Investigate problem in ZIO's promise

[odersky]

In Promise.scala in core/shared/src/main/scala/zio we get on error in the following line:

final class Promise[E, A] private (private val state: AtomicReference[State[E, A]], blockingOn: List[Fiber.Id])

We have to change State[E, A] to Promise.internal.State[E, A] to make it pass. This since the new scheme for
constructor proxies #10784. We should find out the cause and see whether we can address it.

[odersky]

Minimization:

import Promise.internal.State

class Promise(state: State)

object Promise:
  object internal:
    trait State

This gives:

-- [E046] Cyclic Error: test.scala:4:21 ----------------------------------------
4 |class Promise(state: State)
  |                     ^
  |                     Cyclic reference involving val <import>

[odersky]

Here's the observed completion sequence:

completing val <import>
  completing val Promise
  completed Promise in object Test
  completing type Promise$
    completing val <init>
    completed <init> in object Promise
add constr app object Promise
    completing type Promise
      completing val <init>
        completing val state
          completing val <import>
Cyclic reference involving! val <import>

So, it tries to type the import, which means it needs to complete the Promise object. That means we need to complete the Promise class since we need to know what constructor proxies to add. Completing the Promise class implies typechecking the parameters, which means we need to resolve State, which leads to completing the import. Hence the cycle.
It only happens if the import comes first, and if the type State is two levels deep in Promise.

[odersky]

The following variants work:

  import Promise.State

  class Promise(state: State)

  object Promise:
    trait State

and

  object Promise:
    object internal:
      trait State

  import Promise.internal.State

  class Promise(state: State)

and

class Promise(state: Promise.internal.State)

object Promise:
  object internal:
    trait State

and

object Promise:
  object internal:
    trait State

class Promise(state: Promise.internal.State)

[odersky]

At this point, I don't see much we can do. I tried to delay resolving the import but that caused problems in other code. So I guess we have to live with the fact that this orderings of code and imports now gives a cycle. There are of course ways in which this could have given a cycle before, for instance if object Promise had extended class Promise. This is just one more way to produce a cycle. I believe we can live with it.

[som-snytt]

FWIW, this issue feels like a recent duplicate, but I couldn't find where I saw it. The forward-reference style of import from companion is common; the idiom always feels strange to me; maybe it's worth advertising its fragility, perhaps in the error message.

[nafg]

This seems pretty bad to me. Is there no possible solution?

…

On Wed, Dec 16, 2020 at 7:33 AM som-snytt ***@***.***> wrote: FWIW, this issue feels like a recent duplicate, but I couldn't find where I saw it. The forward-reference style of import from companion is common; the idiom always feels strange to me; maybe it's worth advertising its fragility, perhaps in the error message. — You are receiving this because you are subscribed to this thread. Reply to this email directly, view it on GitHub <#10808 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAYAUEA7TNEKVR3YMFUMPTSVCSKRANCNFSM4U44JJVA> .