http4s/http4s

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Christopher Davenport

@ChristopherDavenport

But that’s just a workaround for implicit position.

Ross A. Baker

@rossabaker

Add an argument to BlazeClientBuilder in F that defaults to F.delay(...) and you'll get a taste of the problem being discussed. Though that can minimize to other toy examples quite readily.

Every time I've wanted to do that I've later refactored away from it. Maybe it's good that we can't.

Fabio Labella

@SystemFw

I think that's enough

def foo[F[_]: Sync](log: String => F[Unit] = (x: String) => F.delay(println(x)): Thing

that's the problem right?

Ross A. Baker

@rossabaker

I wish we had a failsafe way to create an SSLContext, although the only way to do so would be if we could create one that was guaranteed to fail, in which case it's not too different from an Option... or a lazily guarded one. It's all in the message.

Yeah, that's another variant.

Fabio Labella

@SystemFw

you also want named arguments, or is withLog enough?

Ross A. Baker

@rossabaker

Because the Sync[F] is really in a parameter list after the log.

Uh... withLog would be sufficient for this use case.

Christopher Davenport

@ChristopherDavenport

def logMe[F[_]: Sync](s: String, logAction: Option[String => F[Unit]]): F[Unit] = {
    val log = logAction.getOrElse({s => Sync[F].delay(println(s))})
    log(s)
  }

Option variant

Ross A. Baker

@rossabaker

All of our other configs are inspectable before they're built, so I don't really like the laziness/optionality of it.

Fabio Labella

@SystemFw

you could build a bunch of different apis.

thing.withFoo(...).withBar(...)
thing { builder => builder(foo = myFoo, bar) }

Christopher Davenport

@ChristopherDavenport

def logMe[F[_]: Sync] = new logMePartiallyApplied[F]
private class logMePartiallyApplied[F[_]: Sync]{
    def apply(s: String, logAction: String => F[Unit] = {s => Sync[F].delay(println(s))}) = 
      logAction(s)
  }

Partially applied approach.

Ross A. Baker

@rossabaker

I'd like to be able to have a default argument that might fall through to an implementation of last resort, but Java doesn't let us define that for this type.

Fabio Labella

@SystemFw

right, ok

Christopher Davenport

@ChristopherDavenport

The intriguing part to me, is that due to the implicit nature, that partially applied variant is always equivalent.

Fabio Labella

@SystemFw

@ChristopherDavenport I'm not sure the partially applied works, you need a different name

as it stands, logMe[IO]("hello") will think that "hello" is Sync[F]

iirc I did make something similar work at some point, with apply, but it's more horrendous in implementation

like you need a magnet and so on

Christopher Davenport

@ChristopherDavenport

:scream: It gets worse.

Fabio Labella

@SystemFw

 found   : String("hello")
[error]  required: cats.effect.Sync[cats.effect.IO]
[error]   logMe[IO]("hello")

yep

Gavin Bisesi

@Daenyth

needs moar scala3 :p

Fabio Labella

@SystemFw

well, an easy fix for your code is renaming apply to of or build

(fun fact, when I originally suggested HttpRoutes.of I got the name while hacking on this exact problem)

(I mean the name HttpRoutes.of, not the idea, ofc)

Christopher Davenport

@ChristopherDavenport

Right, but what I really want is a rather simple syntactic change, and should be able to be automated somehow.

Fabio Labella

@SystemFw

heh, I'm not sure it's that simple, all things considered

Christopher Davenport

@ChristopherDavenport

Well noA => B => C => D to C => B => A => D should be doable.

Due to the implicit nature that’s the rearrangement, but it is baked into the language semantics that the transformation is not simple.

Fabio Labella

@SystemFw

there was a discussion a few days ago about supporting context bounds in higher-rank functions in scala 3, which was touching on a similar problem

I can see a few issues trying to implement mixing of implicits and not implicits lists. Not saying that is nonsensical or impossible, but it doesn't strike as particularly trivial, especially if you want to allow seamless explicit passing of implicit parameters as well

Fabio Labella

@SystemFw

@ChristopherDavenport if you want to indulge in evil

  def logMe[F[_]]: Magnet[F] = new Magnet[F]

  class Magnet[F[_]]
  object Magnet {
    implicit def conv[F[_]: Sync](m: Magnet[F]): LogMePartiallyApplied[F] =
      new LogMePartiallyApplied[F]

    class LogMePartiallyApplied[F[_]: Sync] {
      def apply(s: String, logAction: String => F[Unit] = { s =>
        Sync[F].delay(println(s))
      }) =
        logAction(s)
    }
  }

  def yolo = logMe[IO]("hello")
  // scala> yolo.unsafeRunSync
  // hello

tbh for a builder it's not that bad

changing names a bit

Christopher Davenport

@ChristopherDavenport

:100:

Luka Jacobowitz

@LukaJCB

How did you even come up with that :smile:

Fabio Labella

@SystemFw

I have this small library called eidos to encode IDs

mostly useless but I've learned a lot about bending scala to my will ;)

some of the stuff I have in a branch is truly crazy

like adding some sane error messages to that thing above

Christopher Davenport

@ChristopherDavenport

Sane error messaging, that seems unreasonable.

This is FP. 234sekjfasdf or it didn’t happen.

Fabio Labella

@SystemFw

with just that code if you do logMe("hello") you get "Magnet[F] does not get parameters"

let me see if I can remember how to fix that

(if it's fixable, lost track)

Christopher Davenport

@ChristopherDavenport

To be fair, logMe is definitely incorrect in the way scala does type identification in the first place.

Fabio Labella

@SystemFw

mm I think in the end I settled for just renaming apply to something else to get implicitNotFound-driven errors to work, can't quite remember anymore

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