]

Im using

api platform('io.arrow-kt:arrow-stack:1.1.3-alpha.41')
api "io.arrow-kt:arrow-core"
api 'io.arrow-kt:arrow-core-retrofit'

[Tower Guidev2]

i ended up with this code...

[Tower Guidev2]

override suspend fun doActualWork(): Result =  when(actualWork(45467L)) {
    is Either.Left -> Result.failure()
    is Either.Right -> Result.success()
}

private suspend fun actualWork(artistId: Long) = either {
    val artistResponse = repository.artist(artistId = artistId).tapLeft { networkError -> consumeNetworkError(networkError) }.bind()

    Either.catch {
        repository.database.artistDao().insertAsync(ARTIST_MAPPER.map(artistResponse))
    }.tapLeft { databaseError -> consumeDatabaseError(databaseError) }.bind()
}