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Dead simple validation micro library (or micro framework) for Scala.
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Dead simple validation micro library (or micro framework) for Scala.


Because every project bigger than HelloWorldApp requires data validation at some point. Reinventing the structure and validation handling boilerplate every time you need one is definitely not something developers enjoy. I too had this pain when hacking on

This small utility aims to provide minimal framework to write your validation logic, act as a guard between data and action and collect results in unified way. That's all it does. And it's really, really tiny. No external dependencies and no magic included. Just boilerplate.


Current version released is 0.1. Project package is deployed to Sonatype's OSS repository. To use it in your project, add a dependency:

libraryDependencies += "com.softwaremill.scalaval" %% "scalaval" % "0.1"

ScalaVal works with Scala 2.10+.


  1. Define rules (in Scala code, no insane rules lang etc).
    • Each rule gets freely-defined "label" (usually name of field it validates)
    • Returns pair: (condition, errorMessage)
    • When condition is not met validation rule is considered as failed and errorMessage is collected
  2. Call validate and collect errors or provide optional block of code to execute when validation passes
  3. Handle validation result returned as Either with either errors reported or or block result

Quick example

See full example at the bottom of the page.

// rule
val checkUserNameAvailable = rule("username") {
  val available = // call DB to check if username is available
  (available, s"Username ${form.userName} is already taken")

// validation
validate(checkUserNameAvailable).whenOk {
  // save user

Evaluating rules

Rules are evaluated only once and only when validate is called.

By default all rules are evaluated and all errors collected are reported. You can make particular rule immediately halting validation process if required e.g. when further validation calls are expensive etc. In such case it will evaluate only rules until this failing rule is found (inclusive). This is shown below:

// rule
val checkUserNameAvailable = rule("username", haltOnFail = true) {
  val available = // call DB to check if username is available
  (available, s"Username ${form.userName} is already taken")

Example usage

Take a look at project's specs dir for more detailed examples.

Let's validate some User sign-up form object

case class SignUpForm(email: String, password: String, rePassword: String)

object ScalaValExample extends App {

  import com.softwaremill.scalaval.Validation._

  // invalid email and passwords don't match
  val form = SignUpForm("", "secret", "secre")

  // define rules
  val emailValid = rule("email") {
    val emailOk ="@")  // just pretend it's enough
    (emailOk, s"Provided email ${} is invalid")
  val passwordLength = rule("password")(form.password.length >= 8, "Password is too short")
  val passwordMatch = rule("password")(form.password == form.rePassword, "Passwords don't match")

  // validate against rules (in order)
  val result = validate(emailValid, passwordLength, passwordMatch).whenOk {
    // register user
    // save in DB, trigger email sending, etc

  // handle operation result
  result match {
    case Left(errors) => errors.flatMap(_._2).foreach(println)  // handle errors, grouped by key       
    case _ => // yay! User registered, go celebrating!



Licensed under Apache 2.0 licence. Issues, Pull Requests are more than welcome!

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