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Add Foldable extension collectFirstSomeM #2366

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28 changes: 28 additions & 0 deletions core/src/main/scala/cats/syntax/foldable.scala
Original file line number Diff line number Diff line change
Expand Up @@ -92,4 +92,32 @@ final class FoldableOps[F[_], A](val fa: F[A]) extends AnyVal {
b.toString.dropRight(delim.length)
} + suffix
}

/**
* Monadic version of `collectFirstSome`.
* {{{
* scala> import cats.implicits._
* scala> def parseInt(s: String): Either[String, Int] = Either.catchOnly[NumberFormatException](s.toInt).leftMap(_.getMessage)
* scala> val keys1 = List("1", "2", "4", "5")
* scala> val map1 = Map(4 -> "Four", 5 -> "Five")
* scala> keys1.collectFirstSomeM(parseInt(_) map map1.get)
* res1: scala.util.Either[String,Option[String]] = Right(Some(Four))
* scala> val map2 = Map(6 -> "Six", 7 -> "Seven")
* scala> keys1.collectFirstSomeM(parseInt(_) map map2.get)
* res2: scala.util.Either[String,Option[String]] = Right(None)
* scala> val keys2 = List("1", "x", "4", "5")
* scala> keys2.collectFirstSomeM(parseInt(_) map map1.get)
* res3: scala.util.Either[String,Option[String]] = Left(For input string: "x")
* scala> val keys3 = List("1", "2", "4", "x")
* scala> keys3.collectFirstSomeM(parseInt(_) map map1.get)
* res4: scala.util.Either[String,Option[String]] = Right(Some(Four))
* }}}
*/
def collectFirstSomeM[G[_], B](f: A => G[Option[B]])(implicit F: Foldable[F], G: Monad[G]): G[Option[B]] =
F.foldRight(fa, Eval.now(G.pure(Option.empty[B])))((a, lb) =>
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👍

Eval.now(G.flatMap(f(a)) {
case s @ Some(_) => G.pure(s)
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We could also match on None first and then add case s - we'll avoid an instance check and cast, so it could be good for performance. Not sure if the difference is significant though, we'd need to benchmark. Feel free to ignore this comment ;)

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Got it. Let me perform some jmh benchmarks.

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@kubukoz JMH proved you right :)

case None => lb.value
})
).value
}
10 changes: 9 additions & 1 deletion tests/src/test/scala/cats/tests/FoldableSuite.scala
Original file line number Diff line number Diff line change
Expand Up @@ -140,12 +140,20 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(
fa.toList should === (iterator(fa).toList)
}
}

test(s"Foldable[$name] mkString_") {
forAll { (fa: F[Int]) =>
fa.mkString_("L[", ";", "]") should === (fa.toList.mkString("L[", ";", "]"))
}
}

test(s"Foldable[$name].collectFirstSomeM") {
forAll { (fa: F[Int], n: Int) =>
fa.collectFirstSomeM(x => (x > n).guard[Option].as(x).asRight[String]) should === (fa.toList.collectFirst {
case x if x > n => x
}.asRight[String])
}
}
}

class FoldableSuiteAdditional extends CatsSuite {
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