scala/scala

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Gulshan Singh

@indiagulshan_twitter

https://github.com/makeindiagulshan/newPro/tree/main/src/main/scala

go through with this and make it runnable

johannes karoff

@cornerman

To answer my own question about finding out which member method are abstract (not implemented) in a macro, you can use: TypeTree.of[T].memberMethods.filter(_.flags.is(Flags.Deferred))

Edrick de Guzman

@edrick.deguzman_gitlab

Hello everyone, I would like to ask for some guidance/insight regarding create a jar file/package for my scala test project? I currently don't have a main class since we use test instead of main. Can anyone point me to the right direction?

Luis Miguel Mejía Suárez

@BalmungSan

@edrick.deguzman_gitlab care to elaborate?

Why do you want to create a jar if you don't have a main?

Who is going to be your user?

tgeng

@tgeng

I have a naive question about using type class in scala 3. So I defined a Functor and an Applicative trait in the standard way

trait Functor[F[_]]:
  extension[T] (f: F[T])
    def fmap[S](g: T => S): F[S]

trait Applicative[A[_]] extends Functor[A] :
  def pure[S](s: S): A[S]

  extension[T] (a: A[T])
    infix def <*>[S](f: A[T => S]): A[S]

Then I am trying to add given instance that needs some other applicative:

given[A[_]: Applicative]: Applicative[[T] =>> Foo[A, T]] with
  override def pure[T](t: T): Foo[A, T] = Foo(pure(t))
  // ... rest omitted

where Foo accepts a A[T]. But the above code resolves pure(t) to the pure being defined, rather than the pure available from A[_]: Applicative. How can I provide a hint so resolution goes to the right function?

BTW, I wish I could use map instead of fmap for Functor. But doing that would prevent me from calling the standard map method if I want to provide given instances for, say List. Is there a way to work around this?

Luis Miguel Mejía Suárez

@BalmungSan

@tgeng

given[A[_]](using ev: Applicative[A]): Applicative[[T] =>> Foo[A, T]] with
  override def pure[T](t: T): Foo[A, T] = Foo(ev.pure(t))

BTW, I wish I could use map instead of fmap for Functor. But doing that would prevent me from calling the standard map method if I want to provide given instances for, say List. Is there a way to work around this?

WDYM?

2 replies

Luis Miguel Mejía Suárez

@BalmungSan

@tgeng please don't use threads.

Second, that is not a valid Functor.

At least not one represented using typeclasses.

You need the F parameter.

Ah well no.

Forget about it.

I am not used to extension methods.

tgeng

@tgeng

Thanks @BalmungSan ! I am not sure I follow. Anyway. Let me explain my second question.

Luis Miguel Mejía Suárez

@BalmungSan

No no

You are right.

I am just rusty with Scala 3.

I guess the extension makes it impossible to use the stdlib map

That is probably a bug.

No idea.

tgeng

@tgeng

Right, that's my second question.

Luis Miguel Mejía Suárez

@BalmungSan

Still I don't think people will use inner extensions.

For multiple reasons.

I guess most people will keep using the Scala 2 pattern of having the typeclass and the extensions defined separately.

tgeng

@tgeng

OK, so you mean define fmap just as a non-extension function and then define an extension map calling fmap?

Luis Miguel Mejía Suárez

@BalmungSan

Yup.

Or even just map

(sorry for the late response)

just like cats does as today.

tgeng

@tgeng

Got it! Makes sense! Thank you very much!!

Luis Miguel Mejía Suárez

@BalmungSan

But again, it may be just me not using Scala 3 a lot yet.

Is just that for a couple of things I have seen, that I think that most people will NOT use extension inside the typeclass definition.

tgeng

@tgeng

TBH, I have basically 0 experience with scala 2 so now that I kind of get that extension is a new thing that may not interact well with other language features.

Again, thank you for helping me!

rohith

@rohithbittu33_twitter

hi all, I am getting below error when I am running sbt dependencyBrowseTree. I made sure I am using sbt version=1.5.6 and also added enablePlugins(MiniDependencyTreePlugin) in my build.sbt. Can someone help me to solve this error?
stack trace is suppressed; run last Compile / dependencyBrowseTreeHTML for the full output

ritschwumm

@ritschwumm

@rohithbittu33_twitter what does last Compile / dependencyBrowseTreeHTML say? anything interesting in the stack trace?

Clinton Mead

@clintonmead

What do I need the type of x to be in the following code to be able to refer to x.f in g()? A.B didn't work.

trait A {
    trait B {
        def f: Unit
    }
}

object O {
    def g(x : ?): Unit {
        x.f
    }
}

nova

@nova99:matrix.org

[m]

A#B?

lambook

@lambook

Hello there, i want to write a method to concatenate many Byte arrays
def join(parts : Array[Byte]* ) : Array[Byte] ={...
is there already something to do that in scala ? thnx

Luis Miguel Mejía Suárez

@BalmungSan

@lambook parts.toArray.flatten

@clintonmead Note that your code means that for every instance of A there will be a new different type B so in order to properly reference B you first need an instance of a

Are you sure that is what you want?

Tim Spence

@TimWSpence

I was trying to convince scala 3 to recusrively flatten nested Eithers of some unknown depth but couldn’t figure out how to do it.
I got a match type that describes the expected output type:

case class Inv[T]()

type FlattenR[X] = Inv[X] match
  case Inv[Either[ValidationError, x]] => FlattenR[x]
  case Inv[x]                                                  => Either[ValidationError, x]

but couldn’t figure out a way around erasure to implement it at the value level?

D Cameron Mauch

@DCameronMauch

I can do something like list.sortBy(_.date) where date is java.util.Date, because there is an implicit Ordering[Date] in scope. But where is it?

Luis Miguel Mejía Suárez

@BalmungSan

@DCameronMauch here: https://www.scala-lang.org/api/current/scala/math/Ordering$.html#ordered[A](implicitasComparable:LowPriorityOrderingImplicits.this.AsComparable[A]):scala.math.Ordering[A]

implicit def ordered[A](implicit asComparable: AsComparable[A]): Ordering[A]

And ju.Date extends Comparable[Date]

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