Use `Nothing` for no `Result` #52

armanbilge · 2021-11-19T05:19:05Z

Based on #51.

@bpholt Before cherry-picking 00eb18f I wanted to try and write a test in 06c9eee that demonstrates the compiler issue with Nothing. I tried replicating your example from #45 (comment) but didn't have much luck. When you have chance would you mind poking at this? Thanks :)

bpholt · 2021-11-19T17:03:48Z

This is the best minimization I've found so far:

class LambdaSuite {
  def lambda[F[_]]: Resource[F, Lambda[F, Unit, Nothing]] = ???

  def middleware[F[_], Event, Result, Env](lambda: Resource[F, Lambda[Kleisli[F, Env, *], Event, Result]]): Resource[F, Lambda[F, Event, Result]] = ???

  implicit val nothing: Encoder[Nothing] = _ => ???

  class NothingLambda extends IOLambda[Unit, Nothing] {
    def handler =
      middleware(lambda[Kleisli[IO, Unit, *]].mapK(Kleisli.applyK(())))
  }
}

[error] …/feral/lambda/shared/src/test/scala/feral/lambda/LambdaSuite.scala:35:14: type mismatch;
[error]  found   : cats.effect.kernel.Resource[[+A]cats.effect.IO[A],feral.lambda.Lambda[[γ$1$]cats.data.Kleisli[cats.effect.IO,Unit,γ$1$],Unit,Nothing]]
[error]     (which expands to)  cats.effect.kernel.Resource[[+A]cats.effect.IO[A],(Unit, feral.lambda.Context[[γ$1$]cats.data.Kleisli[cats.effect.IO,Unit,γ$1$]]) => cats.data.Kleisli[cats.effect.IO,Unit,Option[Nothing]]]
[error]  required: cats.effect.kernel.Resource[[+A]cats.effect.IO[A],feral.lambda.Lambda[[γ$0$]cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,γ$0$],Unit,Result]]
[error]     (which expands to)  cats.effect.kernel.Resource[[+A]cats.effect.IO[A],(Unit, feral.lambda.Context[[γ$0$]cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,γ$0$]]) => cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,Option[Result]]]
[error]         .mapK(Kleisli.applyK(())))
[error]              ^
[error] one error found

It doesn't matter if the value passed to middleware is a Resource in F[_] or Kleisli[F, E, *]. If it's the latter, then the mapK in the NothingLambda goes away, but AFAICT it doesn't change the inference failure:

[error] …/feral/lambda/shared/src/test/scala/feral/lambda/LambdaSuite.scala:33:24: type mismatch;
[error]  found   : cats.effect.kernel.Resource[[γ$2$]cats.data.Kleisli[cats.effect.IO,Unit,γ$2$],feral.lambda.Lambda[[γ$2$]cats.data.Kleisli[cats.effect.IO,Unit,γ$2$],Unit,Nothing]]
[error]     (which expands to)  cats.effect.kernel.Resource[[γ$2$]cats.data.Kleisli[cats.effect.IO,Unit,γ$2$],(Unit, feral.lambda.Context[[γ$2$]cats.data.Kleisli[cats.effect.IO,Unit,γ$2$]]) => cats.data.Kleisli[cats.effect.IO,Unit,Option[Nothing]]]
[error]  required: cats.effect.kernel.Resource[[γ$1$]cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,γ$1$],feral.lambda.Lambda[[γ$0$]cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,γ$0$],Unit,Result]]
[error]     (which expands to)  cats.effect.kernel.Resource[[γ$1$]cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,γ$1$],(Unit, feral.lambda.Context[[γ$0$]cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,γ$0$]]) => cats.data.Kleisli[[+A]cats.effect.IO[A],Unit,Option[Result]]]
[error]       middleware(lambda[Kleisli[IO, Unit, *]])
[error]                        ^
[error] one error found

Co-authored-by: Brian Holt <bholt@dwolla.com>

armanbilge · 2021-11-19T17:49:03Z

@bpholt thanks! That's an excellent minimization, exactly what I was looking for :)

Unfortunately, placing the INothing encoder in the companion object for IOLambda doesn't seem to bring it in scope for whatever reason. I'm going to futz with this some more.

armanbilge · 2021-11-19T18:30:58Z

Darn, 2.12 seems to be lacking. Dare I say ... we drop it? 😆

bpholt · 2021-11-19T19:03:05Z

Dropping 2.12 seems reasonable to me. I don't think the common reasons why people are stuck on 2.12 would apply in this context.

