Type class instances for isomorphic types

[ceedubs]

Related Gitter conversation: https://gitter.im/typelevel/cats?at=58f5f7e801d449152eb5c5dc

It may be nice to provide an easy way to create type class instances that simply delegate to the type class instances of an isomorphic type. For example, if I create final case class DecodeJson[A](decode: Kleisli[Either[String, ?], Json, A]), then I can easily create an iso functor (a bidirectional tranformation via a pair of DecodeJson ~> Kleisli[Either[String, ?], Json, ?] and Kleisli[Either[String, ?], Json, ?] ~> DecodeJson). It seems like I should be able to pass in this iso functor to create a Monad[DecodeJson] that simply delegates through to the Kleisli Monad and runs the transformations on both ends.

I mentioned this use-case as motivation for #992, but that ticket seems to be stalled, and I'm not sure whether we'll go through with it. I think that this functionality could be useful whether or not anything happens with #992.

[kailuowang]

👍 The isomorphism defined in scalaz is meticulous. It defines multiple isomorphisms with different kinds

[qingwei91]

Hi ~ is this feature still desirable?
If so, is it currently being work on?

I am happy to help

[ceedubs]

I think that this is still desired, but there are a number of design decisions that would need to be made, such as whether these isomorphism instances are auto-generated by the build. It might make sense to start this out as an experimental module (or even a separate repo) before we consider it to be part of Cats proper.

[qingwei91]

@ceedubs Cool, is there a discussion somewhere around these design consideration?

I'll give it a go if nobody is working on it in a separate repo, do you think following what scalaz did is a good start?

Cheers

[kailuowang]

👍 on an experiment module and following what scalaz did.