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Implement Reducible.reduceM #1193

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Jul 15, 2016
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Implement Reducible.reduceM #1193

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6 changes: 6 additions & 0 deletions core/src/main/scala/cats/Reducible.scala
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Original file line number Diff line number Diff line change
Expand Up @@ -59,6 +59,12 @@ import simulacrum.typeclass
*/
def reduceLeftTo[A, B](fa: F[A])(f: A => B)(g: (B, A) => B): B

/**
* Monadic variant of [[reduceLeftTo]]
*/
def reduceLeftM[G[_], A, B](fa: F[A])(f: A => G[B])(g: (B, A) => G[B])(implicit G: FlatMap[G]): G[B] =
reduceLeftTo(fa)(f)((gb, a) => G.flatMap(gb)(g(_, a)))

/**
* Overriden from Foldable[_] for efficiency.
*/
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18 changes: 18 additions & 0 deletions tests/src/test/scala/cats/tests/ReducibleTests.scala
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@@ -0,0 +1,18 @@
package cats
package tests

class ReducibleTestsAdditional extends CatsSuite {

test("Reducible[NonEmptyList].reduceLeftM stack safety") {
def nonzero(acc: Long, x: Long): Option[Long] =
if (x == 0) None else Some(acc + x)

val n = 100000L
val expected = n*(n+1)/2
val actual = (1L to n).toList.toNel.flatMap(_.reduceLeftM(Option.apply)(nonzero))
actual should === (Some(expected))
}
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This should be fine since (1L to n).toList.toNel should never return None, but I think I'd rather either see it explicitly use Option.get or do something like:

val actual = (1L to n).toList.toNel.flatMap(_.reduceM(Option.apply)(nonzero))
actual should === (Some(expected))


}