typelevel/kittens

Automatic type class derivation for cats | Currently 1.1.1 | Code of conduct http://typelevel.org/conduct.html

Georgi Krastev

@joroKr21

@rzeigler this works:

  import cats.Show
  import cats.instances.all._
  import shapeless.Typeable

  case class Foo[I[_]](a: String, i: I[String])
  object Foo {
    implicit def show[I[_]](implicit show: Show[I[String]], typ: Typeable[I[String]]): Show[Foo[I]] =
      semiauto.show[Foo[I]]
  }

Ryan Zeigler

@rzeigler

nifty

thanks

Lena Brüder

@lenalebt

Hey together, while looking for replacements for a lib that does "equals ignoring some fields" I was looking into cats Eq, which pointed to kittens. Is it easily doable to derive an Eq instance that takes care of all but one field? In my specific use-case, it's a very large case class with about 25 fields, and I'd like to avoid the error-prone boilerplate... I need to compare all but the database ID :D

Miles Sabin

@milessabin

It's doable, but if you can I'd recommend wrapping your db id in a newtype with the desired (universal) equality semantics.

Lena Brüder

@lenalebt

Oh, thank you - I think I can work something out that applies for the case at hand. Thank you!

Paul Snively

@paul-snively

So this seems like something Kittens can likely help with.

Let's say I have an HList of values of some typeclass that forms a Monad, so it has some means of combining or sequencing. In my concrete case, it's Atto Parsers.

Atto Parsers can be sequenced with ~, for example, to yield a Parser that yields a pair of the types of the combined Parsers.

Is it possible to type-safely, say, "reduce" an HList of Parser[T] to get a single Parser with (an unfortunately horribly nested) Tuple?

Georgi Krastev

@joroKr21

You can traverse with a Poly or sequence

That's in the cats.sequence package

But you will not get a tuple

You will get Parser[HList]

You can convert it to a tuple easily though

  test("sequencing Either")(check {
    forAll { (x: Either[String, Int], y: Either[String, String], z: Either[String, Float]) =>
      val expected = (x, y, z) mapN (_ :: _ :: _ :: HNil)
      (x :: y :: z :: HNil).sequence == expected
    }
  })

Paul Snively

@paul-snively

Right. I need to combine elements with ~, and the result should be a Parser[T], where T is some nested Tuple because ~ gives you a Parser of a Pair.

So, reduce.

Georgi Krastev

@joroKr21

x ~ y should be just (x, y).tupled or am I missing something?

Paul Snively

@paul-snively

@joroKr21: I think it's an alias for flatMap. It sequences two Atto Parsers.

@joroKr21: Parser[A] ~ Parser[B] => Parser[(A, B)].

Georgi Krastev

@joroKr21

That doesn't look like flatMap

https://typelevel.org/cats/api/cats/Semigroupal.html

Anyway you can use hlistOfParsers.sequence.map(_.tupled)

Provided there is Applicative[Parser]

Paul Snively

@paul-snively

  implicit class Command[P <: HList : IsHCons : *->*[Parser]#λ](parsers: P) {
    def command[InF, InL <: HList, R, O <: Product, OL <: HList](f: InF)
      (implicit
        fntop: FnToProduct.Aux[InF, InL => R],
        pgood: Comapped.Aux[P, Parser, InL]
      ): String => Either[Throwable, R] = { s =>
        val tough  = parsers.sequence
        val parser = tough.map(flatten).map(_.productElements)
        val args   = parser.parseOnly(s).either.leftMap(s => new RuntimeException(s))
        args.map(a => fntop(f)(a))
      }
  }

This complains that it can't construct a Sequencer.

"!I seq (cannot construct sequencer, make sure that every item of your hlist P is an Apply): Sequencer[P]"

Georgi Krastev

@joroKr21

So do you have an Apply[Parser] in scope?

Paul Snively

@paul-snively

Well, every element is a Parser; every Parser is an Applicative, and every Applicative is an Apply.

Georgi Krastev

@joroKr21

But perhaps the issue is in the generics here

Paul Snively

@paul-snively

Hmm. Maybe I need another import?

Georgi Krastev

@joroKr21

I've never tried to use sequence in a generic context

Paul Snively

@paul-snively

Right. I'm trying to 1) ensure my HList is a stable value, 2) give whatever evidence I need.

Georgi Krastev

@joroKr21

I guess you just need an implicit Sequencer[P] { type F[x] = Parser[x] }

Ah wait

Sequencer.Aux[P, Parser, InL]

Paul Snively

@paul-snively

That goes right back to not finding a Sequencer...

Georgi Krastev

@joroKr21

Can you put it in scastie?

Paul Snively

@paul-snively

Oh wait... I added it to the implicit stanza and I think it worked!

Georgi Krastev

@joroKr21

Yes, that's what I meant you need it as implicit

Paul Snively

@paul-snively

Right, of course. Thanks!

Georgi Krastev

@joroKr21

:thumbsup:

Simão Martins

@Lasering

Hi!
In the following code:

import cats.Show
import cats.derived
import com.comcast.ip4s.{Cidr, IpAddress}

case class Route[+IP <: IpAddress](destination: Cidr[IP], nexthop: IP)
object Route {
  implicit def show[IP <: IpAddress]: Show[Route[IP]] = {
    implicit val ipShow: Show[IP] = IpAddress.show
    implicit val cidrShow: Show[Cidr[IP]] = Cidr.show
    derived.semiauto.show
  }
}

I'm getting the following error:

[error] /home/simon/DSI/openstack/scala-neutron-client/src/main/scala/pt/tecnico/dsi/openstack/neutron/models/Route.scala:30:22: Could not derive an instance of Show[A] where A = pt.tecnico.dsi.openstack.neutron.models.Route[IP].
[error] Make sure that A has a Typeable instance and satisfies one of the following conditions:
[error]   * it is a case class where all fields have a Show instance
[error]   * it is a sealed trait where all subclasses have a Show instance
[error]     derived.semiauto.show
[error]                      ^

Which makes no sense, is this a bug, or am I missing something?

Paul Snively

@paul-snively

I wonder whether not using the IP type variable would help?

Simão Martins

@Lasering

typelevel/kittens#267 seems the problem is caused by the lack of an instance of Typeable[Cidr[IP]], I'm trying to implement one

Paul Snively

@paul-snively

:+1:

Georgi Krastev

@joroKr21

Kittens 2.2.2 with Scala Native support is out: https://github.com/typelevel/kittens/releases/tag/v2.2.2 :tada:

Miles Sabin

@milessabin

:tada:

Georgi Krastev

@joroKr21

Kittens 2.3.0 released: https://github.com/typelevel/kittens/releases/tag/v2.3.0 :tada:

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