kotlinlang #arrow

That small example of S3 + logging I wrote yesterday might be suitable as a demonstration of a practical use-case, as it (potentially) demonstrates moving from Spring DI to composable algebras using Arrow without any framework

:arrow: 2

sam

05/31/2019, 2:22 AM

This is a project that 20 people have spent 2 years working on - it's all kotlin, spring, spring-mvc, jpa, hibernate. Basically your typical enterprise java app but in Kotlin. It's well written, well tested. I've just spent 8 years doing Scala full time and this is now my first Kotlin role and I'm finding it hard to justify ditching spring, etc and doing things in a more functional way. Arrow is just one part of that. No one uses any of that stuff in Scala land.

raulraja

05/31/2019, 11:33 AM

Kotlin already supports DI with delegation and extension functions natively. Not sure why people insist on using DI with a runtime container. An example of DI with delegation and another one with type arg constrains: https://gist.github.com/raulraja/97e2d5bf60e9d96680cf1fddcc90ee67

sam

05/31/2019, 11:54 AM

What would you use for configuration loading? Equiv of pureconfig for scala basically

sam

05/31/2019, 8:06 PM

This is a minor thing, but I find

Option.fromNullable

to be very verbose. Isn't the point of option that you may or may not have a nullable value, and so shouldn't Option(t) accept a nullable ?

pakoito

06/02/2019, 1:09 AM

ReaderT.monad<F, AccountRepository<F>>(this).tupled(debit(from, amount), credit(to, amount))

pakoito

06/02/2019, 1:58 AM

if B doesn’t depend on A then use

applicative().map(val1, val2, val3) { a, b, c -> SomeClass(a, b, c) }

pakoito

06/02/2019, 9:07 AM

in both cases there’s no way of generating an

of type

Int

from the

Invalid

origin, so those lambdas that you pass cannot be called and the internal function calls won’t execute to create a new Validated that accumulates errors

sam

06/02/2019, 10:36 PM

What dependency do I need for

arrow.data.extensions.option.monad.binding

sam

06/03/2019, 10:32 AM

I notice that

val (nuke) = arm()

if you remove the parenthesis it fails to extract

snackycracky

06/03/2019, 10:42 AM

hey @raulraja 🙂 what was the intention for https://github.com/arrow-kt/arrow-ktor ?

raulraja

06/03/2019, 2:57 PM

fun validateExpiryDate(date: LocalDate): ValidatedNel<ValidationError, LocalDate> =
  if (logic) date.validNel() else Expired.invalidNel()

raulraja

06/03/2019, 3:02 PM

import arrow.data.extensions.validated.apply.map

raulraja

06/03/2019, 3:09 PM

so it returns

this

and does not delegate to the platform dependent trace stuff

raulraja

06/03/2019, 3:10 PM

fun <A> Try<A>.toValidatedNel(): ValidatedNel<Throwable, A> =
  fold({ it.invalidNel() }, { it.validNel() })

sam

06/03/2019, 3:11 PM

Is that predefined ?

sam

06/03/2019, 3:16 PM

ok, so in that other method, same question

pakoito

06/04/2019, 7:41 AM

I did a talk about “Headless Android development” where I go through an app, refactor it, and then test it completely from the JVM on the Kotlin REPL. Sadly it wasn’t recorded, so you’ll need to follow the slides, the demo, and the snippet that came with it: https://www.pacoworks.com/2017/03/16/headless-development-at-mobos-librarymaking-updates-and-plans-for-2017-2/

simon.vergauwen

06/04/2019, 7:42 AM

Hi @earroyoron, Raul gave a talk on this https://www.47deg.com/presentations/2018/06/08/better-types-fewer-tests/. I don’t think it was recorded tho. Jullian Traffaut also did a couple of presentations but I have not checked out recording and focuses more on theory than practice compared to Raul’s https://github.com/julien-truffaut/types-vs-tests.

sam

06/04/2019, 12:28 PM

what's the goal of non-fx arrow

😄 1

sam

06/04/2019, 12:45 PM

So, in summary, if the goal is that you want to focus on pure constructs only, that's a good goal, but one that isn't going to work for me (not that I expect your project to focus on me only) but I do think that 99% of kotliners are coming from a Java background, and making it as accessible as possible is important. And what I mean by that is allowing impure FP as a stepping stone.

simon.vergauwen

06/04/2019, 12:45 PM

suspend fun foo()  = start().flatMap { a ->
   bar(a).flatMap { b ->
     update(b).map { result(c) 
   }
}

raulraja

06/04/2019, 2:22 PM

@sam you can always call the function like that inside

suspend

we may even lift that restriction in

fx

down the road but in whatever case the biz about getting rid of Either is that IO is gonna be soon

IO<E, A>

so you don't need

Either

because you can do

just(a)

raiseError(e)

and that is the same as Either. You can also do

raiseError(exception)

Derek Berner

06/04/2019, 3:07 PM

So that concerns me. Even well tested, "pure" code could throw an OutOfMemoryError

raulraja

06/04/2019, 9:27 PM

https://twitter.com/arrow_kt/status/1135961780377726976?s=19

:arrow: 10

Imran/Malic

06/05/2019, 2:34 PM

@Derek Berner good question. Look here https://kotlinlang.slack.com/archives/C5UPMM0A0/p1520103294000092

Robert Menke

06/05/2019, 9:35 PM

Sort of random, but something in the #compose channel made me think of this. Do you think the arrow stdlib should include something like compose or andThen as utility functions?

//left to right
infix fun <A, B, C> ((A) -> B).andThen(right : (B) -> C) : (A) -> C = {
    arg -> right(invoke(arg))
}

//right to left
infix fun <A, B, C> ((B) -> C).compose(right : (A) -> B) : (A) -> C = {
    arg -> invoke(right(arg))
}

Title

Robert Menke

06/05/2019, 9:35 PM

Sort of random, but something in the #compose channel made me think of this. Do you think the arrow stdlib should include something like compose or andThen as utility functions?

//left to right
infix fun <A, B, C> ((A) -> B).andThen(right : (B) -> C) : (A) -> C = {
    arg -> right(invoke(arg))
}

//right to left
infix fun <A, B, C> ((B) -> C).compose(right : (A) -> B) : (A) -> C = {
    arg -> invoke(right(arg))
}

streetsofboston

06/05/2019, 9:36 PM

I think they already are part of (a) Arrow library…

Robert Menke

06/05/2019, 9:37 PM

oh are they?

hmm must have missed that in the docs

streetsofboston

06/05/2019, 9:37 PM

https://github.com/arrow-kt/arrow/blob/master/modules/core/arrow-syntax/src/main/kotlin/arrow/syntax/function/composition.kt

Judging from that, in the arrow-syntax lib…?

Robert Menke

06/05/2019, 9:37 PM

Thanks!

streetsofboston

06/05/2019, 9:39 PM

And other ones can be found in is parent directory (eg currying and such 🙂 ) https://github.com/arrow-kt/arrow/tree/master/modules/core/arrow-syntax/src/main/kotlin/arrow/syntax/function

pakoito

06/05/2019, 10:00 PM

these were brought from Funktionale, so thank @dh44t!!!

👏 4

dh44t

06/06/2019, 4:14 AM

😅

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