Add `traverseCollect` to `TraverseFilter` typeclass by emilhotkowski · Pull Request #4277 · typelevel/cats

emilhotkowski · 2022-07-30T08:23:45Z

Thank you for contributing to Cats!

This is a kind reminder to run sbt +prePR and commit the changed files, if any, before submitting.

armanbilge

Thanks for the PR, looking good! As next steps, do you think we can add a law, syntax (and test for the syntax)?

emilhotkowski · 2022-07-31T07:01:42Z

I added syntax and test for it, but I am not sure if we need laws. Looking at TraverseFilterLaws, we don't have laws for other functions like sequenceFilter. Should I add some. for traverseCollect anyway?

armanbilge · 2022-07-31T07:42:23Z

Looking at TraverseFilterLaws, we don't have laws for other functions like sequenceFilter. Should I add some. for traverseCollect anyway?

Oh that's interesting. I think laws are good to have, in case an implementation overrides this method, and at the very least to exercise this code. It's also often good to have some tests with a concrete implementation testing it works as expected. Here's a recent PR that is a great example:

Added updated to Traverse #4248

emilhotkowski · 2022-07-31T09:58:49Z

By laws you mean we want to have a test proving that traverseCollect works the same as traverse and collect in two operations?

armanbilge · 2022-07-31T22:37:35Z

The law should verify that the actual implementation of the method matches the default implementation—you can reuse the same code. You might find the commentary in #4248 (comment) helpful :) let me know if you have questions!

emilhotkowski · 2022-08-02T16:47:07Z

I added all necessary stuff (I believe so) :)

johnynek · 2022-08-02T23:49:53Z

thanks for your PR!

armanbilge · 2022-08-02T23:58:55Z

 package cats

 import cats.data.State
+import cats.implicits.toTraverseOps


Can we write this without the ops import? Current practice is try to avoid circular dependencies between syntax and typeclasses.

I tried to do manual sequence for this operation. WDYT?

armanbilge · 2022-08-03T00:18:54Z

   */
  def traverseCollect[G[_], A, B](fa: F[A])(f: PartialFunction[A, G[B]])(implicit G: Applicative[G]): G[F[B]] =
-    traverseFilter(fa)(a => f.lift(a).sequence)
+    traverseFilter(fa)(a => f.lift(a).map(G.map(_)(Option(_))).getOrElse(G.pure(None)))


I think sequence was good, we just can't use the syntax :)

Suggested change

traverseFilter(fa)(a => f.lift(a).map(G.map(_)(Option(_))).getOrElse(G.pure(None)))

traverseFilter(fa)(a => Traverse[Option].sequence(f.lift(a)))

actually, while we are at it:

val optF = f.lift traverseFilter(fa)(a => Traverse[Option].sequence(optF(a)))

so we don't reallocate f.lift on every item a.

@emilhotkowski I think Oscar's suggestion still applies :) it's a good optimization.

Co-authored-by: Arman Bilge <armanbilge@gmail.com>

armanbilge · 2022-08-03T11:51:17Z

+  implicit def toTraverseCollectOps[F[_], G[_], A](fa: F[A]): TraverseCollectOps[F, G, A] =
+    new TraverseCollectOps(fa)


Should this just be TraverseFilterOps? We can add more ops in the future; it's not specific to traverseCollect I think.

I followed current implementations like new SequenceFilterOps(fgoa) I don't know what would be the other solution tbh

Ah the situation for SequenceFilter is slighly different, since it only works for F[G[Option[A]]], not F[A]

cats/core/src/main/scala/cats/syntax/traverseFilter.scala

Line 28 in 0cd6d46

implicit def toSequenceFilterOps[F[_], G[_], A](fgoa: F[G[Option[A]]]): SequenceFilterOps[F, G, A] =

Do you know where I could find examples to follow? I am not sure what I need to change here.

