Add ifTrueM, ifFalseM, ensureTrue and ensureFalse

core/src/main/scala-2/cats/syntax/MonadOps.scala

@@ -21,6 +21,7 @@
 
 package cats.syntax
 
+import cats.kernel.Monoid
 import cats.{Alternative, Monad}
 
 final class MonadOps[F[_], A](private val fa: F[A]) extends AnyVal {
@@ -32,4 +33,8 @@ final class MonadOps[F[_], A](private val fa: F[A]) extends AnyVal {
   def iterateUntil(p: A => Boolean)(implicit M: Monad[F]): F[A] = M.iterateUntil(fa)(p)
   def flatMapOrKeep[A1 >: A](pfa: PartialFunction[A, F[A1]])(implicit M: Monad[F]): F[A1] =
     M.flatMapOrKeep[A, A1](fa)(pfa)
+  def ifTrueM[B: Monoid](ifTrue: => F[B])(implicit env: F[A] <:< F[Boolean], M: Monad[F]): F[B] =
+    M.ifTrueM(fa)(ifTrue)
+  def ifFalseM[B: Monoid](ifFalse: => F[B])(implicit env: F[A] <:< F[Boolean], M: Monad[F]): F[B] =
+    M.ifFalseM(fa)(ifFalse)
 }

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

@@ -21,6 +21,8 @@
 
 package cats
 
+import cats.kernel.Monoid
+
 /**
  * Monad.
  *
@@ -162,6 +164,18 @@ trait Monad[F[_]] extends FlatMap[F] with Applicative[F] {
     tailRecM(branches.toList)(step)
   }
 
+  /**
+   * If the `F[Boolean]` is `true` then return `ifTrue` otherwise return `Monoid[A].empty`
+   */
+  def ifTrueM[B: Monoid](fa: F[Boolean])(ifTrue: => F[B]): F[B] =
+    ifM(fa)(ifTrue, pure(Monoid[B].empty))
+
+  /**
+   * If the `F[Boolean]` is `false` then return `ifFalse` otherwise return `Monoid[A].empty`
+   */
+  def ifFalseM[B: Monoid](fa: F[Boolean])(ifFalse: => F[B]): F[B] =
+    ifM(fa)(pure(Monoid[B].empty), ifFalse)
+
   /**
    * Modifies the `A` value in `F[A]` with the supplied function, if the function is defined for the value.
    * Example:

core/src/main/scala/cats/MonadError.scala

@@ -40,6 +40,18 @@ trait MonadError[F[_], E] extends ApplicativeError[F, E] with Monad[F] {
   def ensureOr[A](fa: F[A])(error: A => E)(predicate: A => Boolean): F[A] =
     flatMap(fa)(a => if (predicate(a)) pure(a) else raiseError(error(a)))
 
+  /**
+   * Ensures that a `F[Boolean]` is `true`, otherwise raises an error.
+   */
+  def ensureTrue(fa: F[Boolean])(error: => E): F[Boolean] =
+    ensure(fa)(error)(identity)
+
+  /**
+   * Ensures that a `F[Boolean]` is `false`, otherwise raises an error.
+   */
+  def ensureFalse(fa: F[Boolean])(error: => E): F[Boolean] =
+    ensure(fa)(error)(bool => !bool)
+
   /**
    * Inverse of `attempt`
    *

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

@@ -33,12 +33,19 @@ trait MonadErrorSyntax {
 }
 
 final class MonadErrorOps[F[_], E, A](private val fa: F[A]) extends AnyVal {
+
   def ensure(error: => E)(predicate: A => Boolean)(implicit F: MonadError[F, E]): F[A] =
     F.ensure(fa)(error)(predicate)
 
   def ensureOr(error: A => E)(predicate: A => Boolean)(implicit F: MonadError[F, E]): F[A] =
     F.ensureOr(fa)(error)(predicate)
 
+  def ensureTrue(error: => E)(implicit env: F[A] <:< F[Boolean], F: MonadError[F, E]): F[Boolean] =
+    F.ensureTrue(fa)(error)
+
+  def ensureFalse(error: => E)(implicit env: F[A] <:< F[Boolean], F: MonadError[F, E]): F[Boolean] =
+    F.ensureFalse(fa)(error)
+
   /**
    * Turns a successful value into the error returned by a given partial function if it is
    * in the partial function's domain.

tests/shared/src/test/scala/cats/tests/MonadErrorSuite.scala

@@ -95,4 +95,14 @@ class MonadErrorSuite extends CatsSuite {
   test("rethrow returns the successful value, when applied to a Right of a specialized successful value") {
     assert(successful.attempt.asInstanceOf[Try[Either[IllegalArgumentException, Int]]].rethrow === successful)
   }
+
+  test("ensureTrue raise an error only when the value is false") {
+    Try(true).ensureTrue(failedValue) === Success(true)
+    Try(false).ensureTrue(failedValue) === Failure(failedValue)
+  }
+
+  test("ensureFalse raise an error only when the value is true") {
+    Try(true).ensureFalse(failedValue) === Failure(failedValue)
+    Try(false).ensureFalse(failedValue) === Success(true)
+  }
 }

tests/shared/src/test/scala/cats/tests/MonadSuite.scala

@@ -154,4 +154,19 @@ class MonadSuite extends CatsSuite {
     assert(actual.value === 2)
   }
 
+  test("ifTrueM") {
+    val actual1: Eval[Int] = Eval.later(true).ifTrueM(Eval.later(1))
+    assert(actual1.value === 1)
+
+    val actual2: Eval[Int] = Eval.later(false).ifTrueM(Eval.later(1))
+    assert(actual2.value === 0)
+  }
+
+  test("ifFalseM") {
+    val actual1: Eval[Int] = Eval.later(true).ifFalseM(Eval.later(1))
+    assert(actual1.value === 0)
+
+    val actual2: Eval[Int] = Eval.later(false).ifFalseM(Eval.later(1))
+    assert(actual2.value === 1)
+  }
 }