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A standalone Validation data type, called Result. (see How do I error handle thee? for more on the general idea of a Validation)

The Result type is very similar to scalaz.Validation,, or org.scalactic.Or with the following differences:

  • no dependencies besides scala and no additional baggage

that is no Monads, Applicative Functors, Arrows, or Categories. The only thing that is provided besides Result is a NonEmptyVector which is similar to a regular Vector, only that there will always be at least one element in it.

The main incentive is, that Result can be used in places where a full-fledged dependency on scalaz and co. is undesired, e.g. in projects with many people that are unfamiliar with or intimidated by scalaz.

  • explicit symbolic and unsafe operators

By default, all operations are safe, meaning they are referentially transparent if their input is. Unsafe operations (foreach, get) can be enabled by importing validation.Result.unsafe._

All methods are named aptly (or so I hope) and with ascii characters. Symbolic names (such as | for getOrElse or |@| for and) can be enabled by importing validation.Result.symbolic._

  • implied underlying Vector to accumulate data

Scalaz differentiates between Validation[E, A] and type ValidationNel[E, A] = Validation[NonEmptyList[E], A] where as Result[E, A] has an implied NonEmptyVector accumulating the E. It behaves mostly as a permanent (imaginary) ResultNev[E, A].

What Validation is not

This library is not an implementation of a validation framework, just a general data structure.


// scala 2.11.x
libraryDependencies += "de.knutwalker" %% "validation" % "0.2.0"

// scala 2.12.x
libraryDependencies += "de.knutwalker" %% "validation" % "0.3.0"


import validation._

type Validated[A] = Result[String, A]

case class Person(name: String, age: Int)

def parseName(s: String): Validated[String] =
  if (s.trim.nonEmpty) Result.valid(s.trim)
  else Result.invalid(s"invalid name: '$s'")

def parseAge(s: String): Validated[Int] =
    .filter(_ >= 0, s"invalid age: '$s'")

def parsePerson(name: String, age: String): Validated[Person] =
   (parseName(name) and parseAge(age)) apply Person

parsePerson("Bernd", "42")
// res0: Validated[Person] = Valid(Person(Bernd,42))

parsePerson("Bernd", "fortytwo")
// res1: Validated[Person] = Invalid(For input string: "fortytwo")

parsePerson("Bernd", "-1337")
// res2: Validated[Person] = Invalid(invalid age: '-1337')

parsePerson("", "")
// res3: Validated[Person] = Invalids(NonEmptyVector(invalid name: '',For input string: ""))