armanbilge · 2021-11-19T19:14:10Z

Yeah, apparently we are "future-forward" or something 😆

If it becomes an issue we can probably bring back 2.12 very easily anyway. Those users would just need to explicitly import this implicit and we'd need to make this test 2.13/3.x-only. FTR if sbt supported this config out-of-the-box that's exactly what I would have done.

armanbilge · 2021-11-19T19:55:23Z

Ummm ... Scala 3 is broken too? Seems like a compiler bug 🤔

[error] -- Error: /workspace/feral/lambda/shared/src/test/scala/feral/lambda/LambdaSuite.scala:35:54 
[error] 35 |  class NothingLambda extends IOLambda[Unit, INothing] {
[error]    |                                                      ^
[error]    |ambiguous implicit arguments: both feral.lambda.nothingEncoder and feral.lambda.nothingEncoder match type io.circe.Encoder[feral.lambda.INothing] of parameter encoder of constructor IOLambda in class IOLambda

Update: tracking in scala/scala3#13981.
Update 2: it was a combination of weird incremental compiling issues, and the fact that implicit scopes have changed in Scala 3. So, this needed difference sources as I suspected.

kubukoz · 2021-12-09T00:07:48Z

tl;dr but I like the idea of Nothing in the result :D

armanbilge · 2021-12-09T00:42:44Z

@kubukoz you're not stuck on Scala 2.12 are you? The tl;dr here is mostly about implicit scope for Nothing not working on 2.12.

kubukoz · 2021-12-09T00:49:00Z

I'm on 2.13, Nothing was giving me a missing Decoder instance error.

armanbilge · 2021-12-09T00:51:30Z

Try INothing.

kubukoz · 2021-12-09T00:54:55Z

man, I can only have one experimental branch in my dependencies at a time 😂

kubukoz · 2021-12-09T00:55:57Z

lambda/shared/src/main/scala-3/feral/lambda/INothing.scala

+   * userland code returns `Some`.
+   */
+  @nowarn("msg=dead code following this construct")
+  implicit val nothingEncoder: Encoder[INothing] = (_: INothing) => ???


not 100% sure but can't this just return its argument? (a: INothing) => a. INothing <: Nothing and Nothing <: Json.

Good idea, this does work :)

armanbilge · 2021-12-09T00:56:31Z

Actually you should have this in your branch, it's split off :)

kubukoz · 2021-12-09T00:59:27Z

we're reaching levels of stable that shouldn't be possible!

kubukoz · 2021-12-13T02:52:36Z

lambda/shared/src/test/scala/feral/lambda/LambdaSuite.scala

+  def middleware[F[_], Event, Result](lambda: Lambda[Kleisli[F, Unit, *], Event, Result])
+      : Resource[F, Lambda[F, Event, Result]] = ???
+
+  class NothingLambda extends IOLambda[Unit, INothing] {


if I'm not mistaken this should even work with just Nothing, at least on 2.13

kubukoz · 2021-12-13T02:52:40Z

This seems mergeable as well...

armanbilge · 2021-12-13T02:54:24Z

Yep, I'm just dealing with the conflicts rn :)

Try to write a regression test

06c9eee

armanbilge marked this pull request as draft November 19, 2021 05:44

armanbilge mentioned this pull request Nov 19, 2021

Tracing doodles #50

Closed

armanbilge and others added 2 commits November 19, 2021 17:39

Add (broken) test showing Nothing inference issue

3c17409

Co-authored-by: Brian Holt <bholt@dwolla.com>

the CloudFormationCustomResource lambda should output nothing

1942aed

Fix INothing encoder implicit scope

6608970

armanbilge added 2 commits November 19, 2021 19:40

Drop 2.12, sorry not sorry

4d1bf88

Merge branch 'main' into topic/nothing

c149ceb

Seperate Scala 2/3 INothing

744013e

kubukoz reviewed Dec 9, 2021

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armanbilge mentioned this pull request Dec 9, 2021

MVP / 0.1.0 #69

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6 tasks

armanbilge marked this pull request as ready for review December 13, 2021 02:39

kubukoz reviewed Dec 13, 2021

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Merge branch 'main' into topic/nothing

454e753

armanbilge mentioned this pull request Dec 13, 2021

Add a regression test for INothing implicit resolution #73

Open

armanbilge merged commit 7b494d3 into typelevel:main Dec 13, 2021

armanbilge added the lambda label Jan 14, 2022

armanbilge mentioned this pull request Apr 4, 2022

Deprecate/remove INothing? #191

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Use `Nothing` for no `Result` #52

Use `Nothing` for no `Result` #52

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Use Nothing for no Result #52

Use Nothing for no Result #52

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Use `Nothing` for no `Result` #52

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