typelevel/cats-tagless

Soren

@srnb_gitlab

"An aspect weaver is a metaprogramming utility for aspect-oriented languages designed to take instructions specified by aspects (isolated representations of significant concepts in a program) and generate the final implementation code."

This sounds really cool

ritschwumm

@ritschwumm

@srnb_gitlab AOP is just a generalized COME FROM :)

Soren

@srnb_gitlab

I don't know what that is either :sweat_smile: @ritschwumm

ritschwumm

@ritschwumm

@srnb_gitlab GOTO's even more evil twin, see https://en.wikipedia.org/wiki/COMEFROM

Soren

@srnb_gitlab

hmm

Ben Spencer

@dangerousben

is it / should it be possible to use autoFunctorK on algebras that return Resource[F]?

Ben Spencer

@dangerousben

ah... I suspect not because Resource.mapK requires an instance of Defer for the target functor?

Ben Spencer

@dangerousben

oh, and Applicative... G: Applicative[H] is pretty confusing

Georgi Krastev

@joroKr21

Yeah I think those restrictions are lifted in CE3

typelevel/cats-effect#1285

Ben Spencer

@dangerousben

oh nice

Keir Lawson

@keirlawson

I it possible to use cats-tagless to derive a FunctorK instance for a type you don't own?

Georgi Krastev

@joroKr21

Yes, definitely - as long as it's a trait with a type parameter F[_] that appears in covariant positions

You can use Derive.functorK (all derivations are available in that object)

Keir Lawson

@keirlawson

import cats.tagless._

@autoFunctorK
trait Foo[F[_], T] {
  def a(i: Int): F[T]
}

^ this is giving me Cannot expand @autoFunctorK with Scala 2.13.4, clearly something in my setup is messed up, anyone any ideas what it could be?

I'm using "org.typelevel" %% "cats-tagless-macros" % "0.12"

Keir Lawson

@keirlawson

ok so its me being a numpty :D

didn't have -Ymacro-annotations

though in my defence whilst the install instructions on the github mention this, those on the microsite do not

Georgi Krastev

@joroKr21

yep sorry about that and thanks for the PR :thumbsup:

Jonas Chapuis

@jchapuis

hey, how difficult would it be to add BiFunctorK and the corresponding derivation for algebras with two parameters?

Diego E. Alonso Blas

@diesalbla

@jchapuis What would be the key operation of that BiFunctorK ?

I suppose you could just try copying and adapting the code of FunctorK... would it be fairly symmetrical?

Bifunctor being something like F[_, _], and a BifunctorK being for algebra types of the form A[ _[_, _]] ,
you may still have function mapK[G[_, _]](fk: F ~> G) ...

Jonas Chapuis

@jchapuis

thanks, yeah so I thought, I might give it a shot. Indeed I would like to have it to "mapK" bifunctorial algebras

Jonas Chapuis

@jchapuis

well, not so easy. Simulacrum only support single-parameter type constructors, and cats FunctionK likewise is for unary parameter types, I can't express a natural transformation between bifunctors using it

Andrey Patseev

@patseev

Hey guys. How to solve the situation when I need both ApplyK and Aspect for my algebra? Can't auto-derive because both extend FunctorK and this leads to ambiguity.
I've kinda solved this by deriving Aspect and SemigroupalK instead and on "call-site" requiring FunctorK and SemigroupalK and re-implementing map2K manually there. But it's ugly and doesn't feel like a proper solution

Georgi Krastev

@joroKr21

Hmm good question - do you mean that you can't use them? I don't think deriving would be a problem.

Andrey Patseev

@patseev

Yeah, I meant that I can't use it due to ambiguity of FunctorK in the scope. Deriving works.

Georgi Krastev

@joroKr21

That's the same problem with Applicative and Traverse in cats

I guess a workaround would be to use mapK from the companion object or directly from the typeclass

It's not ideal

The former if you're using the macro annotations

Andrey Patseev

@patseev

Gotcha. Thank you!

Ben Spencer

@dangerousben

in this first faq, what would the type of the generated FunctorK instance look like (in kind-projector syntax)? https://typelevel.org/cats-tagless/faq.html#does-cats-tagless-support-algebras-with-extra-type-parameters

Ben Spencer

@dangerousben

FunctorK[Foo[*[_], T forSome { type T }]] ?

Ben Spencer

@dangerousben

or does it generate a different FunctorK for each value of T?

Georgi Krastev

@joroKr21

@dangerousben it would generate a method implicit def functorK[T]: FunctorK[Foo[*[_], T]] = ???

Ben Spencer

@dangerousben

right, thanks

ivan-klass

@ivan-klass

Guys, perhaps I'm missing something, but out of curiosity, why does autoSemigroupalK require all F[_] positions to be covariant? I think that given two algebras (say, algG: Alg[G], algH: Alg[H]) a product Alg[Tuple2K[G, H, *]] can handle input argument t2k: Tuple2K[G, H, A] with passing t2k.first to algG and t2k.second to algH correspondingly. Am I wrong with something here?

Georgi Krastev

@joroKr21

Hmm interesting, I never thought about it - do you want to give it a try?

Georgi Krastev

@joroKr21

I've released 0.14.0 with the SemigroupalK changes: https://github.com/typelevel/cats-tagless/releases/tag/v0.14.0

Brian P. Holt

@bpholt

We have a lot of generated code for working with Thrift services that has a tagless-style trait with implementations in Twitter Futures. When we pull those into a cats-effect app, we’re writing a lot of boilerplate implementations that delegate the actual work to the Future-based impl, wrapping each of those calls in Async[F].async and Sync[F].delay.

Does that sound like the kind of thing that Aspect / Codomain could help with?

Georgi Krastev

@joroKr21

@bpholt I think mapK is enough - you just need def async[F[_]: Async]: Future ~> F if I understand correclty

Oh no wait, I guess the problem with mapK is that it will call the service before mapping it, thus triggering the side effect

Georgi Krastev

@joroKr21

I guess your best bet is Derive.readerT[Service, Future].mapK[F](provide(service)) where

def provide[F[_], R](service: R)(implicit F: Async[F]): ReaderT[Future, R, *] ~> F =
  λ[ReaderT[Future, R, *] ~> F](r => F.fromFuture(F.delay(r(service))))

Georgi Krastev

@joroKr21

I even made it into a test: #250

Brian P. Holt

@bpholt

Oh wow, that’s great. Thanks!

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