DValidation is a little, opinionated domain object validation toolkit on top of scalaz. While scalaz already offers the great Validation abstractions for building, composing and transforming validations it is a very general tool.

DValidation tries to be more opinionated with the goal to offer tools for common validation situations including:

• A set of built-in specific and generic validators
• A custom error aggregation type (DomainErrors)
• An abstract, translation friendly error type (DomainError)
• A set of classes representing common error cases (IsEmptyStringError, IsNotGreaterThenError, ...)
• A Path abstraction for locating errors in a class hierarchy
• Converters from other error representations
• Tools for creating domain specific error types
• Utilities to support testing validations

DValidation is available via the Sonatype OSS repository:

resolvers += Resolver.sonatypeRepo("releases")

The current release targets the Scala 2.11.x, 2.12.x and 2.13.x series together with scalaz 7.3.2.

libraryDependencies += "net.atinu" %% "dvalidation" % "0.4"
libraryDependencies += "org.scalaz" %% "scalaz-core" % "7.3.2"

Lets start with this simple domain model:

trait Classification
case object StringInstrument extends Classification
case object Keyboard extends Classification

abstract class Instrument(val classification: Classification)
case object BassGuitar extends Instrument(StringInstrument)
case object Guitar extends Instrument(StringInstrument)
case object Piano extends Instrument(Keyboard)

case class Musician(name: String, age: Int, instruments: Seq[Instrument])

A way to get started with DValidation is to define an ad-hoc validation using the predefined combinators:

val mikael = Musician("Mikael Åkerfeldt", 40, List(Guitar, BassGuitar))
val martin = Musician("Martin Mendez", 17, List(BassGuitar))

val res: DValidation[Musician] = mikael.validateWith(
  notBlank(mikael.name) forAttribute "name",
  ensure(mikael.age)("error.dvalidation.legalage", 18)(_ > 18) forAttribute "age",
  hasElements(mikael.instruments) forAttribute "instruments"
)

The object mikael is valid and therefore the validation will result in a Success:

Success(User(Mikael Åkerfeldt,40))

martin on the other hand is too young which is also reflected in the validation result:

Failure(DomainError(path: /age, value: 17, msgKey: error.dvalidation.legalage, args: 18))

The definition of reusable validations can be achieved using a DValidator which can be defined as follows:

val musicianValidator: DValidator[Musician] = Validator.template[Musician] { musician =>
  musician.validateWith(
  notBlank(musician.name) forAttribute "name",
  ensure(musician.age)(key = "error.dvalidation.legalage", args = 18)(_ > 18) forAttribute "age",
  hasElements(musician.instruments) forAttribute "instruments"
 )
}
musicianValidator(mikael)
musicianValidator(martin)

A common issue when validating business objects is the validation of sequences since errors are either global or have to be mapped to a specific element. In the example the hasElements combinator is used to define a global validation rule. validSequence is a combinator which applies a given validation to all elements of a sequence. The withValidations combinator will include the resulting sequence of DValidations in the parent validation context.

val max = Musician("Max Mustermann", 29, List(Piano, Guitar))

val stringInstrumentValidator = Validator.template[Instrument](i =>
  ensure(i)(
   key = "error.dvalidation.stringinstrument", 
   args = i.classification)(_.classification == StringInstrument)
)

max.validateWith(
  notBlank(max.name) forAttribute "name",
  ensure(max.age)("error.dvalidation.legalage", 18)(_ > 18) forAttribute "age",
  hasElements(max.instruments) forAttribute "instruments"
).withValidations(
  validSequence(max.instruments, stringInstrumentValidator) forAttribute "instruments"
)

This validation will result in one DomainError with a path indicating the position in the sequence.

