scala/scala

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econoraptor

@econoraptor

ok, this article has convinced me that reasoning about runtime types is difficult

Rob Norris

@tpolecat

It’s easy if you pretend it’s 100% impossible!

Matthew Rooney

@javax-swing

@Woody88 the issue you were facing is that for a Monad ap was being defined in terms of fmap and fmap was being defined in terms of ap. by overriding the definition of fmap in your Monad trait to be defined in terms of flatMap it fixes the issue

     def fmap[B](f : A => B) :M[B] = ma.flatMap(a => pure(f(a)))

Justin Reeves

@justinallenreeves

I guess the time to revive my old ("gamer") discord profile has come.

@BalmungSan
If you have gmail or protonmail, you can use a alias when you signup for an account ${user}@gmail.com => ${user}+${alias}@gmail.com
The aliases still get sent to you, but they have alias in the TO field so you can properly filter via domain
Some thing I wish I knew about 20 years ago for setting up spam filters.

Woodson Delhia

@Woody88

@javax-swing Thank you. It works. I just want to have a better understanding so because Monad trait was defining ap using fmap it causes the loop. I guess that is because Monad trait is reference to the fmap within Applicative. However, why does @main def main = println(List(1).fmap(_ * 2)) refer to the Monad version of instead of directly using the Functor implementation?

Luis Miguel Mejía Suárez

@BalmungSan

@justinallenreeves :+1:

@Woody88 note that you also shouldn't provide the three givens.

You should just provide the instance of the most specific type (in your example Monad)

Woodson Delhia

@Woody88

@BalmungSan Hmm... So, the instance that will always be pass will be the monad instance? What if I have a generalized function that just needs a Functor constraint, how will the compiler find that instance?

For example:

def someGeneralizedFunc [F[_]: Functor](fa: F[String]): F[Int] = fa.fmap(x => 3)

someGeneralizedFunc(List("yo"))  // I don't think this will work with just a Monad instance for List

Luis Miguel Mejía Suárez

@BalmungSan

@Woody88 yes it will find it.

From a theoretical point of view, any type that forms a Monad can form a Functor

Because you can always provide a lawful implementation of map in terms of flatMap and pure

However, in Scala is even simpler than that. Monad[F] extends Functor[F], as such, it is a subtype.

And due Liskov we can always pass a subtype.

Woodson Delhia

@Woody88

@BalmungSan I get that but it's because you Monad directly extends Functor. I have Monad only extends Applicative, and Applicatve extends Functor. I just gave it try and it doesn't work unless I have Monad extends Applicative with Functor.

3 replies

However, isn't it repetitive since Applicative already extends Functor?

Colin Fraizer

@cfraizer

Where can one download the 3.0 API documentation as a tar/zip file?

Swoorup Joshi

@Swoorup

does scala have a dbg! macro as in rust?

heksesang

@heksenlied:matrix.org

[m]

What does this mean when I try to call map on a tuple?

[error]    |                         Found:    Object with PolyFunction {...}
[error]    |                         Required: PolyFunction{apply: [t](x$1: t): Any}

1 reply

heksesang

@heksenlied:matrix.org

[m]

So this kind of error doesn't have a specific meaning? Kinda makes poly functions a pain to use if there is no concise way to interpret compile errors related to them. :S

            summonForT[A].map(
              [r] =>
                (reader: r) =>
                  inline reader match {
                    case reader: ConfigReader[t] =>
                      val key = fieldMapping("field")
                      reader.isInstanceOf[ReadsMissingKeys] match {
                        case true => reader.from(objCur.atKeyOrUndefined(key))
                        case false => objCur.atKey(key).flatMap(reader.from(_))
                      }
                }
            )

where summonForT is an inline method which returns a Tuple.Map[m.MirroredElemTypes, ConfigReader]

heksesang

@heksenlied:matrix.org

[m]

But if there is no good way to interpret the errors, I think it might be better to just not use poly functions, and rather find other ways to achieve my goals.

1 reply

Fabio Epifani

@epifab

I can't figure out why this code wouldn't compile:

case class Wrapper[A <: Singleton](a: A)(using ValueOf[A])

def test[A <: Singleton](blabla: Wrapper[A]): Unit = ()

test(Wrapper("yo"))

and fail with a:

No singleton value available for A
where: A is a type variable with constraint >: ("yo" : String) and <: Singleton

if I extract Wrapper("yo") then it works:

case class Wrapper[A <: Singleton](a: A)(using ValueOf[A])

def test[A <: Singleton](blabla: Wrapper[A]): Unit = ()

val value = Wrapper("yo")
test(value)

is this a bug in Scala 3?

