Add when and unless to OptionT (#3233)

* Add when and unless to OptionT

* Fix comments

core/src/main/scala/cats/data/OptionT.scala

@@ -223,6 +223,46 @@ object OptionT extends OptionTInstances {
    */
   def liftK[F[_]](implicit F: Functor[F]): F ~> OptionT[F, *] =
     λ[F ~> OptionT[F, *]](OptionT.liftF(_))
+
+  /**
+   * Creates a non-empty `OptionT[F, A]` from an `A` value if the given condition is `true`.
+   * Otherwise, `none[F, A]` is returned. Analogous to `Option.when`.
+   */
+  def when[F[_], A](cond: Boolean)(a: => A)(implicit F: Applicative[F]): OptionT[F, A] =
+    if (cond) OptionT.some[F](a) else OptionT.none[F, A]
+
+  /**
+   * Creates a non-empty `OptionT[F, A]` from an `F[A]` value if the given condition is `true`.
+   * Otherwise, the empty `OptionT[F, A]` is returned. Analogous to `Option.when`.
+   */
+  def whenF[F[_], A](cond: Boolean)(fa: => F[A])(implicit F: Applicative[F]): OptionT[F, A] =
+    if (cond) OptionT.liftF(fa) else OptionT(F.map(fa)(_ => Option.empty))
+
+  /**
+   * Same as `whenF`, but expressed as a FunctionK for use with mapK.
+   */
+  def whenK[F[_]](cond: Boolean)(implicit F: Applicative[F]): F ~> OptionT[F, *] =
+    λ[F ~> OptionT[F, *]](OptionT.whenF(cond)(_))
+
+  /**
+   * Creates a non-empty `OptionT[F, A]` from an `A` if the given condition is `false`.
+   * Otherwise, `none[F, A]` is returned. Analogous to `Option.unless`.
+   */
+  def unless[F[_], A](cond: Boolean)(a: => A)(implicit F: Applicative[F]): OptionT[F, A] =
+    OptionT.when(!cond)(a)
+
+  /**
+   * Creates an non-empty `OptionT[F, A]` from an `F[A]` if the given condition is `false`.
+   * Otherwise, the empty `OptionT[F, A]` is returned. Analogous to `Option.unless`.
+   */
+  def unlessF[F[_], A](cond: Boolean)(fa: => F[A])(implicit F: Applicative[F]): OptionT[F, A] =
+    OptionT.whenF(!cond)(fa)
+
+  /**
+   * Same as `unlessF`, but expressed as a FunctionK for use with mapK.
+   */
+  def unlessK[F[_]](cond: Boolean)(implicit F: Applicative[F]): F ~> OptionT[F, *] =
+    λ[F ~> OptionT[F, *]](OptionT.unlessF(cond)(_))
 }
 
 sealed abstract private[data] class OptionTInstances extends OptionTInstances0 {

tests/src/test/scala/cats/tests/OptionTSuite.scala

@@ -1,7 +1,7 @@
 package cats
 package tests
 
-import cats.data.{Const, OptionT}
+import cats.data.{Const, IdT, OptionT}
 import cats.kernel.{Monoid, Semigroup}
 import cats.kernel.laws.discipline.{EqTests, MonoidTests, OrderTests, PartialOrderTests, SemigroupTests}
 import cats.laws.discipline._
@@ -278,6 +278,46 @@ class OptionTSuite extends CatsSuite {
     }
   }
 
+  test("OptionT.when[Id, A] consistent with the same implementation of Option.when") {
+    val when = (c: Boolean, j: Int) => if (c) Some(j) else None
+    forAll { (i: Int, b: Boolean) =>
+      OptionT.when[Id, Int](b)(i).value should ===(when(b, i))
+    }
+  }
+
+  test("OptionT.whenF[F, A] consistent with the same implementation of Option.when") {
+    val when = (c: Boolean, j: Int) => if (c) Some(j) else None
+    forAll { (li: List[Int], b: Boolean) =>
+      OptionT.whenF(b)(li).value should ===(li.map(when(b, _)))
+    }
+  }
+
+  test("OptionT.whenK and OptionT.whenF consistent") {
+    forAll { (li: List[Int], b: Boolean) =>
+      IdT(li).mapK(OptionT.whenK(b)).value should ===(OptionT.whenF(b)(li))
+    }
+  }
+
+  test("OptionT.unless[Id, A] consistent with the same implementation of Option.unless") {
+    val unless = (c: Boolean, j: Int) => if (!c) Some(j) else None
+    forAll { (i: Int, b: Boolean) =>
+      OptionT.unless[Id, Int](b)(i).value should ===(unless(b, i))
+    }
+  }
+
+  test("OptionT.unlessF[F, A] consistent with the same implementation of Option.unless") {
+    val unless = (c: Boolean, j: Int) => if (!c) Some(j) else None
+    forAll { (li: List[Int], b: Boolean) =>
+      OptionT.unlessF(b)(li).value should ===(li.map(unless(b, _)))
+    }
+  }
+
+  test("OptionT.unlessK and OptionT.unlessF consistent") {
+    forAll { (li: List[Int], b: Boolean) =>
+      IdT(li).mapK(OptionT.unlessK(b)).value should ===(OptionT.unlessF(b)(li))
+    }
+  }
+
   test("orElse and orElseF consistent") {
     forAll { (o1: OptionT[List, Int], o2: OptionT[List, Int]) =>
       o1.orElse(o2) should ===(o1.orElseF(o2.value))