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arrow

tavish pegram

05/27/2020, 12:50 AM

Hi all! For Arrow.Fx does it make sense to have all impure functions have IO return types (in addition to the

suspend

modifier) or is it best practice to just use IO to run the effects at the boundary of the system and stick to

suspend

to indicate impurity? For example

suspend fun printAString(s: String): Unit

suspend fun printAString(s: String): IO<Unit>

At the moment I am attempting to stick to a ports and adapters architecture with

suspend

for impure functions generally,

IO

return types at the service level, and actually unwrapping/running the IO at the controllers. The data is also the boundary but it didn't make a lot of sense to me to unwrap anything there so I've just been using

suspend

in the repos. As a follow-up, does anyone know if it is possible to custom alias the modifier to something like

impure

to distinguish from coroutine suspend methods? Or am I thinking about that incorrectly? Like

impure fun printAString(s: String): Unit

Is that something we might want to use Arrow.Meta for? Thanks!

Peter

05/27/2020, 3:30 PM

hi - is there a

foreach

equivalent for

Either

in arrow?

streetsofboston

05/27/2020, 3:39 PM

The

fold

works, but it always seems to be a bit wrong. The

fold

method returns a value; it doesn’t seem fit to execute a side-effect like a

forEach

would do….

Marcin Gryszko

05/27/2020, 4:22 PM

@pakoito thanks for pointing me to

arrow-fx-mtl

. After writing my message I started to implement my own

Concurrent

for

EitherT

🙂

😄 1

pakoito

05/27/2020, 4:44 PM

suspend fun bla(): Either<SomeError, Result> = Either.fx {
  val ble = somethingWithEither().bind()
  somethingElse(ble)
}

pakoito

05/27/2020, 4:44 PM

that's either-based flows

pakoito

05/27/2020, 7:49 PM

for 0.11 we’re bringing them together, rewriting some APIs from IO to use suspend () -> Either instead

Fred Friis

05/28/2020, 12:00 AM

Hi, I have a question about bracket

<https://arrow-kt.io/docs/0.10/fx/typeclasses/bracket>

Now, let's say we want to open a file, do some work with it, and then close it. With Bracket, we could make that process look like this: val safeComputation = openFile("data.json") .bracket( release = { file -> closeFile(file) }, use = { file -> fileToString(file) }) This would ensure the file gets closed right after completing the use operation, which would be fileToString(file) here. If that operation throws an error, the file would also be closed.

I understand this, but what if the risky thing we were doing was not in the fileToString() but also in openFile? Eg if the underlying file library throws an exception because the file doesn't exist?

raulraja

05/28/2020, 12:49 PM

https://twitter.com/JorgeCastilloPr/status/1265895432129056768

🤩 7

👏 17

🔝 1

kushalp

05/28/2020, 8:51 PM

Are there any common examples that show how to use

Valid

Invalid

and

Either

seamlessly? I'm thinking that this must be a fairly common pattern and would like to understand how others have done this in a simple way: 1. Make request to remote API that returns

Either<ErrorException, ResponseData>

where

ResponseData

is some

data class

2. Validate all of the fields in

ResponseData

such that they can be converted into

Invalid

Valid

3. Operate on the happy/sad paths

pakoito

05/29/2020, 1:46 AM

mySet.k().sequence(Either.applicative())

may just work, it should be a member or extension function actually

Marcin Gryszko

05/29/2020, 6:46 AM

Continuing my exploration of

Concurrent

- I tried in my program replacing

Concurrent.parMapN

with a

ParApplicative

instance (obtained by

Concurrent.parApplicative(ctx)

). Then, I replaced

parMapN

with

mapN

and checked which threads effects passed to

mapN

were executed in. To my surprise, they were executed in the caller's thread:

C.parApplicative(dispatchers().io()).run {
    mapN(getAthleteActivities(), getAthlete()) { (apiActivities, apiAthlete) -> /* transform */ }
}

I checked quickly the implementation; to have a concurrent execution, I have to use

map2

extension function:

C.parApplicative(dispatchers().io()).run {
    getAthleteActivities().map2(getAthlete()) { (apiActivities, apiAthlete) -> /* transform */ }
}

Is this a conscious design decision? Should

mapN

be delegated to

parMapN

than_

05/29/2020, 10:39 AM

hi, am I missing something or is this a bug?

val list = listOf(1,2,3,4).k()

ListK.monadFilter().fx.monadFilter {
    list.bindWithFilter { it % 2 == 0 }
}.let(::println)

crashes with NPE. But this works fine

val list = listOf(1,2,3).k()

ListK.monadFilter().fx.monadFilter {
    list.bindWithFilter { it % 2 == 0 }
}.let(::println)

Jörg Winter

05/29/2020, 11:05 AM

Is there an example of the Haskell analog 'mapM with IO' in arrow core or arrow fx ? I want to build a List (or Traversable, ..) of IO actions, and run them by mapM'ing over that data. is it only foldMapM from ListKFoldable ? or is there a more compact way somewhere ?

