Hi, it’s me once more :stuck_out_tongue: As I am w...

# arrow

[Hiosdra]

Hi, it’s me once more 😛
As I am writing https://github.com/arrow-kt/arrow-integrations/pull/28, I simultaneously write code for my sample arrow project and encountered case. Do you know if there is any way to “hack” this, or another contrubution 😄 is needed 😛

More will be in comment not to throw too much code in here 😛

[Hiosdra]

I have retrofit repository with Tagless (IO) integration (from PR above):

@Multipart
  @POST("/calculate")
  fun calculate(@Part file: MultipartBody.Part): IO<Nothing, FaceVectorsDto>

and error ADT:

sealed class Error
...
sealed class RepositoryError : Error()
class ExternalFailure(val error: Throwable): RepositoryError()

and wish to handle exception flow into domain error ExternalFailure

I have code in service calling above retrofit code:

override fun calculatePhoto(photo: ByteArray): IO<RepositoryError, FaceVectors> =
    IO.fx<RepositoryError, FaceVectors> {
      val photoPart = createPhotoPart(photo)
      val faceVectorsDto = !recognitionClient
        .calculate(photoPart)
        .handleErrorWith({ IO.raiseError(ExternalFailure(it)) }, ::identity) //1*
      faceVectorsDto.toDomainUnsafe()
    }

// some space to get small breath 😛

In line with //1* I get type mismatch error in Intellij syntax highlightning
it is because handleErrorWith have signature:

fun <E, A, E2 : E> IOOf<E, A>.handleErrorWith(f: (Throwable) -> IOOf<E2, A>, fe: (E) -> IOOf<E2, A>): IO<E2, A>

And if E is Nothing (getting it from retrofit client), then E2 must be Nothing too, because NOTHING 😛 is below Nothing in the hieratchy of inheritance.

// and another some space for some breath 😉

Simple fix will be making E2 independent from E
Other handleErrorWith is even worse for this case, because it has fixed IO<Nothing, A> type in arrow code
Is there maybe another way to handle such case?

[pakoito]

checking

[Hiosdra]

If it will be easier for you, I can push that code to github

[pakoito]

I have a guess, this is because the current encoding requires subtyping and it shouldn’t be necessary

[pakoito]

https://github.com/arrow-kt/arrow-fx/pull/156

[pakoito]

nah it isn’t fixed there either

[pakoito]

so what you can do with what’s in master is mapError then handleErrorWith

[pakoito]

https://bit.ly/3bjAK0m

[pakoito]

mapError doesn’t require a subtyping relationship

[Hiosdra]

It looks not so good but works:

val faceVectorsDto = !recognitionClient
        .calculate(photoPart)
        .mapError<Nothing, FaceVectorsDto, RepositoryError>(::identity)
        .handleErrorWith({ IO.raiseError(ExternalFailure(it)) }, { IO.raiseError(it) })

Thanks, also for commenting in ticket 😄