Welcome! Got a question? Do you have -Ypartial-unification turned on? Other FAQs: http://typelevel.org/cats/faq.html
considering there is no
Forall
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
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
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?
ah, I know. It would, I was just checking in
CofreeSuite where it's defined explicitly :joy:
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.
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
if there are so many drawbacks
for the record, in Haskell the recommended choice is STM unless you need fairness, which MVar gives you
@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
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)