Add iterateWhileM and iterateUntilM (#1809)

* Limit runtime of monad loop tests

* Do not use Either syntax in monad loops

* Add iterateWhileM and iterateUntilM

* Implement iterateWhile in terms of iterateWhileM

core/src/main/scala/cats/Monad.scala

@@ -1,7 +1,7 @@
 package cats
 
 import simulacrum.typeclass
-import syntax.either._
+
 /**
  * Monad.
  *
@@ -30,7 +30,7 @@ import syntax.either._
           Left(G.combineK(xs, G.pure(bv)))
         }
       },
-      ifFalse = pure(xs.asRight[G[A]])
+      ifFalse = pure(Right(xs))
     ))
   }
 
@@ -76,28 +76,37 @@ import syntax.either._
    * Execute an action repeatedly until its result fails to satisfy the given predicate
    * and return that result, discarding all others.
    */
-  def iterateWhile[A](f: F[A])(p: A => Boolean): F[A] = {
+  def iterateWhile[A](f: F[A])(p: A => Boolean): F[A] =
     flatMap(f) { i =>
-      tailRecM(i) { a =>
-        if (p(a))
-          map(f)(_.asLeft[A])
-        else pure(a.asRight[A])
-      }
+      iterateWhileM(i)(_ => f)(p)
     }
-  }
 
   /**
    * Execute an action repeatedly until its result satisfies the given predicate
    * and return that result, discarding all others.
    */
-  def iterateUntil[A](f: F[A])(p: A => Boolean): F[A] = {
+  def iterateUntil[A](f: F[A])(p: A => Boolean): F[A] =
     flatMap(f) { i =>
-      tailRecM(i) { a =>
-        if (p(a))
-          pure(a.asRight[A])
-        else map(f)(_.asLeft[A])
-      }
+      iterateUntilM(i)(_ => f)(p)
     }
-  }
+
+  /**
+    * Apply a monadic function iteratively until its result fails
+    * to satisfy the given predicate and return that result.
+    */
+  def iterateWhileM[A](init: A)(f: A => F[A])(p: A => Boolean): F[A] =
+    tailRecM(init) { a =>
+      if (p(a))
+        map(f(a))(Left(_))
+      else
+        pure(Right(a))
+    }
+
+  /**
+    * Apply a monadic function iteratively until its result satisfies
+    * the given predicate and return that result.
+    */
+  def iterateUntilM[A](init: A)(f: A => F[A])(p: A => Boolean): F[A] =
+    iterateWhileM(init)(f)(!p(_))
 
 }

core/src/main/scala/cats/syntax/monad.scala

@@ -3,6 +3,9 @@ package syntax
 
 trait MonadSyntax {
   implicit final def catsSyntaxMonad[F[_], A](fa: F[A]): MonadOps[F, A] = new MonadOps(fa)
+
+  implicit final def catsSyntaxMonadIdOps[A](a: A): MonadIdOps[A] =
+    new MonadIdOps[A](a)
 }
 
 final class MonadOps[F[_], A](val fa: F[A]) extends AnyVal {
@@ -13,3 +16,16 @@ final class MonadOps[F[_], A](val fa: F[A]) extends AnyVal {
   def iterateWhile(p: A => Boolean)(implicit M: Monad[F]): F[A] = M.iterateWhile(fa)(p)
   def iterateUntil(p: A => Boolean)(implicit M: Monad[F]): F[A] = M.iterateUntil(fa)(p)
 }
+
+final class MonadIdOps[A](val a: A) extends AnyVal {
+
+  /**
+    * Iterative application of `f` while `p` holds.
+    */
+  def iterateWhileM[F[_]](f: A => F[A])(p: A => Boolean)(implicit M: Monad[F]): F[A] = M.iterateWhileM(a)(f)(p)
+
+  /**
+    * Iterative application of `f` until `p` holds.
+    */
+  def iterateUntilM[F[_]](f: A => F[A])(p: A => Boolean)(implicit M: Monad[F]): F[A] = M.iterateUntilM(a)(f)(p)
+}

tests/src/test/scala/cats/tests/MonadTest.scala

@@ -7,33 +7,35 @@ import org.scalacheck.Gen
 class MonadTest extends CatsSuite {
   implicit val testInstance: Monad[StateT[Id, Int, ?]] = StateT.catsDataMonadForStateT[Id, Int]
 
