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Add filterFold and collectFold to Foldable #2452

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merged 9 commits into from
Sep 12, 2018
Merged

Add filterFold and collectFold to Foldable #2452

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ed49557
add filterFold and collectFold to Foldable
kun-song Sep 1, 2018
a716cbd
add syntax for collectFold
kun-song Sep 1, 2018
05f5c9b
remove unwanted test file
kun-song Sep 1, 2018
1d82827
add properties for collectFold and filterFold using toList
kun-song Sep 1, 2018
11202e0
fix filterFold
kun-song Sep 2, 2018
0ca4ba7
fix conflict
kun-song Sep 2, 2018
417a5ce
move filterFold and collectFold to syntax for binary compatibility
kun-song Sep 2, 2018
8b232bd
use a more consistent implicit style
kun-song Sep 4, 2018
46d92cc
rename filterFold to collectSomeFold
kun-song Sep 4, 2018
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33 changes: 33 additions & 0 deletions core/src/main/scala/cats/syntax/foldable.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -158,4 +158,37 @@ final class FoldableOps[F[_], A](val fa: F[A]) extends AnyVal {
case Some((a, src)) => G.map(p(a))(if (_) Right(Some(a)) else Left(src.value))
case None => G.pure(Right(None))
})

/**
* Tear down a subset of this structure using a `PartialFunction`.
*{{{
* scala> import cats.implicits._
* scala> val xs = List(1, 2, 3, 4)
* scala> xs.collectFold { case n if n % 2 == 0 => n }
* res0: Int = 6
*}}}
*/
def collectFold[M: Monoid](f: PartialFunction[A, M])(implicit F: Foldable[F]): M = {
val m = Monoid[M]
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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])

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

Thanks!

F.foldLeft(fa, m.empty)((acc, a) ⇒ m.combine(acc, f.applyOrElse(a, (_: A) ⇒ m.empty)))
}

/**
* Tear down a subset of this structure using a `A => Option[M]`.
*{{{
* scala> import cats.implicits._
* scala> val xs = List(1, 2, 3, 4)
* scala> def f(n: Int): Option[Int] = if (n % 2 == 0) Some(n) else None
* scala> xs.filterFold(f)
* res0: Int = 6
*}}}
*/
def filterFold[M: Monoid](f: A ⇒ Option[M])(implicit F: Foldable[F]): M = {
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def filterFold[M: Monoid](f: A ⇒ Option[M])(implicit F: Foldable[F]): M =
  collectFold(Function.unlift(f))

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or, the other way around:

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

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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 :)

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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.

val m = Monoid[M]
F.foldLeft(fa, m.empty)((acc, a) ⇒ f(a) match {
case Some(x) ⇒ m.combine(acc, x)
case None ⇒ acc
})
}

}
14 changes: 14 additions & 0 deletions tests/src/test/scala/cats/tests/FoldableSuite.scala
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Expand Up @@ -85,6 +85,20 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(
}
}

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

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