tailRec instances aren't visible by default

[kubukoz]

As far as I know, import cats.syntax.all.* is now the recommended mega-import. However, it doesn't bring the instances for scala.util.control.TailCalls.TailRec - those still require import cats.instances.tailRec.*.

These instances should probably be included in the companion objects of the typeclasses themselves so that they're auto-discoverable like most other instances.

[aluscent]

@kubukoz I'm interested in helping on this issue.
As I assumed from your explanation, we need to add implicit instances of the TailRecInstances into Defer and StackSafeMonad, right? Please give me more details and help me understand the problem.

[kubukoz]

I just checked and Defer[TailRec] already is in the companion of Defer: implicit def catsDeferForTailRec: Defer[TailRec].

StackSafeMonad isn't a typeclass per se, I'm actually not sure why it appears in the type annotation (should be just Monad IMO)...

but there's a Monad[TailRec] in the companion object of Invariant too: implicit def catsMonadForTailRec: Monad[TailRec] - so I don't see why I opened this issue in the first place...

This works fine on 2.12, 2.13 and 3.x:

//> using dep org.typelevel::cats-core:2.12.0
import scala.util.control.TailCalls.TailRec
import cats.Monad
import cats.Defer

object Demo extends App {

  println(Monad[TailRec])
  println(Defer[TailRec])

}

[kubukoz]

Maybe there's a combination of compiler flags that causes the instances not to be picked up, but so far this seems like a non-issue.

[aluscent]

@kubukoz Thank you for checking!
As you discussed this is not an issue anymore. Do you think this thread should be closed?

[kubukoz]

yeah, I don't think I can reproduce it now.