kotlinlang #arrow

There is not much difference between cats and Arrow. Arrow is inspired among others by cats but the internal implementations are quite different. The main different is that Arrow uses only extension functions in it’s type class definitions for all operations. For example

Kind<F, A>.map(f: (A) -> B): Kind<F, B>

map(fa: F[A], f: (A) -> B): F[B]

in cats

arrow 1

raulraja

06/12/2019, 10:13 AM

Cats has additional syntax also enabled by implicits to achieve what extension functions do and I suppose coming Scala 3 they will encode type classes like we do since Scala has copied Extension functions syntax from Kotlin

raulraja

06/12/2019, 10:14 AM

Also the IO runtime of Cats and our IO has now deviated considerably in some optimisations and the way concurrency etc is handled since we have suspension

raulraja

06/12/2019, 10:17 AM

AS for Fx and binding, Scala cats uses for comprehensions which is an enforced construct with different semantics compared to regular syntax but our binding since it’s based currently in suspension can happen with

imperatively in any fx block and that is a huge difference in terms of user experience and syntax.

earroyoron

06/12/2019, 3:41 PM

Hmmmm… a Coproduct store one of the given types or can store one of each????…. I mean given

typealias LoginResponse = Coproduct4<FatalLoginProblem, WrongCredentials, LoginNeedsMore, SuccessCompletedLogin>

LoginResponse

is only 1 of the 4 types or I should create

Unit

onto the useless. PD: I think as we use

First(FatalLoginProblem())

the answer is it holds just 1, but I am not sure because of the fold syntax where I always should use all “branches” for the coproduct.

thanerian

06/12/2019, 3:44 PM

Only store one of the given types, you could imagine a Coproduct as an anonymous sealed class and fold is like the exaustive use of when expresion over it

👍 1

raulraja

06/12/2019, 4:01 PM

if it could store multiple types it would be a product since coproduct represents a disjunction like

Either

and a product represents all declared properties (tuples, hlist, data classes)

👍 2

Bob Glamm

06/13/2019, 2:35 PM

From https://blog.softwaremill.com/final-tagless-seen-alive-79a8d884691d : in Kotlin, how often do signatures similar to the following occur using final tagless?

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def genFeed[F[_]: Monad:
  Logging: UserDatabase:
  ProfileDatabase: RedisCache:
  GeoIPService: AuthService:
  SessionManager: Localization:
  Config: EventQueue: Concurrent:
  Async: MetricsManager]: F[Feed] = ???

Bob Glamm

06/13/2019, 2:35 PM

I see complexity arising from interface graph dependencies but I wouldn't expect such an obnoxious signature to arise directly

pakoito

06/13/2019, 2:40 PM

that needs an interface

pakoito

06/13/2019, 2:40 PM

definitely

pakoito

06/13/2019, 2:40 PM

ModuleBla with all of them

Bob Glamm

06/13/2019, 2:42 PM

well, my thinking based on what I have so far is that composition would occur piecemeal: e.g.

S3Ops<F>

LoggingOps<F>

LoggedS3Ops<F>: S3Ops<F>, LoggingOps<F>

instead of all at once like the above

Bob Glamm

06/13/2019, 2:43 PM

maybe I don't work on software complex enough to warrant such a signature, though

Bob Glamm

06/13/2019, 3:07 PM

oh I see, Scala 2.x doesn't have type parameter bounds on traits

carbaj0

06/15/2019, 12:17 PM

are_the_same_thing_.txt

simon.vergauwen

06/15/2019, 12:23 PM

Not really since the first definition makes sure it runs on the

ctx

while the second doesn't and leaves the

ctx

unused.

andreworobator

06/16/2019, 11:03 PM

I'm trying to add the latest version of Arrow (0.9.0) to my project, but it seems like the latest published version is 0.8.2 https://search.maven.org/artifact/io.arrow-kt/arrow-core/0.8.2/jar Is the latest version published somewhere else?

pakoito

06/17/2019, 12:00 AM

arrow-core-data, for now

pakoito

06/17/2019, 12:00 AM

arrow-core will be back for 0.10 🙂

👌🏾 1

pakoito

06/17/2019, 12:00 AM

it was a mistake when publishing we cannot fix at the moment

kluck

06/17/2019, 9:01 AM

Hi, I'm trying to find the correct abstraction for continuously decoding barcodes. I'm currently using

IO.async()

which works great for a single barcode scanning.

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fun startScanning(): IO<ScanItem> = IO.async()
        .async { callback: (Either<Throwable, ScanItem>) -> Unit ->
            resume()

            decodeContinuous(object : BarcodeCallback {
                override fun possibleResultPoints(resultPoints: MutableList<ResultPoint>?) {}
                override fun barcodeResult(result: BarcodeResult) {
                    callback(ScanItem(result.text, result.barcodeFormat.toScanItemFormat()).right())
                }
            })
        }

// inside my activity
scanner.startScanning()
                .map {
                    Timber.d("Scanned item: $it")
                    scanner.setStatusText(it.data)
                }
                .unsafeRunAsync { result -> result.mapLeft { Timber.e(it) } }

What is the idiomatic way to handle multiple barcode scans?

pakoito

06/17/2019, 9:08 AM

things I’d do to that code 😄

pakoito

06/17/2019, 9:12 AM

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scanner.startScanning()
                .flatMap { item ->
                     IO { Timber.d("Scanned item: $item") }.map {  item }
                }.flatMap {
                    scanner.setStatusText(it.data)
                }
                .handleErrorWith { err ->
                  IO { Timber.e(err) }
                }.unsafeRunAsync { }

which means you can convert to

fx

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IO.fx {
  val (item) = scanner.startScanning()
  Timber.d("Scanned item: $item")
  !scanner.setStatusText(item.data)
}.handleErrorWith { err ->
  IO { Timber.e(err) }
}.unsafeRunAsync { }

pakoito

06/17/2019, 9:13 AM

now, if you want a stream of data, you’ll need to use Observables or Flow or another multi-data datatype while we build our own

arrow-streams

pakoito

06/17/2019, 9:14 AM

if you have 2-N codes to scan in parallel, you have map and parMap

pakoito

06/17/2019, 9:16 AM

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fun scanOne() =
  scanner.startScanning()

map(scanOne(), scanOne(), scanOne()) { codeA, codeB, codeC ->
    scanner.setStatusText(...) 
}.flatten()
.handleErrorWith { err ->
  IO { Timber.e(err) }
}.unsafeRunAsync { }

pakoito

06/17/2019, 9:19 AM

parMap requires using a CoroutineContext that has the threading info, and using the extfun

parMap

on it. Same code otherwise

kluck

06/17/2019, 9:49 AM

Thanks, I'll check this out

pakoito

06/17/2019, 10:15 AM

if you have a list of any size of IOs, use the function

sequence

for it