Sure, check out any of the syntax implementations :)
https://github.com/typelevel/cats/tree/main/core/src/main/scala/cats/syntax

I'm just proposing that we rename TraverseCollectOps to TraverseFilterOps so that it matches the name of the typeclass and not the operation.

I understand your point now. I thought it was a remark about implementation details. Thanks for your work on this PR !

Ah, sorry I wasn't so clear on that. Great, thank you too!

…cala Co-authored-by: Arman Bilge <armanbilge@gmail.com>

armanbilge

LGTM, thanks for this!

Emil Hotkowski added 2 commits July 30, 2022 09:34

#4276 - Added traverseCollect to TraverseFilter typeclass

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Formatting

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armanbilge reviewed Jul 30, 2022

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Comment thread core/src/main/scala/cats/TraverseFilter.scala Outdated

armanbilge reviewed Jul 30, 2022

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Comment thread core/src/main/scala/cats/TraverseFilter.scala

Added syntax for traverseCollect

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armanbilge changed the title ~~#4276 - Added traverseCollect to TraverseFilter typeclass~~ Add traverseCollect to TraverseFilter typeclass Jul 31, 2022

armanbilge added the enhancement label Jul 31, 2022

armanbilge linked an issue Jul 31, 2022 that may be closed by this pull request

Add traverseCollect to TraverseFilter typeclass #4276

Closed

armanbilge added this to the 2.9.0 milestone Jul 31, 2022

Emil Hotkowski added 2 commits August 1, 2022 23:47

Added proper example

f73a186

Added reference law

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johnynek requested changes Aug 2, 2022

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Comment thread laws/src/main/scala/cats/laws/TraverseFilterLaws.scala Outdated

Emil Hotkowski added 2 commits August 2, 2022 23:39

Added test for a law

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Formatting

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emilhotkowski requested review from armanbilge and johnynek August 2, 2022 22:11

Fixed unidoc example

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johnynek reviewed Aug 2, 2022

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Comment thread laws/src/main/scala/cats/laws/discipline/TraverseFilterTests.scala Outdated

Comment thread laws/src/main/scala/cats/laws/discipline/TraverseFilterTests.scala Outdated

Emil Hotkowski added 2 commits August 3, 2022 00:59

Removed unnused props

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Added implicit arbitrary

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emilhotkowski requested a review from johnynek August 2, 2022 23:18

formatting

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johnynek previously approved these changes Aug 2, 2022

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armanbilge reviewed Aug 2, 2022

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Remove dependency on ops

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emilhotkowski dismissed johnynek’s stale review via cdc49db August 3, 2022 00:15

armanbilge reviewed Aug 3, 2022

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Update core/src/main/scala/cats/TraverseFilter.scala

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Co-authored-by: Arman Bilge <armanbilge@gmail.com>

emilhotkowski requested a review from armanbilge August 3, 2022 05:39

small optimisation for optF

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armanbilge reviewed Aug 3, 2022

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Change to more general name

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emilhotkowski requested review from armanbilge and johnynek August 16, 2022 20:16

armanbilge reviewed Aug 17, 2022

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Comment thread laws/src/main/scala/cats/laws/discipline/TraverseFilterTests.scala Outdated

Update laws/src/main/scala/cats/laws/discipline/TraverseFilterTests.s…

8b7dcb6

…cala Co-authored-by: Arman Bilge <armanbilge@gmail.com>

emilhotkowski dismissed a stale review via 8b7dcb6 August 17, 2022 08:49

armanbilge approved these changes Aug 17, 2022

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johnynek approved these changes Aug 17, 2022

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armanbilge merged commit 791c65d into typelevel:main Aug 17, 2022

	traverseFilter(fa)(a => f.lift(a).map(G.map(_)(Option(_))).getOrElse(G.pure(None)))
	traverseFilter(fa)(a => Traverse[Option].sequence(f.lift(a)))

		implicit def toTraverseCollectOps[F[_], G[_], A](fa: F[A]): TraverseCollectOps[F, G, A] =
		new TraverseCollectOps(fa)

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armanbilge Aug 16, 2022 •

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