Extend Functor implementation in the Traverse instance of EitherT, OptionT and Tuple2K

[barambani]

This is about using the Functor instance in Traverse for the monad transformers EitherT and OptionT and in Tuple2K so to use its map implementation. This follows @peterneyens 's comment #2182 (comment) related to WriterT.

[kailuowang]

this breaks binary compatibility. unfortunately these traits were not sealed. We might need to extend the functor trait at the instance creation site, rather than the traits.

[barambani]

Thanks for the pointer, I was trying funny things. I will keep trying that way and see if I can make it keep the binary compatibility. Otherwise I will close it.

[codecov-io]

Codecov Report

Merging #2191 into master will increase coverage by 0.22%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master    #2191      +/-   ##
==========================================
+ Coverage   94.73%   94.95%   +0.22%     
==========================================
  Files         330      333       +3     
  Lines        5582     5787     +205     
  Branches      209      222      +13     
==========================================
+ Hits         5288     5495     +207     
+ Misses        294      292       -2

Impacted Files	Coverage Δ
core/src/main/scala/cats/data/Kleisli.scala	96.9% <ø> (-0.99%)	⬇️
...in/scala/cats/data/IndexedReaderWriterStateT.scala	99.04% <100%> (-0.02%)	⬇️
core/src/main/scala/cats/data/EitherK.scala	93.47% <100%> (-4.35%)	⬇️
core/src/main/scala/cats/data/OptionT.scala	97.46% <100%> (ø)	⬆️
core/src/main/scala/cats/data/Nested.scala	94.28% <100%> (-1.43%)	⬇️
core/src/main/scala/cats/data/WriterT.scala	91.37% <100%> (ø)	⬆️
core/src/main/scala/cats/data/IorT.scala	97.76% <100%> (ø)	⬆️
core/src/main/scala/cats/data/EitherT.scala	97.65% <66.66%> (+0.05%)	⬆️
core/src/main/scala/cats/data/Tuple2K.scala	90.69% <83.33%> (-4.66%)	⬇️
... and 20 more

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[peterneyens]

Thanks for the follow up.

I think we can do something similar (if you are interested) for :

• EitherT: Bitraverse we can override bimap.
• Tuple2K : ContravariantMonoidal extend Contravariant (can drop its contamap definition).
• RWST: Alternative extend SemigroupK
• IorT: Traverse extends Functor

I might have missed some other opportunities.

[barambani]

Sure, I'm happy to cover as much as I can. I will give a look. Thanks for the great support

[barambani]

I couldn't find a way to remove this. If I do it I get this

[error]  * in current version, classes mixing cats.data.WriterTContravariantMonoidal needs to update body of method contramap(cats.data.WriterT,scala.Function1)cats.data.WriterT
[error]    filter with: ProblemFilters.exclude[UpdateForwarderBodyProblem]("cats.data.WriterTContravariantMonoidal.contramap")

from Mima. @kailuowang , @peterneyens can you think to anything I can try ? Thanks a lot

[barambani]

There is still a problem with IndexedReaderWriterStateT as most of the derivations are inside sealed abstract class so I cannot compose with mixin. An option could be change them to trait (even though I'm not sure if it break the compatibility), but involving IRWSTFunctor the change will affect most of the instances so I wanted to double check here before trying it. Do you see it as feasible ? Thanks a lot for the help.

[barambani]

I'm not sure what happened with this build, it looks like it was killed. Do I have any way to restart the CI ? Thanks

[barambani]

Could the CI be reran ? Thanks

[barambani]

This has been quiet for a while. I think I changed any case I could find. It should be ready for a review. Many thanks.

[LukaJCB]

Sorry for not reviewing sooner, this looks good to me, apart from the one snippet in WriterTContravariant that you mentioned 👍 Thanks very much! :)

[barambani]

No problem at all 😄 , thank you. I will see if I can do something then.

[kailuowang]

Thanks so much! sorry about the delay.

[barambani]

No worries. Thank you all 👍