Add filterFold and collectFold to Foldable by kun-song · Pull Request #2452 · typelevel/cats

kun-song · 2018-09-01T09:44:20Z

johnynek · 2018-09-01T18:25:40Z

+  def filterFold[A, M: Monoid](fa: F[A])(f: A ⇒ Option[M]): M = {
+    val m = Monoid[M]
+    foldLeft(fa, m.empty)((acc, a) ⇒ f(a) match {
+      case Some(x) ⇒ m.combine(x, acc)


I think the order on combine is wrong. I think this won’t match collectFold for a non commutative Monoid.

Yes, that's right, thanks, I will fix it soon :)

johnynek · 2018-09-01T18:26:19Z

+  test(s"Foldable[$name] partial summation") {
+    forAll { (fa: F[Int], f: Int ⇒ Boolean) ⇒
+      val m: Monoid[Int] = Monoid[Int]
+


Let’s test with a non commutative Monoid like String.

Yes, l will do it, thanks.

kun-song · 2018-09-02T06:32:36Z

The CI complains an binary compatibility issue:

[error]  * method filterFold(java.lang.Object,scala.Function1,cats.kernel.Monoid)java.lang.Object in trait cats.Foldable is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.Foldable.filterFold")
[error]  * method collectFold(java.lang.Object,scala.PartialFunction,cats.kernel.Monoid)java.lang.Object in trait cats.Foldable is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.Foldable.collectFold")
[error]  * method filterFold(scala.Function1,cats.kernel.Monoid)java.lang.Object in trait cats.Foldable#Ops is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.Foldable#Ops.filterFold")
[error]  * method collectFold(scala.PartialFunction,cats.kernel.Monoid)java.lang.Object in trait cats.Foldable#Ops is present only in current version
[error]    filter with: ProblemFilters.exclude[ReversedMissingMethodProblem]("cats.Foldable#Ops.collectFold")

Should I implement these two methods as syntax extensions to avoid this problem?

codecov-io · 2018-09-02T12:38:02Z

Codecov Report

Merging #2452 into master will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #2452     +/-   ##
=========================================
+ Coverage   95.29%   95.39%   +0.1%     
=========================================
  Files         351      357      +6     
  Lines        6371     6519    +148     
  Branches      279      275      -4     
=========================================
+ Hits         6071     6219    +148     
  Misses        300      300

Impacted Files	Coverage Δ
core/src/main/scala/cats/syntax/foldable.scala	`100% <100%> (ø)`	⬆️
core/src/main/scala/cats/instances/list.scala	`100% <0%> (ø)`	⬆️
core/src/main/scala/cats/instances/option.scala	`100% <0%> (ø)`	⬆️
core/src/main/scala/cats/data/Const.scala	`100% <0%> (ø)`	⬆️
core/src/main/scala/cats/instances/vector.scala	`100% <0%> (ø)`	⬆️
.../src/main/scala/cats/laws/TraverseFilterLaws.scala	`100% <0%> (ø)`
...s/src/main/scala/cats/laws/FunctorFilterLaws.scala	`100% <0%> (ø)`
...cala/cats/laws/discipline/FunctorFilterTests.scala	`100% <0%> (ø)`
...ala/cats/laws/discipline/TraverseFilterTests.scala	`100% <0%> (ø)`
core/src/main/scala/cats/FunctorFilter.scala	`100% <0%> (ø)`
... and 7 more

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LukaJCB

I think it looks good for now, I'd like to add more laws like the one I mentioned in the issue, but since we can't add these two methods to the type class I'm more than content to just merge this as is :)

PeterPerhac · 2018-09-03T07:20:28Z

+    * res0: Int = 6
+    *}}}
+    */
+  def filterFold[M: Monoid](f: A ⇒ Option[M])(implicit F: Foldable[F]): M = {


def filterFold[M: Monoid](f: A ⇒ Option[M])(implicit F: Foldable[F]): M = collectFold(Function.unlift(f))

or, the other way around:

def collectFold[M: Monoid](f: PartialFunction[A, M])(implicit F: Foldable[F]): M = filterFold(f.lift)

We could indeed define these in terms of each other. I'm not sure if they'd be more performant than just using foldLeft, but if we do this, then it should be filterFold in terms of collectFold, since A => Option[M] has to allocate an extra Option :)

Up to you, @satansk :)

Hi @PeterPerhac @LukaJCB , I think define them in terms of each other is good, but people are more familiar with foldLeft, so I personally prefer to define them by foldLeft :)

But if there are big performance differences, we can change to the better one.

kailuowang · 2018-09-04T14:16:38Z

+    *}}}
+    */
+  def collectFold[M: Monoid](f: PartialFunction[A, M])(implicit F: Foldable[F]): M = {
+    val m = Monoid[M]


totally nitpick - there are two styles of implicit type class instance summoning in this one method (and the filterFold method below). I think it would help readability a little bit if we just use (implicit F: Foldable[F], M: Monoid[M])

This is a nice advice, it will be more readable, and I'll fix it :)

Thanks!

kailuowang · 2018-09-04T15:40:41Z

+    * res0: Int = 6
+    *}}}
+    */
+  def filterFold[M](f: A ⇒ Option[M])(implicit F: Foldable[F], M: Monoid[M]): M =


The word filter, to me, kind of implies using a boolean value.
We already have a collectFirstSome https://github.com/typelevel/cats/blob/master/core/src/main/scala/cats/Foldable.scala#L233
how about naming this one collectSomeFold?

A reasonable advice, I'll fix it.

We do have mapFilter which does use Option though.
Maybe this should be mapFilterFold? I don't know

Right, I am fine with mapFilterFold too. Sorry about the bikeshedding @satansk , your call.

LukaJCB · 2018-09-12T16:28:32Z

👍

He-Pin · 2018-09-12T17:01:58Z

@satansk +100 ：）

kun-song · 2018-09-13T00:55:07Z

@hepin1989 :)

kun-song added 4 commits September 1, 2018 17:43

add filterFold and collectFold to Foldable

ed49557

add syntax for collectFold

a716cbd

remove unwanted test file

05f5c9b

add properties for collectFold and filterFold using toList

1d82827

johnynek reviewed Sep 1, 2018

View reviewed changes

kun-song added 2 commits September 2, 2018 09:20

fix filterFold

11202e0

fix conflict

0ca4ba7

move filterFold and collectFold to syntax for binary compatibility

417a5ce

LukaJCB previously approved these changes Sep 2, 2018

View reviewed changes

PeterPerhac reviewed Sep 3, 2018

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kailuowang reviewed Sep 4, 2018

View reviewed changes

use a more consistent implicit style

8b232bd

kun-song dismissed LukaJCB’s stale review via 8b232bd September 4, 2018 14:42

kailuowang reviewed Sep 4, 2018

View reviewed changes

rename filterFold to collectSomeFold

46d92cc

kailuowang approved these changes Sep 12, 2018

View reviewed changes

LukaJCB merged commit 30c29e8 into typelevel:master Sep 12, 2018

kailuowang added this to the 1.4 milestone Sep 12, 2018

kailuowang added the enhancement label Sep 12, 2018

kun-song deleted the add-partial-folding-to-Foldable branch September 13, 2018 00:55

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LukaJCB commented Sep 12, 2018

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