@jtonic forgot also to mention:

OptionT.monad<EitherKindPartial<String>>()  
//is now
OptionT.monad(Either.monad<String>())

Dear arrow community, Yesterday I was a presenter at a kotlin meetup event in Bucharest, Romania. And the topic was about how friendly is kotlin programming language to FP approach, and, moreover, how amazing is the kotlin

FP companion library, Arrow

. Preparing this event was not easy at all, considering I just started, the FP journey, more than a month ago. Needless to say that

Functional Kotlin

book was the main source of information, w/o it, I was definitely lost. Reading it was a pleasure, and it easy the FP path I am on. I want to congratulate the authors for their amazing work, and I am looking forward to reading some other books of them. On the other hand, there were moments I was really stuck with understanding some concepts or not figuring out why some arrow based test code refuse to compile. In these moments this channel was my salvation, and I witnessed how amazing the kotlin/arrow community is. *A big THANK YOU to each and every of you for how amazing you are.

@jtonic thank you for your kind words, writing that book was hard as nails for both Rivu and me. But knowing that people find it enjoyable and useful leave me with a feeling of gratitude

hi folks, I am relatively new to Kotlin and very new to functional programming... as a learning exercise I am trying to convert a scala parser library to Kotlin and I was wondering if there is anyway to do this with Kotlin/Arrow: sealed trait FooBar[L <: HList, T <: HList, R <: HList] { type Out <: HList } object FooBar { implicit def foobar[L <: HList, T <: HList, R <: HList, Out0 <: HList] (implicit ts: Aux[L, L, T, T, R, HNil, Out0]): FooBar[L, T, R] { type Out = Out0 } = throw IllegalStateException("Compile-time only call") I am having trouble with the "type" parameter which is reassigned with the function "foobar" (ie. fb.Out can be assigned a different type)...

You can follow progress here https://github.com/arrow-kt/arrow/issues/808

well, I can try, I'm not very familiar with the arrow codebase yet nor all the new concepts that it introduces, but this seems like a good opportunity to start

@rawtoast https://github.com/RawToast/go-north-and-die/blob/master/src/main/kotlin/gonorth/free/GameEffect.kt awesome stuff! 👏

I tried but cannot figure out what should be in place of … to make it work 🙂

@xenomachina what is the parser target? narses is sort of a production system input format

"i would expect any library throwing around the term FP and "purity" in the README elevator pitch would make some inroads toward safer type system and operators than the ones that make a dual type hierarchy for nulls and ternary by null/elvis", leveraging kotlin's almighty Unit and Nothing Monads

whats the difference between Either, Option, Validated? besides naming?

like the Combined instances IIRC

@jacob no dates yet, I'm gonna focus on finishing the keep-87 and then move into those integrations. But if people need it soon we can discuss implementations and take it from there with whomever wants to help and it's available.

does kotlin have the concept of forall in its type system?

@jacob forall support in Kotlin type system is limited. Generic functions in Kotlin have forall type signature like

fun <T> foo(a: T): List<T>

(the type of

foo

in Haskell terms would be

forall t. t -> List t

), but you cannot directly capture such a functional type in a plain variable. You’ll have to always capture both

foo

and

T

with another generic function like

fun <T> bar(a: T) = foo(foo(a))

or with a generic class like

class A<T> { fun bar(a: T) = foo(a) }

Not really. There is no generic way to know if the current thread already has event loop unless you are programming in some kind of a framework that guarantees that any thread has an event loop. Why do you care about threads anyway?

It’s not easy to know without a bit of context 😄

and I’m not sure whether you’ll get the right ::plus, so you may still need the lambda

I was trying to do something like this:

val foo = myEither.getOrElse { return false }

but Kotlin won't allow this because the lambda to

getOrElse

is

crossinline

. Why is it declared

crossinline

?

@zmarkan we call it “Functional companion to Kotlin’s Standard Library” on www.arrow-kt.io so FP toolkit is the right terminology I think

if we don’t inline we can’t have local or non-local returns

ask Simon lol

I like Reason a lot

Is there a way to flatten a Try?

Try {
            when (accessChecker.loggedInUserHasAccess()) {
                true -> Try.just(Unit)
                false -> Try.raise(FeatureAccessDeniedException("No access for feature"))
            }
        }

Or is better to just throw the error in the false branch. Note that

accessChecker.loggedInUser...

can also throw an exception.

Try {
            when (accessChecker.loggedInUserHasAccess()) {
                true -> Unit
                false -> throw FeatureAccessDeniedException("No access for feature")
            }
        }

for non-maintainers, how’s this tutorial?

if any one

enrichValue

fails, the whole operation fails. Else it aggregates the successes on the right, which you map to the update of your previous value.