Using CpsMonadConversion with async/await without Direct-syntax?

[benhutchison]

Hi Ruslan,

I noticed [CpsMonadConversion][F[_], G[_]], which apparently permits await on a F[A] within a async[G] block.

However, my attempts to get it to compile have been unsuccessful.

The use case I have is Cats Effect. We usually have IO (probably expressed as F[_]: Async) as our overall effect type, but it's common for my subexpressions to have type [A] =>> Either[Throwable, A] or [A] =>> Either[String, A] (ie pure computation may fail, but dosnt do IO effects).

Sometimes I'd like to "auto-homogenize" these weaker effects by lifting them to IO. We can do with liftTo[F].await but writing that repeatedly gets boring. I was hoping to teach the system how to await on them and auto-lift.

The problem I see seems to related to Direct syntax. I don't and can't use Direct-syntax, because the compiler plugin requirement is impossible in my context. But somehow using CpsMonadConversion wants to pull in Direct syntax typeclass, via CpsMonadContext.

Can they be separated out, so that people using basic explicit async/await can also enjoy benefits of CpsMonadConversion?

[rssh]

Sorry, I can't understand the problem from the description. Perhaps a concrete example can help clarify the point.

CpsMonadConversion does not require DirectSyntax. We use conversion in projects that are compiled without a plugin.

Maybe the problem is a message during implicit search: when CpsMonadConversion is not found, the compiler reports that the implicit value is not found and tells that CpsDirect was looking as a candidate(?).

But it's hard to see something without an example.

[benhutchison]

Yes exactly, I was guided by the compiler error. However the missing implicit in my case is not the Conversion, as I've defined that locally myself. It's the CpsMonadContext implicit. I assumed from the error msg that it was related to, or depends on, Direct syntax. Also noticed there is a field in the Context trait that mentions Direct as well.

…

On Mon, 28 July 2025, 6:30 am Ruslan Shevchenko, ***@***.***> wrote: Sorry, I can't understand the problem from the description. Perhaps a concrete example can help clarify the point. CpsMonadConversion does not require DirectSyntax. We use conversion in projects that are compiled without a plugin. Maybe the problem is a message during implicit search: when CpsMonadConversion is not found, the compiler reports that the implicit value is not found and tells that CpsDirect was looking as a candidate(?). But it's hard to see something without an example. — Reply to this email directly, view it on GitHub <#106 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AAAXJZDLTNG64SOLOC4MF6D3KUZFDAVCNFSM6AAAAACCPGLHAKVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTGOJQGM2TQNA> . You are receiving this because you authored the thread.Message ID: <dotty-cps-async/dotty-cps-async/repo-discussions/106/comments/13903584@ github.com>

[rssh]

CpsMonadContext exists only in async block and inside direct functions.
I.e. most common situation -- await is outside async block.

[benhutchison]

OK, resolved and working :) There were two issues.

Some of the compiler error messages led me in the wrong direction. Combined with my inexperience with Dotty CPS I drew wrong conclusion about what the true problem was (that's common with Dotty CPS for me I find).

Here's the code in question:

  def convertEitherStringToF[F[_]: Async] = new CpsMonadConversion[[A] =>> Either[String, A], F]:
    def apply[A](e: Either[String, A]) = e.leftMap(err => new RuntimeException(err)).liftTo[F]
  given eitherStringToFAsyncGiven[F[_]: Async]: CpsMonadConversion[[A] =>> Either[String, A], F] = convertEitherStringToF

  def convertEitherThrowableToF[F[_]: Async, Ex <: Throwable] = new CpsMonadConversion[[A] =>> Either[Ex, A], F]:
    def apply[A](e: Either[Ex, A]) = e.liftTo[F]
  given eitherThrowableToFAsyncGiven[F[_]: Async, Ex <: Throwable]: CpsMonadConversion[[A] =>> Either[Ex, A], F] = convertEitherThrowableToF

  def lookup[F[_]: Async](asset: Asset, time: Instant): F[BigDecimal] = async[F]:
    val coinId = assetCoindIdMap.get(asset).toRight(s"Asset not supported $asset").await
    val uri = Uri.unsafeFromString(s"https://api.coingecko.com/api/v3/coins/$coinId/history")
      .withQueryParams(Map("date" -> yyMMyyyyFormat.format(time), "localization" -> "false"))

    EmberClientBuilder.default[F].build.use: client =>
      val json = parse(client.expect[String](uri).await).await
      json.hcursor.downField("market_data").downField("current_price").get[BigDecimal]("aud").liftTo[F]
    .await //when this left out, "all hell breaks loose"

1. the final await was missing. My mistake, forgot I need there for some reason. But weirdly the errors that result say things like

[error] value await is not a member of Either[String, String].
[error] An extension method was tried, but could not be fully constructed:
[error]
[error]     cps.await[[B] =>> Either[String, B], T, G](
[error]       stakecrunch.CoinGeckoPriceQuery.assetCoindIdMap.get(asset).toRight[String](
[error]         _root_.scala.StringContext.apply(["Asset not supported ","" : String]*).s(
[error]           [asset : Any]*)
[error]       )
[error]     )(
[error]       cps.CpsMonadContext.monadContext[G](
[error]         cps.CpsDirect.byInclusion[F, G](
[error]           cps.CpsMonadContext.monadContext[F](
[error]             /* missing */summon[cps.CpsDirect.Direct[F]]),
[error]         ???)
[error]       ),
[error]     ???)
[error]
[error]     failed with:
[error]
[error]         No given instance of type cps.CpsMonadContext[G] was found for parameter ctx of method await in package cps.

Which led away from the true cause.

2. second issue was more subtle. But again the errors did not lead me to the problem.

Initially my given instant for listing Either[Throwable, *] had signature given eitherThrowableToFAsyncGiven[F[_]: Async]: CpsMonadConversion[[A] =>> Either[Throwable, A], F] . But CpsMonadConversion is invariant, so it won't type match for an expression Either[RuntimeException, A].

Once I added the Ex <: Throwable type param that problem got fixed.

PS Finally, you'll notice I use non-direct syntax for the Resource block ie EmberClientBuilder.default[F].build.use: client =>. I'm not sure what the direct syntax version would be here. Is there an example in the docs? Anyway, it reads fine how it is.