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Fields is a Scala validation library that you should use because it is:

  • Final Tagless. Choose any Effect, Validated, or Error types.
  • Informative. Error paths help understanding where the error occurred.
  • Expressive. Rich, extendable validation syntax.
  • Lightweight. The core module has no-dependencies.
  • Dauntless. Have no fear of complex validations with Rule type.
  • Short-circuit. Avoid running undesired validation side-effects.


// Here we pass validation dependencies using implicits, but you could be doing this the way you prefer
def policy(implicit tokenService: TokenService, userService: UserService): Policy[RegisterRequest] = Policy
  .subRule(_.age)(_ >= 18, _ <= 110)
  .subRule(_.password)(_.nonEmpty, _.minSize(4), _.maxSize(100))
  .subRule(_.password, _.passwordRepeat)(_ equalTo _)
  .subRule(_.token)(_.ensureF(tokenService.validateToken, _.failMessage("invalid-token")))

// Extract complex validations to reusable methods.
def validateUsername(username: Field[String])(implicit userService: UserService): MRule =
    username.minSize(1) &&
    username.maxSize(10) &&
    MRule.flatten {
      userService.findByUsername(username.value).map {
        case Some(_) => username.failMessage("username-already-exists")
        case None    => MRule.valid