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##### Activity
Alexander Konovalov
@alexknvl
I doubt it.
considering there is no Forall
Jakub Kozłowski
@kubukoz
ohh
so with that in mind
implicit def forListOfA[A](implicit A: Eq[A]): Eq[List[A]] = ???
//here!
implicitly[implicit Eq[Int] => Eq[List[Int]]]
would it be the first or the second line that needed e.g. forall?
to me it looks like valid scala
it might even work if A: Eq[A] in the first line isn't marked implicit
aaaand the implicit keyword disappears in the second line
but that's just an implicit view then
Rob Norris
@tpolecat
@kubukoz you might look at how these are defined in matryoshka
Jakub Kozłowski
@kubukoz

so my reasoning for this working in dotty is that:
given

implicit def forList[A](A: Eq[A]): Eq[List[A]] = ???
implicitly[Eq[Int] => Eq[List[Int]]]

compiles in scala, then

implicit def forList[A]: implicit Eq[A] => Eq[List[A]] = ???
implicitly[implicit Eq[Int] => Eq[List[Int]]]

should also compile

@tpolecat I'll take a look (later), thanks
EqualT?
I was thinking about sth like trait EqK[F[_]] { def eqv[A](x: F[A], y: F[A]): Boolean } before
Jakub Kozłowski
@kubukoz
you know what, this could actually work... an EqK[F[_]], Eq[A] => Eq[F[A]] derivation would be trivial
or EqK could just be a type alias: type EqK[F[_]] = Eq ~> λ[α => Eq[F[α]]]
with a derivation of Eq: implicit def eqKToEq[F[_] : EqK, A : Eq]: Eq[F[A]] = implicitly[EqK[F]].apply(Eq[A])
and instead of defining an Eq[List[A]] we'd define EqK[List], and the rest would be done by the conversion
massive incompatibilities though... without breaking the best we could do is probably define EqK as a separate typeclass, and duplicate the instances for Option, List etc. (I guess)
WDYT?
Jakub Kozłowski
@kubukoz
still confused why Eq[Cofree[List, Int]] breaks and Eq[Cofree[Option, Int]]doesn't
ah, I know. It would, I was just checking in CofreeSuite where it's defined explicitly :joy:
Jakub Kozłowski
@kubukoz
oh, and it looks like we'd need the same for Order and Show...
Mateusz Górski
@goral09
Hi, I am confused about use case for MVar and Ref. Looking at their interface they both provide methods for get: F[A] and set/put(a: A): F[Unit]. Can't MVar in place of Ref? I understand that MVar can be used as a communication channel ( blocking on get and put ) which is not possible with Ref.
Derek Williams
@derekjw
@goral09 A Ref wont block on get, but an MVar might if no value is available. So an MVar cannot be used in place of a Ref if a value is always expected.
Fabio Labella
@SystemFw
@goral09 a very, very important difference is that modify on Ref is atomic
just to give a bit of perspective, Ref and Deferred are coming from fs2, MVar is coming from Monix (and of course from Haskell, although in Haskell you also have STM)
when designing the new concurrency scheme in fs2, I made a decision to not go with MVar, separating the functionality into Ref and Deferred instead

Can't MVar in place of Ref?

Anyway if you do set/put, it will break in the presence of concurrency

so use Ref.modify
a Ref is also lighter weight
Fabio Labella
@SystemFw
(I guess I should specify, take/put on MVar for modification does work, but you end up having to worry about the state of the MVar, whether empty or full, and are at risk of deadlock)
Jakub Kozłowski
@kubukoz
what's MVar useful for, then? (in Scala)
if there are so many drawbacks
Fabio Labella
@SystemFw
well, I'm biased obviously since I consciously used to not go there. I think when dealing with synchronisation it might give slightly more control than Ref + Deferred, however at the price of significantly increased complexity as a design tool
for the record, in Haskell the recommended choice is STM unless you need fairness, which MVar gives you
Derek Williams
@derekjw
it's like a better SynchronousQueue
Jakub Kozłowski
@kubukoz
gotcha
@LukaJCB/@maintainers if you have a moment to look at my rant about Eq/diverging implicits (https://gitter.im/typelevel/cats?at=5b31861bce3b0f268d4311fa), please let me know if the idea to add a higher kinded Eq variant sounds appealing, I could come up with a proposal
Mateusz Górski
@goral09
Thanks @SystemFw and @derekjw
Ionuț G. Stan
@igstan
As far as I see, Ref + Deferred seems to be write-once, whereas MVar supports multiple writes (always alternated with reads, which makes it a single-element queue effectively).
Fabio Labella
@SystemFw
@igstan nope
Deferred is write-once
but with Ref + Deferred you can implement structures that aren't
e.g. Semaphore and Queue in fs2 are implemented with Ref + Deferred (used to be called Promise)
Mateusz Górski
@goral09
you could put new Deferred instances inside Ref right?
Fabio Labella
@SystemFw
that's exactly what you do
Mateusz Górski
@goral09
I will look at fs2's Queue implementation then
Fabio Labella
@SystemFw
the insight behind the design is this: separating synchronisation from concurrent state