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Powered by
Title
t

thanh

07/04/2022, 10:21 AM
Do we have something like this in Arrow? I tried to find but it seems missing.
public fun <A, B, C> Either<A, B>.combine(SGA: Semigroup<A>, b: Either<A, C>): Either<A, Pair<B, C>>
s

stojan

07/04/2022, 10:35 AM
the function exists for
Validated
and it's called
zip
https://arrow-kt.io/docs/apidocs/arrow-core/arrow.core/zip.html AFIK it doesn't exist for Either
t

thanh

07/04/2022, 10:38 AM
thanks, I knew that, but also I feel like, the way we have to switch between
Validated
and
Either
is a bit annoying sometime ;(
s

simon.vergauwen

07/04/2022, 11:46 AM
I would honestly not write such a function for
Either
. Typically I work with
Validated
in parts where I am actually validating, and then I simply
bind
them from
either
blocks. For example here, https://github.com/nomisRev/ktor-arrow-example/blob/e35fbd8e24c253eecf1200b3024caafedf70c908/src/main/kotlin/io/github/nomisrev/validation.kt#L57 https://github.com/nomisRev/ktor-arrow-example/blob/e35fbd8e24c253eecf1200b3024caa[…]0c908/src/main/kotlin/io/github/nomisrev/service/UserService.kt
👍 1
Do you have different use-cases?
t

thanh

07/04/2022, 11:52 AM
Thanks Simon. I'm using similar strategy as your example. It works great. But in my experience there are some difficulty when introduce
Validated
to beginners.
s

simon.vergauwen

07/04/2022, 12:05 PM
Always interested in improving on that, but I personally don't think introducing such an API for Either would make this more clear.
👍 2
t

thanh

07/04/2022, 12:14 PM
Agree, this api also introduce some complication as well.
r

raulraja

07/04/2022, 12:43 PM
I think in the future the use cases for validated like error accumulation can be added directly over Either when there is a NonEmptyList<E> on the Left side. Other than that Validated and Either are the same. This is something I think we should re-visit. Is there other use case that Validated covers and Either does not beside the operators for Error accumulation?
s

simon.vergauwen

07/04/2022, 12:46 PM
If we opt for such a API I would argue that we should drop
zip
for
Either
completely. Since
zip
is simply the following, but the snippet below doesn't even suffer from the
n-arity
issue
either {
   transform(a.bind(), b.bind(), c.bind(), ...)
}
That raises another issue though. What about
traverse
does it accumulate, or not for "Either/Validated".
If we can remove API surface, and/or data types whilst still covering the same features/behaviour in a clear way I am pro.
t

thanh

07/04/2022, 1:43 PM
I think
traverse
should work as normal, and additional
parTraverse
for error accumulation.
But maybe that will be confused with some other concurrency functionality.
s

simon.vergauwen

07/04/2022, 1:49 PM
It would conflict with
parTraverse
from Arrow Fx yes
👍 1
I am personally not a huge fan of
parMap
/
parTraverse
naming for error accumulation.
I always expect parallelism
p

phldavies

07/05/2022, 8:18 AM
would `nelMap`/`nelTraverse` work?
🤔 2
s

simon.vergauwen

07/05/2022, 8:57 AM
Hmm, I like this discussion thread! 🥳 To make a suggestion, and to stay within the Kotlin Std naming spirit I think something like
mapAccumulate
or
mapCollect
or something along those might be a good API name for such an API. If we can close the gap between
Validated
and
Either
we can seriously decrease the API surface 🤔 Is there a use-case for
Validated
except for
traverse w/ Monoid
or
zip w/ Monoid
? cc\\ @Alejandro Serrano Mena
a

Alejandro Serrano Mena

07/05/2022, 10:01 AM
I’ve never used it in any other place than either
zip
or `traverse`ing some data structure (maybe not a list, maybe a tree or something like that)
but indeed one could minimize the surface a lot by making something like
Validated<A, B> = Either<NonEmptyList<A>, B>
or something like that, and provide specific
mapAccumulate
which only work when
Either
has a list inside
s

stojan

07/05/2022, 10:03 AM
right now
Validated
support any type on the left.... wouldn't that limit the left side to
NonEmptyList
?
🔝 1
s

simon.vergauwen

07/05/2022, 11:23 AM
Yes, introducing such a restriction seems unnecessary.
s

stojan

07/05/2022, 11:30 AM
the way I see it, in terms of the data they carry
Validated
and
Either
are the same. We can convert between both of them without information loss. The differences are: •
Validated
lacks
flatMap
by design (it used to have
andThen
which was similar to
flatMap
) •
Either
has a
zip
that does NOT accumulate errors,
Validated.zip
accumulates errors •
Validated
accumulates errors and
Either
short circuits on the first error The short circuit VS error accumulation behaviour right now is based on the container type. If we were to merge both containers we would need some other way to trigger error accumulation VS short circuiting. I'm not sure what that would be and if it would actually make things simpler. 🙂
👍 1
t

thanh

07/05/2022, 5:28 PM
So maybe I'm naive, but if we introduce two new functions for
Either
for errors accumulation, which can be
zipAcccumulate
and
traverseAccumulate
then we don't have to use
Validated
anymore. At least for my use cases and Simon's examples above.
3
s

stojan

07/10/2022, 3:17 PM
I was curious how ZIO handles this as ZIO also doesn't have typeclasses and it's not constrained by them... ZIO itself doesn't have error accumulation, it only has the failing fast
zip
method. For accumulating errors they have
Validation
which is similar to
ValidatedNel
and
ZValidation
which is similar to
Validated
in ZIO Prelude https://zio.github.io/zio-prelude/docs/functionaldatatypes/
a

Alejandro Serrano Mena

07/10/2022, 3:59 PM
yes, having different
Either
and
Validated
is a common pattern, not only in ZIO, Haskell also follows that trend. But there are also reasons to keep the API simple and have just a single one