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Inconsistent `ApplicativeError` behaviour for `OptionT[F[_], A]` / `EitherT[F[_], A, B]` if `F[_]` also has an `ApplicativeError` instance

4

For example, if `F[_]` is `Either[Throwable, *]`, there would exist 2 instances of `ApplicativeError` for `OptionT[Either[Throwable, *], *]`: ```scala ApplicativeError[OptionT[Either[Throwable, *], *], Throwable] // higher priority instance derived from `F[_]`...

yangzai

absorption/lifting typeclass to generalize a few concepts

12

We have FunctorFilter and TraverseFilter. I have in the past proposed a generalization (FunctorFlatten, later AlternativeFlatten: #1337). But it seems to me the above are actually doing something like: ```scala...

johnynek

cats foldable syntax unnecessarity has Foldable implicit in its signature

7

when you import: ``` cats.syntax.foldable._ ``` You got two methods which look very similar in terms in what they do: ``` .foldMap[B](f: A => B)(implicit B: Monoid[B]): B .collectFold[B](f: A...

SimY4

Add a documentation page on error handling

8

There's some major gaps around error handling in the official docs, especially painful because the whole ecosystem relies on them - library users are often instructed to use MonadError/ApplicativeError error...

Daenyth

alleycats - no default instance for Empty[Option[T]]

2

There is no default instance for `Empty[Option[T]]` if T has no Monoid. But there is one for collections. Would it make sense to add an `Empty[Option[T]]` instance that behaves like...

tnielens

add toValidated and co. to TryOps

1

IMO it is cumbersome to use `toEither.toValidated` thus I suggest to provide the same shortcuts we provide for EitherOps. Surely only some sugar but intuitively I am looking for `toValidated`...

robstoll

FreeT is stack-unsafe

2

...somehow. ``` at cats.free.FreeT.$anonfun$step$1(FreeT.scala:163) at scala.Function1.$anonfun$andThen$1(Function1.scala:85) at cats.free.FreeT.$anonfun$step$1(FreeT.scala:163) at scala.Function1.$anonfun$andThen$1(Function1.scala:85) at cats.free.FreeT.$anonfun$step$1(FreeT.scala:163) at scala.Function1.$anonfun$andThen$1(Function1.scala:85) at cats.free.FreeT.$anonfun$step$1(FreeT.scala:163) at scala.Function1.$anonfun$andThen$1(Function1.scala:85) at cats.free.FreeT.$anonfun$step$1(FreeT.scala:163) at scala.Function1.$anonfun$andThen$1(Function1.scala:85) at cats.free.FreeT.$anonfun$step$1(FreeT.scala:163) at scala.Function1.$anonfun$andThen$1(Function1.scala:85) ``` That goes on...

djspiewak

Positive integers and non-empty collections

2

# Positive integers and non-empty collections Hi all. `cats` implements non-empty collections such as `NonEmptyList`. I think we might squeeze a little more out of the concept. Wouldn't it be...

akopich

contramapN should come from syntax.contravariant._

2

When using a la carte imports, I need to `import cats.syntax.apply._` to be able to call `contramapN`, but I expected to `import cats.syntax.contravariant._`. Here's a scastie to demonstrate: https://scastie.scala-lang.org/mrdziuban/IZmraJ4BRlugdmXfeUWoLg/3

mrdziuban

Add `attemptOption` to Alternative

6

```scala def attemptOption[A](fa: F[A]): F[Option[A]] = fa.map(_.some) Option.empty[A].pure[F] ```

LukaJCB