Failure(DomainError(path: /instruments/[0], value: Piano, msgKey: error.dvalidation.stringinstrument, args: Keyboard))

Validation often get applied to data which lives in a context. The state of the context now becomes a requirement for a successful validation. If for example an Option[String] is used to represent an optional academic title a validation expects the String to be non empty if the Option is a Some. To support those use cases higher order validators have been introduces for Option and Try. With this tool the academic title example can be solved like this:

val academicTitle: Option[String] = Some("Dr.")
validOptRequired(academicTitle)(title => notBlank(title)) 

The ensure combinator offers a simple approach how to define a custom validator.

 def isEqual[T](valueCheck: T, valueExcept: T): DValidation[T] =
    ensure(valueCheck)("error.dvalidation.isequal", valueExcept)(a => a == valueExcept)

DValidation offers generic validators which are only useful when applied to a concrete class or use case. For example the isSmallerThan validator works for all instances with a defined scalaz.Order type class. A validation for Java 8 java.time.LocalDateTime for example would look like this:

  object DateValidation {
    private implicit val lDtOrder = scalaz.Order.order[LocalDateTime]((a, b) =>
      if (a.isBefore(b)) Ordering.LT else Ordering.GT)
    
    val inPast = Validator.template[LocalDateTime](_ is_< LocalDateTime.now())
    val inFuture = Validator.template[LocalDateTime](_ is_> LocalDateTime.now())
  }

Since isSmallerThen does not know for which class the validation was done, the validator can only yield an IsNotLowerThenError. This is not an optimal situation if specific translations have to be presented to the user or if business logic should is bound to this error scenario.

DValidation offers so called ErrorMaps to map the generic error value of a validator to a use case specific error representation. In the context of the date validation this could look like this:

  object DateValidation {
    private implicit val lDtOrder = scalaz.Order.order[LocalDateTime]((a, b) =>
      if (a.isBefore(b)) Ordering.LT else Ordering.GT)
    private implicit val toInPastError = ErrorMap.mapKey[IsNotLowerThenError]("dvalidaiton.dateInPast")
    private implicit val toInFutureError = ErrorMap.mapKey[IsNotGreaterThenError]("dvalidaiton.dateInFuture")

    val inPast = Validator.template[LocalDateTime](_ is_< LocalDateTime.now())
    val inFuture = Validator.template[LocalDateTime](_ is_> LocalDateTime.now())
  }

scalaz Validation offers an applicative validation style with which validations can be composed and mapped to a larger validation. In this aspect the applicative validation is similar to the common validation style of DValidation, with the difference that the mapping from element validations to a bigger validation is done explicitly via a mapping function. Because of this the applicative validation style is potentially more type safe, if the apply function of a case class is used. The cost of this style is the fixed structure of the expected validations, which can lead to more complicated validation code.

val musicianValidatorApplicative = Validator.template[Musician] { musician =>
  val stringInstrument = validSequence(musician.instruments, stringInstrumentValidator).collapse
  val atLeastOneString = stringInstrument.disjunction
      .flatMap(value => hasElements(value).disjunction).validation
  val hasLegalAge = ensure(musician.age)(key = "error.dvalidation.legalage", args = 18)(_ > 18)

  ((notBlank(musician.name) forAttribute "name") |@|
   (hasLegalAge forAttribute "age") |@|
   (atLeastOneString forAttribute "instruments"))(Musician.apply)
}

DValidation offers conversions between Scala standard library types and DValidation.

Some(1).asValidation
// => Success(1)

val opt: Option[Int] = None
opt.asValidation
// => Failure(DomainError(path: /, value: None, msgKey: error.dvalidation.isNone))

scala.util.Success(1).asValidation
// => Success(1)

val exception = new IllegalArgumentException
scala.util.Failure(exception).asValidation
// => Failure(DomainError(path: /, value: java.lang.IllegalArgumentException, msgKey: error.dvalidation.isTryFailue))

A DValidation and DValidator are defined as follows:

type DValidation[T] = scalaz.Validation[DomainErrors, T]
type DValidator[T] = T => DValidation[T]

In other words, in the failure case of a scalaz.Valdation the DomainErrors type is used, which itself is a list of at least one DomainError. This type has attributes which allow clients to handle errors appropriately. The path attribute helps to map an error to the domain object which is responsible for the error. The value attribute will return the value of the validated attribute. Together with the msgKey and args attributes clients can build language specific error messages, which can be represented to users.

The library packages some predefined type of DomainErrors like IsEmptySeqError which already chooses appropriate value and msgKey values.