Michał Pałka

@prolativ

@epifab this seems ok. When you call test(value) then the most exact type of value is value.type which is a singleton type but the type of Wrapper("yo") would be Wrapper["yo"] which is not a singleton type by itself

Vojtech Letal

@letalvoj

Hi :wave: I have a question about quotes and splices. In scala2 I was able to create an Expr[CC] for a case class CC(...) which was equivalent to instantiating the class CC.apply(arguments:_*) as

 val symbol = weakTypeOf[CC].typeSymbol
 val constructor = q"reify(${c.mirror.staticModule(symbol.fullName)}).tree"
 q"Apply($constructor, List(..$arguments))"

The thing which I am struggling with is that in scala2 I could interpolate instances of Tree into a quasiquote and reify and use quotes without getting staging level errors.

Please, anyone has a quick hint on how to replicate the three lines of scala2 in scala3? 🙏🏻

Fabio Epifani

@epifab

@prolativ no I don't believe I'm requiring Wrapper to be a singleton, only its type parameter

Michał Pałka

@prolativ

@epifab that's the point so it makes sense that this doesn't compile

Fabio Epifani

@epifab

@prolativ it doesn't make sense to me yet, sorry. why would it fail saying that it can't find a singleton for "yo": String which is, obviously, a singleton?

heksesang

@heksenlied:matrix.org

[m]

test["yo"](Wrapper("yo")) works.

The problem is that the compiler infers the type of "yo" to be String, not "yo".

Fabio Epifani

@epifab

but that's impossible because I'm explicitly declaring a type bound for A <: Singleton

@heksenlied:matrix.org also the error message suggests that the compiler knows that A is indeed a singleton

Michał Pałka

@prolativ

@epifab sorry, I misread

def test[A <: Singleton](blabla: Wrapper[A])

def test[A <: Singleton](blabla: A)

Fabio Epifani

@epifab

yes I thought so :)

heksesang

@heksenlied:matrix.org

[m]

@prolativ: Thanks! I will try to see if I do something similar to what you did, if I can extract a more exact error. 🙂

@epifab: Unsure why, but it obviously doesn't interpret the type as "yo" (as manually specifying that type makes it work). But I do mean to recall there has been such a problem mentioned somewhere (some issue on the tracker?) where one had to store things in a val and pass that val to the method for it to type it correctly.

Fabio Epifani

@epifab

@heksenlied:matrix.org yeah, something definitely wrong there. I opened a bug lampepfl/dotty#12583

heksesang

@heksenlied:matrix.org

[m]

The problem that happens is that A in Wrapper is somehow being "altered" by test, which makes summoning ValueOf[A] fails. What seems to also work is moving the using-clause to test, so it obviously is a case of the test-method making the parameter less specific.

Michał Pałka

@prolativ

test(Wrapper("yo")) works if you redefine Wrapper as

case class Wrapper[A <: Singleton](a: A)

so it looks like using ValueOf[A] is the problem here, not Singleton itself

heksesang

@heksenlied:matrix.org

[m]

Yeah, the type of A within test doesn't resolve to "yo", I guess it resolves to String & Singleton instead.

Which means that, yes, it's a singleton, but there is no way to summon a ValueOf for it.

Michał Pałka

@prolativ

This works

case class Wrapper[A <: Singleton](a: A)(using ValueOf[a.type])

def test[A <: Singleton](blabla: Wrapper[A]): Unit = ()

test(Wrapper("yo"))

Fabio Epifani

@epifab

yeah, something is wrong with the type inference. I also just tested it in Scala 2, and this is a regression

it's interesting that this last example works. I think it defeats my understanding of Singleton

Vojtech Letal

@letalvoj

Please, anyone has a quick hint on how to replicate the three lines of scala2 in scala3? 🙏🏻

I will try to give a more specific example. I would like to have an Expr[CC] which is equivalent to instantiating a new instance of CC, yet generic.

    case class CC(i:Int)
    val expected = '{ new CC(1) }

Attempts to generalize that:

    val ts = TypeRepr.of[CC] // or Type[CC] ...
    val attempt1 = '{new ts(1)} // something as simple as this fails
    val attempt2 = Apply(
      Select.unique(New(Ident(TermRef(ts,"???"))),"<init>"),
      List(Literal(IntConstant(1)))
    ).asExprOf[CC] // I have no idea how to instantiate this

Pls, any chance I can create a quote equivalent to instantiating a new class, given I have

the necessary constructor parameters (List[Expr[_]])
type of the class (Type[CC])
? 🙏🏻

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