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Add/fix Foldable extensions: findM and collectFirstSomeM #2421

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merged 4 commits into from
Aug 28, 2018
Merged

Add/fix Foldable extensions: findM and collectFirstSomeM #2421

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2421f78
Add findM to Foldable extensions
catostrophe Aug 18, 2018
8dfad2e
Empty commit (trigger Travis CI build)
catostrophe Aug 18, 2018
deed9ea
Reimplement monadic fold extensions stack-safely
catostrophe Aug 18, 2018
ce25400
Add test for .collectFirstSomeM short-circuiting
catostrophe Aug 18, 2018
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28 changes: 28 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 @@ -120,4 +120,32 @@ final class FoldableOps[F[_], A](val fa: F[A]) extends AnyVal {
case s => G.pure(s)
})
).value

/**
* Find the first element matching the effectful predicate, if one exists.
*
* If there are no elements, the result is `None`. `findM` short-circuits,
* i.e. once an element is found, no further effects are produced.
*
* For example:
* {{{
* scala> import cats.implicits._
* scala> val list = List(1,2,3,4)
* scala> list.findM(n => (n >= 2).asRight[String])
* res0: Either[String,Option[Int]] = Right(Some(2))
*
* scala> list.findM(n => (n > 4).asRight[String])
* res1: Either[String,Option[Int]] = Right(None)
*
* scala> list.findM(n => Either.cond(n < 3, n >= 2, "error"))
* res2: Either[String,Option[Int]] = Right(Some(2))
*
* scala> list.findM(n => Either.cond(n < 3, false, "error"))
* res3: Either[String,Option[Int]] = Left(error)
* }}}
*/
def findM[G[_]](p: A => G[Boolean])(implicit F: Foldable[F], G: Monad[G]): G[Option[A]] =
F.foldRight(fa, Eval.now(G.pure(Option.empty[A])))((a, lb) =>
Eval.now(G.flatMap(p(a))(if (_) G.pure(Some(a)) else lb.value))
).value
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I’m worried about stack safety here. Note findM, foldM and existsM use tailRecM to be stack safe.

Can we make Source package private so that we can use it here with tailRecM?

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Good point. I assumed stack-safety is provided by safe flatMap implementation. Will rework via tailRecM.

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Also added stack-safety tests for other monadic folds.

}
9 changes: 8 additions & 1 deletion tests/src/test/scala/cats/tests/FoldableSuite.scala
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Expand Up @@ -85,11 +85,12 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(
}
}

test(s"Foldable[$name].find/exists/forall/existsM/forallM/filter_/dropWhile_") {
test(s"Foldable[$name].find/exists/forall/findM/existsM/forallM/filter_/dropWhile_") {
forAll { (fa: F[Int], n: Int) =>
fa.find(_ > n) should === (iterator(fa).find(_ > n))
fa.exists(_ > n) should === (iterator(fa).exists(_ > n))
fa.forall(_ > n) should === (iterator(fa).forall(_ > n))
fa.findM(k => Option(k > n)) should === (Option(iterator(fa).find(_ > n)))
fa.existsM(k => Option(k > n)) should === (Option(iterator(fa).exists(_ > n)))
fa.forallM(k => Option(k > n)) should === (Option(iterator(fa).forall(_ > n)))
fa.filter_(_ > n) should === (iterator(fa).filter(_ > n).toList)
Expand Down Expand Up @@ -324,6 +325,12 @@ class FoldableSuiteAdditional extends CatsSuite {
assert(F.forallM[Id, Boolean](false #:: boom)(identity) == false)
}

test(".findM short-circuiting") {
implicit val F = foldableStreamWithDefaultImpl
def boom: Stream[Int] = sys.error("boom")
assert((1 #:: boom).findM[Id](_ > 0) == Some(1))
}

test("Foldable[List] doesn't break substitution") {
val result = List.range(0,10).foldM(List.empty[Int])((accum, elt) => Eval.always(elt :: accum))

Expand Down