Trait providing an implicit class that adds a validating method to Future, which takes one or more validation functions and returns either the same Future if either the Future had already failed or its value passes all the functions, or ValidationFailedException containing an error message describing the first validation that failed.

Here's an example validation method, which passes if the given Int is evenly divisible by 10 (i.e., the result will be Pass). If the value does not pass this test, the result is a Fail containing a helpful error message string.

scala> import org.scalactic._
import org.scalactic._

scala> import FutureSugar._
import org.scalactic.FutureSugar._

scala> import scala.concurrent.Future
import scala.concurrent.Future

scala> import scala.concurrent.ExecutionContext.Implicits.global
import scala.concurrent.ExecutionContext.Implicits.global

scala> def isRound(i: Int): Validation[ErrorMessage] =
     |   if (i % 10 == 0) Pass else Fail(i + " was not a round number")
isRound: (i: Int)org.scalactic.Validation[org.scalactic.ErrorMessage]

Validation will be attempted on a successful Try. If the validation succeeds, the resulting Future will be the same successful Future with the same value. (A "validation" only transforms the Future if the validation fails, otherwise it is the same Future. The only difference is its value has now been proven valid.) In the following example, a successful Future[Int] with the value 100 passes the validation (which checks whether 100 is evenly divisible by 10), therefore the result of the validating call is the same successful Future with the same value.

scala> val fut100 = Future(100)
fut100: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@67f9c9c6

scala> fut100.value
res0: Option[scala.util.Try[Int]] = Some(Success(100))

scala> val round100 = fut100.validating(isRound)
round100: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@1ac2f0d1

scala> round100.value
res1: Option[scala.util.Try[Int]] = Some(Success(100))

If validation fails, the successful Future will be transformed into a failed one, with a ValidationFailedException that contains the error message returned by the validation function. In the following example, 42 fails the validation because it is not evenly divisible by 10:

scala> val fut42 = Future(42)
fut42: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@19c6e4d1

scala> fut42.value
res2: Option[scala.util.Try[Int]] = Some(Success(42))

scala> val round42 = fut42.validating(isRound)
round42: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@b5175d

scala> round42.value
res3: Option[scala.util.Try[Int]] = Some(Failure(org.scalactic.exceptions.ValidationFailedException: 42 was not a round number))

If validating is called on a failed Future, it just returns the same failed Future:

scala> val futEx = Future[Int] { throw new Exception("oops!") }
futEx: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@3ba0299c

scala> futEx.value
res4: Option[scala.util.Try[Int]] = Some(Failure(java.lang.Exception: oops!))

scala> val roundEx = futEx.validating(isRound)
roundEx: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@22bf1acf

scala> roundEx.value
res5: Option[scala.util.Try[Int]] = Some(Failure(java.lang.Exception: oops!))

The validating method accepts one or more validation functions. If you pass more than one, they will be tried in order up until the first failure, whose error message will appear in the ValidationFailedException. In other words, validating will short circuit at the first error and return that. It will not accumulate errors. For example, the following validation will short circuit after the isDivBy3 function fails:

scala> def isDivBy3(i: Int): Validation[ErrorMessage] =
     |   if (i % 3 == 0) Pass else Fail(i + " was not divisible by 3")
isDivBy3: (i: Int)org.scalactic.Validation[org.scalactic.ErrorMessage]

scala> def isAnswerToLifeTheUniverseAndEverything(i: Int): Validation[ErrorMessage] =
     |   if (i == 42) Pass else Fail(i + " did not equal 42")
isAnswerToLifeTheUniverseAndEverything: (i: Int)org.scalactic.Validation[org.scalactic.ErrorMessage]

scala> val futShort = fut100.validating(isRound, isDivBy3, isAnswerToLifeTheUniverseAndEverything)
futShort: scala.concurrent.Future[Int] = scala.concurrent.impl.Promise$DefaultPromise@30bb943e

scala> futShort.value
res11: Option[scala.util.Try[Int]] = Some(Failure(org.scalactic.exceptions.ValidationFailedException: 100 was not divisible by 3))