+  val smallPosInt = Gen.choose(1, 5000)
+
   val increment: StateT[Id, Int, Unit] = StateT.modify(_ + 1)
   val incrementAndGet: StateT[Id, Int, Int] = increment >> StateT.get
 
   test("whileM_") {
-    forAll(Gen.posNum[Int]) { (max: Int) =>
+    forAll(smallPosInt) { (max: Int) =>
       val (result, _) = increment.whileM_(StateT.inspect(i => !(i >= max))).run(0)
       result should ===(Math.max(0, max))
     }
   }
 
   test("whileM") {
-    forAll(Gen.posNum[Int]) { (max: Int) =>
+    forAll(smallPosInt) { (max: Int) =>
       val (result, aggregation) = incrementAndGet.whileM[Vector](StateT.inspect(i => !(i >= max))).run(0)
       result should ===(Math.max(0, max))
       aggregation should === ( if(max > 0) (1 to max).toVector else Vector.empty )
     }
   }
 
   test("untilM_") {
-    forAll(Gen.posNum[Int]) { (max: Int) =>
+    forAll(smallPosInt) { (max: Int) =>
       val (result, _) = increment.untilM_(StateT.inspect(_ >= max)).run(-1)
       result should ===(max)
     }
   }
 
   test("untilM") {
-    forAll(Gen.posNum[Int]) { (max: Int) =>
+    forAll(smallPosInt) { (max: Int) =>
       val (result, aggregation) = incrementAndGet.untilM[Vector](StateT.inspect(_ >= max)).run(-1)
       result should ===(max)
       aggregation should === ((0 to max).toVector)
@@ -51,7 +53,7 @@ class MonadTest extends CatsSuite {
   }
 
   test("iterateWhile") {
-    forAll(Gen.posNum[Int]) { (max: Int) =>
+    forAll(smallPosInt) { (max: Int) =>
       val (result, _) = incrementAndGet.iterateWhile(_ < max).run(-1)
       result should ===(Math.max(0, max))
     }
@@ -63,7 +65,7 @@ class MonadTest extends CatsSuite {
   }
 
   test("iterateUntil") {
-    forAll(Gen.posNum[Int]) { (max: Int) =>
+    forAll(smallPosInt) { (max: Int) =>
       val (result, _) = incrementAndGet.iterateUntil(_ == max).run(-1)
       result should ===(Math.max(0, max))
     }
@@ -74,4 +76,28 @@ class MonadTest extends CatsSuite {
     result should ===(50000)
   }
 
+  test("iterateWhileM") {
+    forAll(smallPosInt) { (max: Int) =>
+      val (n, sum) = 0.iterateWhileM(s => incrementAndGet map (_ + s))(_ < max).run(0)
+      sum should ===(n * (n + 1) / 2)
+    }
+  }
+
+  test("iterateWhileM is stack safe") {
+    val (n, sum) = 0.iterateWhileM(s => incrementAndGet map (_ + s))(_ < 50000000).run(0)
+    sum should ===(n * (n + 1) / 2)
+  }
+
+  test("iterateUntilM") {
+    forAll(smallPosInt) { (max: Int) =>
+      val (n, sum) = 0.iterateUntilM(s => incrementAndGet map (_ + s))(_ > max).run(0)
+      sum should ===(n * (n + 1) / 2)
+    }
+  }
+
+  test("iterateUntilM is stack safe") {
+    val (n, sum) = 0.iterateUntilM(s => incrementAndGet map (_ + s))(_ > 50000000).run(0)
+    sum should ===(n * (n + 1) / 2)
+  }
+
 }