typelevel/cats

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Billzabob

@Billzabob

@SystemFw Interesting how often things seem to tie together like that.

Fabio Labella

@SystemFw

yet another way to look at Free (especially evident in the first version) is as a tree with a Functorful f of children, and values at the leaves (the Pure constructor is Leaf a, and Roll is the branch, with f children). If you look at Cofree, that's also a Tree with values at the branches instead, and again whose structure is determined by f, and that's a Rose Tree

data Free f a = Leaf a | Node f (Free f a)
data Cofree f a = Node a (f (Cofree f a))

type Rose = Cofree []
-- replace and you get
data Rose a = Node a [Rose a]

Igor Postanogov

@ipostanogov

Hi!

If I have Map[String, Int] & f: Int => ValidatedNec[String, Bool], I can get Map[String, ValidatedNec[String, Bool]] via mapValues.
Is there any way to get ValidatedNec[String, Map[String, Bool]] from it?

Oleg Pyzhcov

@oleg-py

import alleycats.std.map._ and then traverse?

Igor Postanogov

@ipostanogov

@oleg-py, it works, thanks!

Soren

@srnb_gitlab

How do I dropWhile a List[F[T]] based on F[T] => F[Boolean]?

A solution using sequencing is not what I want because I don't want to evaluate the F[T]s later in the list

Gavin Bisesi

@Daenyth

@srnb_gitlab I recommend fs2

Oleg Pyzhcov

@oleg-py

or manual recursion

Gavin Bisesi

@Daenyth

val lst: List[F[T]] = ???
fs2.Stream.emits(lst)  // Stream[Pure, F[T]]
  .evalMap(identity)   // Stream[F, T]
  .evalMap(t => cond(t).map(b => (t, b))) // Stream[F, (T, B)]
  .dropWhile(tb => tb._2)
  .map(_._1) // Stream[F, T]

Oleg Pyzhcov

@oleg-py

def dropWhileM[F[_]: Monad, A](list: List[F[A]])(fn: F[A] => F[Boolean]): F[List[F[A]]] = list match {
  case fa :: fas => fn(fa).ifM( dropWhileM(fas)(fn), list.pure[F] )
  case _ => list.pure[F]
}

wonder why you'd want F[A] => F[Boolean] instead of A => F[Boolean] tho

Luis Martinez

@luis3m

or even A => Boolean I'd say

Oleg Pyzhcov

@oleg-py

Well, that's quite common. "filename => file.exists" needs to go through an effect type

Luis Martinez

@luis3m

right

Oleg Pyzhcov

@oleg-py

Hmm...

def dropWhileM[F[_]: Monad, A](list: List[F[A]])(fn: F[A] => F[Boolean]): F[List[F[A]]] = {
  list.tails.toStream
    .findM { _.headOption.existsM(fn).map(!_) }
    .map(_.orEmpty)
}

Gavin Bisesi

@Daenyth

needs moar tailRecM

Ivano Pagano

@ivanopagano

any way to generically build a Traversable instance from a collection ?

e.g. I have a List[A] and would like to build a T[A] where there's a Traversable[T]

this is some form of CanBuildFrom, semantically, yet I would rather avoid doing it procedurally

but I guess I need to be able to "unfold" rather than "fold" as in traverse

owww, sorry, I meant Traverse[T] of course

Gavin Bisesi

@Daenyth

I'm not sure there's a typeclass describing unfold

Ivano Pagano

@ivanopagano

I guess not, indeed

I guess this means that I may stop searching

Rob Norris

@tpolecat

List is already traversable … what is your intent?

Gavin Bisesi

@Daenyth

@tpolecat I think he wants def unfoldWhatever[F[_]: Foldable, G[_]: ?Unfoldable?, A](as: F[A]): G[A] ( @ivanopagano is that right)

something something recursion schemes?

Greg Pfeil

@sellout

👀

Rob Norris

@tpolecat

Haha

We don’t have such a thing but you can do it with Foldable and MonoidK

JsonFn

@JsonFn

When trying Applicative example e.g. Applicative[List].product(List(3), List(4)).flatten It throws No implicit view available from (Int, Int) => scala.collection.GenTraversableOnce[B]. How can I create GenTraversableOnce? The only way is to create a new such instance? for instance new GenTraversableOnce { ... }

Fabio Labella

@SystemFw

that's a red herring

Rob Norris

@tpolecat

What is your intent? The product is a List of pairs of Int.

Fabio Labella

@SystemFw

there no implicit views needed in cats, you must be getting flatten from collections

Rob Norris

@tpolecat

If you want a list of Int you can flatMap the pairs to lists.

Fabio Labella

@SystemFw

random info: Applicative cannot flatten, you need monad. you can use syntax and do (List(3), List(4)).product

or mapN and many other things

JsonFn

@JsonFn

@SystemFw no wonder. my fault. thanks for the advice.

Soren

@srnb_gitlab

wonder why you'd want F[A] => F[Boolean] instead of A => F[Boolean] tho

@oleg-py I'm checking the state of whether creating a Resource[F, ?] failed or not

So I want to use dropWhileF(_.attemptT.isLeft)

then take the head on it

Jakub Kozłowski

@kubukoz

We don’t have such a thing but you can do it with Foldable and MonoidK

MonoidK or Alternative?

Soren

@srnb_gitlab

@Daenyth On the usage of fs2, how should I get the head from the stream? Is calling compile in the middle of my program ok?

Jakub Kozłowski

@kubukoz

it's fine (stream.head.compile.lastOrError should do it)

Gavin Bisesi

@Daenyth

Sure. You can .head.compile.last and get F[Option[A]]

Jakub Kozłowski

@kubukoz

or that yeah

Gavin Bisesi

@Daenyth

But if your F[A] is a resource you don't want this

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