Add existsM and forallM to Foldable

core/src/main/scala/cats/Foldable.scala

@@ -326,6 +326,78 @@ import simulacrum.typeclass
       if (p(a)) lb else Eval.False
     }.value
 
+  /**
+    * Check whether at least one element satisfies the effectful predicate.
+    *
+    * If there are no elements, the result is `false`.  `existsM` short-circuits,
+    * i.e. once a `true` result is encountered, no further effects are produced.
+    *
+    * For example:
+    *
+    * {{{
+    * scala> import cats.implicits._
+    * scala> val F = Foldable[List]
+    * scala> F.existsM(List(1,2,3,4))(n => Option(n <= 4))
+    * res0: Option[Boolean] = Some(true)
+    *
+    * scala> F.existsM(List(1,2,3,4))(n => Option(n > 4))
+    * res1: Option[Boolean] = Some(false)
+    *
+    * scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) Option(true) else Option(false))
+    * res2: Option[Boolean] = Some(true)
+    *
+    * scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) Option(true) else None)
+    * res3: Option[Boolean] = Some(true)
+    *
+    * scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) None else Option(true))
+    * res4: Option[Boolean] = None
+    * }}}
+    */
+  def existsM[G[_], A](fa: F[A])(p: A => G[Boolean])(implicit G: Monad[G]): G[Boolean] = {
+    G.tailRecM(Foldable.Source.fromFoldable(fa)(self)) {
+      src => src.uncons match {
+        case Some((a, src)) => G.map(p(a))(bb => if (bb) Right(true) else Left(src.value))
+        case None => G.pure(Right(false))
+      }
+    }
+  }
+
+  /**
+    * Check whether all elements satisfy the effectful predicate.
+    *
+    * If there are no elements, the result is `true`.  `forallM` short-circuits,
+    * i.e. once a `false` result is encountered, no further effects are produced.
+    *
+    * For example:
+    *
+    * {{{
+    * scala> import cats.implicits._
+    * scala> val F = Foldable[List]
+    * scala> F.forallM(List(1,2,3,4))(n => Option(n <= 4))
+    * res0: Option[Boolean] = Some(true)
+    *
+    * scala> F.forallM(List(1,2,3,4))(n => Option(n <= 1))
+    * res1: Option[Boolean] = Some(false)
+    *
+    * scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) Option(true) else Option(false))
+    * res2: Option[Boolean] = Some(false)
+    *
+    * scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) Option(false) else None)
+    * res3: Option[Boolean] = Some(false)
+    *
+    * scala> F.forallM(List(1,2,3,4))(n => if (n <= 2) None else Option(false))
+    * res4: Option[Boolean] = None
+    * }}}
+    */
+  def forallM[G[_], A](fa: F[A])(p: A => G[Boolean])(implicit G: Monad[G]): G[Boolean] = {
+    G.tailRecM(Foldable.Source.fromFoldable(fa)(self)) {
+      src => src.uncons match {
+        case Some((a, src)) => G.map(p(a))(bb => if (!bb) Right(false) else Left(src.value))
+        case None => G.pure(Right(true))
+      }
+    }
+  }
+
   /**
    * Convert F[A] to a List[A].
    */

tests/src/test/scala/cats/tests/FoldableTests.scala

@@ -35,11 +35,13 @@ abstract class FoldableCheck[F[_]: Foldable](name: String)(implicit ArbFInt: Arb
     }
   }
 
-  test(s"Foldable[$name].find/exists/forall/filter_/dropWhile_") {
+  test(s"Foldable[$name].find/exists/forall/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.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)
       fa.dropWhile_(_ > n) should === (iterator(fa).dropWhile(_ > n).toList)
       fa.takeWhile_(_ > n) should === (iterator(fa).takeWhile(_ > n).toList)
@@ -239,6 +241,13 @@ class FoldableTestsAdditional extends CatsSuite {
     assert(concatUntil("Zero" #:: "One" #:: "STOP" #:: boom, "STOP") == Left("ZeroOne"))
   }
 
+  test(".existsM/.forallM short-circuiting") {
+    implicit val F = foldableStreamWithDefaultImpl
+    def boom: Stream[Boolean] = sys.error("boom")
+    assert(F.existsM[Id, Boolean](true #:: boom)(identity) == true)
+    assert(F.forallM[Id, Boolean](false #:: boom)(identity) == false)
+  }
+
   test("Foldable[List] doesn't break substitution") {
     val result  = List.range(0,10).foldM(List.empty[Int])((accum, elt) => Eval.always(elt :: accum))