update Foldable.existsM/forallM to short-circuit

typelevel · Aug 7, 2017 · 9800e4a · 9800e4a
1 parent 6ff5f12
commit 9800e4a
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Showing 2 changed files with 45 additions and 16 deletions.
diff --git a/core/src/main/scala/cats/Foldable.scala b/core/src/main/scala/cats/Foldable.scala
@@ -327,11 +327,10 @@ import simulacrum.typeclass
     }.value
 
   /**
-    * Check whether at least one element satisfies the predicate.
+    * Check whether at least one element satisfies the effectful predicate.
     *
-    * If there are no elements, the result is `false`.
-    *
-    * All effects are sequenced, i.e. later effects aren't skipped even if a `true` is encountered.
+    * 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:
     *
@@ -341,22 +340,33 @@ import simulacrum.typeclass
     * 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))
-    * res1: Option[Boolean] = Some(true)
+    * res2: Option[Boolean] = Some(true)
     *
     * scala> F.existsM(List(1,2,3,4))(n => if (n <= 2) Option(true) else None)
-    * res2: Option[Boolean] = 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] =
-    foldM(fa, false)((z, a) => G.map(p(a))(b => b || z))
+  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 predicate.
-    *
-    * If there are no elements, the result is `true`.
+    * Check whether all elements satisfy the effectful predicate.
     *
-    * All effects are sequenced, i.e. later effects aren't skipped even if a `false` is encountered.
+    * 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:
     *
@@ -366,15 +376,27 @@ import simulacrum.typeclass
     * 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 => if (n <= 2) Option(true) else Option(false))
+    * 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)
-    * res2: Option[Boolean] = 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] =
-    foldM(fa, true)((z, a) => G.map(p(a))(b => b && z))
+  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].

diff --git a/tests/src/test/scala/cats/tests/FoldableTests.scala b/tests/src/test/scala/cats/tests/FoldableTests.scala
@@ -241,6 +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))