Add EitherT.cond, which is a friend of Either.cond

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

@@ -293,6 +293,30 @@ private[data] trait EitherTFunctions {
     def apply[E, A](opt: Option[A], ifNone: => E)(implicit F: Applicative[F]): EitherT[F, E, A] =
       EitherT(F.pure(Either.fromOption(opt, ifNone)))
   }
+
+  /**  If the condition is satisfied, return the given `A` in `Right`
+    *  lifted into the specified `Applicative`, otherwise, return the
+    *  given `E` in `Left` lifted into the specified `Applicative`.
+    *
+    * {{{
+    * scala> import cats.data._
+    * scala> import cats.instances.all._
+    * scala> import scala.concurrent.ExecutionContext.Implicits.global
+    * scala> val userInput = "hello world"
+    * scala> EitherT.cond[Future](
+    *   userInput.forall(_.isDigit) && userInput.size == 10,
+    *   userInput,
+    *   s"The input $userInput does not look like a phone number")
+    * res0: EitherT[Future,String,String] =
+    *   EitherT(Future(Success(Left(The input hello world does not look like a phone number))))
+    * }}}
+    */
+  final def cond[F[_]]: CondPartiallyApplied[F] = new CondPartiallyApplied
+
+  final class CondPartiallyApplied[F[_]] private[EitherTFunctions] {
+    def apply[E, A](test: Boolean, right: => A, left: => E)(implicit F: Applicative[F]): EitherT[F, E, A] =
+      EitherT(F.pure(Either.cond(test, right, left)))
+  }
 }
 
 private[data] abstract class EitherTInstances extends EitherTInstances1 {

tests/src/test/scala/cats/tests/EitherTTests.scala

@@ -156,6 +156,12 @@ class EitherTTests extends CatsSuite {
     }
   }
 
+  test("cond") {
+    forAll { (cond: Boolean, s: String, i: Int) =>
+      Either.cond(cond, s, i) should === (EitherT.cond[Id](cond, s, i).value)
+    }
+  }
+
   test("isLeft negation of isRight") {
     forAll { (eithert: EitherT[List, String, Int]) =>
       eithert.isLeft should === (eithert.isRight.map(! _))