Missing instances for Cofree

[kubukoz]

https://hackage.haskell.org/package/free-4.1/docs/Control-Comonad-Cofree.html

There are some instances/derivations in the Haskell version of Cofree that might be useful for some people (I for one would welcome Eq):

• Eq
• Ord
• Show
• Alternative f => Monad (Cofree f) (needs tailRecM)
• Distributive

Possibly more, but these are the ones I noticed missing now.

I'd love to take a shot at implementing them, if it sounds like a good idea :)

[kubukoz]

I'm working on this (slowly), so far got the Monad instance, should have the rest in the upcoming week

[kubukoz]

Blocked by #2308

[kubukoz]

I suggest waiting until 2.13 is released until we go forward with this - lazy implicits make it possible to implement the recursive instances (Eq, Show, Order and maybe more).

If anyone stumbles upon this issue and needs an instance before that, I suggest copying code from https://github.com/typelevel/shapeless-contrib/pull/9/files and porting it to Cats.

[kubukoz]

@kailuowang mind adding an "on hold" label?

[kailuowang]

added. thanks @kubukoz

[kubukoz]

Thanks :)