05/29/2020, 2:16 PM

Is there a shortcut to create a lens for a generic class? I guess I’m looking for plens, but I’d like to avoid writing everything manually

julian

05/29/2020, 2:35 PM

Scala has implicit parameter injection that provides typeclasses to methods. Is there a way with arrow not to have to explicitly provide typeclasses in function calls?

kushalp

05/29/2020, 8:23 PM

What is a

Kind<ForListK, String>

and how can I convert it into a

List<String>

Chris Cordero

05/30/2020, 2:29 AM

Is it possible to compose one

IO.fx

object in another

IO.fx

object?

Chris Cordero

05/30/2020, 2:41 AM

I'm a little confused as to when you would use

IO.fx

and when you would just use

IO

simon.vergauwen

06/01/2020, 2:22 PM

If you’d want an additional delay first.

IO.sleep(5.seconds).flatMap { 
  IO.effect { println("${Instant.now()}: test") }
    .repeat(IO.concurrent(), Schedule.spaced(IO.monad(), 5.seconds)) 
}

Chris Cordero

06/01/2020, 4:15 PM

huh, what is that

.k()

doing?

Chris Cordero

06/02/2020, 4:10 AM

If you have a

IO<List<A>>

is there a great way to transform it into a

List<IO<A>>

instead of doing:

val foo: IO<List<A>> = ...
val (bar) = foo
val final = bar.map { IO.just(it) }

tim

06/02/2020, 9:12 AM

Hi folks, can anyone recommend a good article on flatMap vs map? I'm finding that i'm almost always using map in my code and so suspect I'm missing something as I learn FP

pakoito

06/02/2020, 9:17 PM

If you change the first line to

val oe: Eq<Option<Int>> =

instead of the inferred `OptionEq<A>`then the second line will stop working

julian

06/03/2020, 1:31 AM

There's a pattern that appears a lot when I try to code more functionally.

sealed class A {
    data class B(val b: String) : A()
    data class C(val c: Int) : A()
}

class D

val a: A = TODO()

fun f(a: A): D {
    when (a) {
        is A.B -> { TODO() }
        is A.C -> { TODO() }
    }
}

I end up with so many of these `when`s throughout my code. Am I doing something wrong? Or is my allergy to it due to coming from doing only OOP for years? With OOP

would have some abstract method that

and

would implement.

CLOVIS

06/03/2020, 9:53 AM

I guess the same style could be used to define

Validated<E, A>

Union2<Success<A>, Collection<E>>

thanerian

06/03/2020, 9:54 AM

I see Union like anonymous sealed class

👍🏾 1

raulraja

06/03/2020, 2:39 PM

As soon as we have stream we can model those more sane

tim

06/03/2020, 5:37 PM

Hi all, another newb question sorry 🙂 I'm trying to figure out how to combine multiple Eithers into one Either where their Rs form a list... Something like this:

val a = Either.right(1) // or left
val b = Either.right(2) // or left
val c = Either.right(3) // or left

val d = // magic

d // Either<Nothing, List<Int>>

I've tried playing with flatMap, but i end up with flatMaps inside flapMaps ... which isn't very nice:

val d = a.flatMap { a ->
 b.flatMap {
   Either.right(listOf(a, it))
 }
}
...

I think I'm just missing something basic here as I cant imagine my use case here is unique?

tim

06/03/2020, 6:46 PM

@julian the one from my scratch pad is this:

val a = Either.right(1)
val b = Either.right(2)
val c = Either.right(3)

val d = Either
    .applicative<Nothing>()
    .mapN(a, b, c) { (a, b, c) ->
        listOf(a, b, c)
    }
    .fix()
    .fold(
        ifLeft = { println("try again :(") },
        ifRight = { println("success: $it") }
    )

But I'm now integrating it into my actual use case which is about parsing variables from a url path

Title

tim

06/03/2020, 6:46 PM

@julian the one from my scratch pad is this:

val a = Either.right(1)
val b = Either.right(2)
val c = Either.right(3)

val d = Either
    .applicative<Nothing>()
    .mapN(a, b, c) { (a, b, c) ->
        listOf(a, b, c)
    }
    .fix()
    .fold(
        ifLeft = { println("try again :(") },
        ifRight = { println("success: $it") }
    )

But I'm now integrating it into my actual use case which is about parsing variables from a url path

julian

06/03/2020, 6:49 PM

Thanks @tim!

👍 1

tim

06/03/2020, 6:53 PM

Hmm I've tried to expand this a bit but no joy:

val a = getEither(1)
val b = getEither(2)
val c = getEither(3)

val d = Either
    .applicative<Int>()
    .mapN(a, b, c) { (a, b, c) ->
        listOf(a, b, c)
    }
    .fix()
    .fold(
        ifLeft = { println("try again :(") },
        ifRight = { println("success: $it") }
    )

fun getEither(value: Int) : Either<String, Int> {
    return if(Random.nextBoolean()) {
        Either.left("ouch!")
    } else {
        Either.right(value)
    }
}

I'm getting this .. its fine with Left = Nothing

pakoito

06/03/2020, 6:53 PM

^^^ That last one is

Either.conditionally

that warning, if you squint

is telling you you’re expecting string on the left

and the applicative expects Int on the left instead

tim

06/03/2020, 6:56 PM

Ahhh

So whats in applicative is the Left value of the either

haha yep that fixed it straight away thanks!

I'll adjust my original code above as that's not correct then

👍🏾 1

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